CA2028028A1 - Intrinsically safe system - Google Patents
Intrinsically safe systemInfo
- Publication number
- CA2028028A1 CA2028028A1 CA 2028028 CA2028028A CA2028028A1 CA 2028028 A1 CA2028028 A1 CA 2028028A1 CA 2028028 CA2028028 CA 2028028 CA 2028028 A CA2028028 A CA 2028028A CA 2028028 A1 CA2028028 A1 CA 2028028A1
- Authority
- CA
- Canada
- Prior art keywords
- voltage
- circuits
- conductor
- power supply
- current
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/008—Intrinsically safe circuits
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/02—Details
- H02H3/021—Details concerning the disconnection itself, e.g. at a particular instant, particularly at zero value of current, disconnection in a predetermined order
- H02H3/023—Details concerning the disconnection itself, e.g. at a particular instant, particularly at zero value of current, disconnection in a predetermined order by short-circuiting
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/02—Details
- H02H3/04—Details with warning or supervision in addition to disconnection, e.g. for indicating that protective apparatus has functioned
- H02H3/044—Checking correct functioning of protective arrangements, e.g. by simulating a fault
Abstract
ABSTRACT
A system for rendering intrinsically safe the electrical supply to field devices in a hazardous area includes redundant crowbar circuits coupled to the power supply which energizes the field devices and resistors in series with the electrical lines supplying the field devices. In this way, large numbers of field devices may be rendered intrinsically safe without use of expensive intrinsic safety barriers in each field circuit.
A system for rendering intrinsically safe the electrical supply to field devices in a hazardous area includes redundant crowbar circuits coupled to the power supply which energizes the field devices and resistors in series with the electrical lines supplying the field devices. In this way, large numbers of field devices may be rendered intrinsically safe without use of expensive intrinsic safety barriers in each field circuit.
Description
2~28028 ~xpress Mail LB143189563 t~nVdrYlbr~ trinslVptnt.~plcn , Oc~ ~ 09:~:02 19~9 INTRINS ICALLY SAFE SYSTEM
Field Of The Inventio~
This invention relates to electrical systems, such as instrument systems, which may operate in an environment which may contain combustible materials. More particularly, this invention relates to methods and apparatus for ensuring that electrical energy and power supplied to an environment which may contain combustible materials are limited so as to minimize tne likelihood of i~nition of comhustible materials.
BacXaround of the_Invention Many loc~tions, such as in and around industrial processes, ar2 or are subject to becoming "hazardous"
areas, i.e. areas in which the concentration of flammable gases i8 or may become in the range which will present an explosion and~or ~re hazard. Such hazardous areas often require electrical apparatus and/or wiring to be present, such as apparatus and wiring for instrumentation systems to monitor and con~rol an lndustrial process.
~0 Several approaches have been developed to cope with the explo6ion hazard in ~uch Areas. Initially, apparatl~s was placed in heavy and expensive metal casings and wiring was run in heavy and expensive metal conduit, so that any axplosion ignited by ~he apparatus would be ~5 con1ned to the interlor of the hou~ing and conduit system 2~28~28 and unable to ignite hazardous atmospheres surrounding the ~ystem. Such "explosion proof" systems are extremely expensive, and are inconvenient because they require that the area be made non-hazardous prior to opening an explosion proo~ housing such as to test, calibrate, or inspect apparatus in the housing.
With the advent of solid state electronics, it became possible to design instrumentation and control systems operable at power levels low enough to be intrinsically unable to ignite specified hazardous atmospheres. So long as the energy storage capacity of circuits supplied from energy and power limited supplies is sufficiently low, such apparatus will be incapable of releasing sufficient energy to ignite hazardous atmospheres and the entire system, including both wiring and connect~d electrical apparatus, will be safe for u~e in the hazardous atmosphere.
Since all real~world 3ystems are s~bject to component ailure, systems are generally desi~ned to tolerate B certain number of events which are considered to be faults in the system, while still maintai~ing energy and power lavals su~ficiently low as to preclude ignition of a hazardous atmosphere. According to generally accepted standards, a ~ystem i~ deemed "intrinsically safeN for a ~peciied hazardous atmosphPre if it can withstand any combination of two ~aults in tha ~y~tem while maintaining en~rgy and power levels below the limits necessary to ignita that ha~ardous atmosphere.
Intrinsic ~afety "barriers" have been developed to prevent intrusion of hazardous voltages and currents into circuitry which is intended to be intrinsically safe.
Such barriers have an input terminal adapted to be coupled to a power ~ource, an output terminal adapted to be coupled to intrinsically safe wiring and circuity, means for limiting the voltage which may be applied to the output terminal, and means for limiting the current which may be supplied to the output terminal. Such barr~ers typically include a fuse to protect the voltage an~
current limiting components from ~ailure in the event of high power being supplied to the input terminal.
Such barriers generally c~ntain components whose failure modes, arrangement, and redundancy are such that the output terminal is considered intrinsically safe. The circuit components and terminals are typically supplied as a potted or encapsulated assembly in which the components, including the fuse, are inaccessibl~. By supplying one such barrier in eac~ non-grounded line entering a 20 hazardous area, those lines will be rendered intrinsically ~afe provided that intrinsically ~afe wiring practices are followed and devices coupled to those lines have appropriately low ener~y storage capacity.
However, ~uch intrinsic ~afety barriers are expen~ive items, and can considerably afect the cost of an intrinsically safe circuit or system. This i5 particularly the case with systems involving a large number of intrinsically safe circuits. Moreover, when a fault cccur8 which blows a fuse in a barrier, it typically must be replaced, leading to additional expense and inconvenience. Some intrinsic saf2ty barriers also laçk the ability to have each limiting component tested individually to verify that the barri~r is fully unctional with t~e intended degree of redundancy. Even in barriers whera quch tests may be made, testing typically requires removal of the barrier from service, a procedure which may involve considerable expense, time, and inconvenienc~.
S mmarv Of The Invention It it therefore an object of the invention to provide inexpensive intrinsic safety means.
It is a further object of the invention to provide intrinsic sa~ety means suitable for use with a lar~e number of circuits.
It is another ob;ect of the invention to provid~
intrinsic sa~ety means usable with a wide variety of ~ield devices.
It i3 another object o~ the invention to provide intrinsic sa~ety means using only passive components in the ~ntrinsically sa~e lines.
It ~s ~nother object o~ the invention to provide intrinsic safety means which does not degrade a ~ignal ~asslng through ~t.
- 2~28~28 It is another object of the invention to provide intrinsic safety means which permits great separation between the electrical supply and hazardous area equipment it supplies.
It is anoth~r object sf the invention to provide intrinsic safety means which is simple and convenient.
It is another object of the invention to provide intrinsic safety means which may be tested without removal from service and without interruption in operation of the circuits protected by the intrinsic safety means.
In accordance with the foregoing objects, the intrinsic &afety means of the present invention includes a plurality of lntrinsically afe conductors each of which is coupled to a common power supply, current limiting me~ns associated with each intrinsically safe conductor, and voltage limiting means coupled to and effecti~e to limit the voltage o~ the common ~upply. In accordance with the pr~ferred embodiment o~ the invention, the voltage limiting means comprises redundant crowbar circuits.
These and other objects and features of the inv~ntion will b~ understood wi~h re~erence ~o the ~ollowing description, the drawings, and th~ claims.
2~2~2~
Briaf Description_of the Drawinqs Figure l is a schematic diagram of an intrinsic ~afety barrier typical of the prior art.
Figure 2 is a block diagram o a typical intrinsically sae system according to the prior art.
Figure 3 is a block diagram of an intrinsically safe system in accordance with the present invention.
Figures 3a and 3b show preferred current interrupting means and current limiting means, respectively, for use in the present invention.
Figure 4 is a schematic diagram of a crowbar circuit useul in the system o~ the present invention.
Figure 5 is a ~lock diagram of an Pmbodiment of 2~ the invention which includes means for testing certain protective components.
Detailed~DescriptiQ~ of the Preferred Embodiment Figure 1 shows a schematic representation of an intrinsic ~afety barrier used in accordance with the prior art to protect a ~lngle line lntended to ~e run in a hazardous area. The barrier includes an input terminal 24 intended t~ be coupled to a source of power and an output terminal 26 intended to be coupled to ~azardous area wirin~ ~nd circuitry. The barrler includes redundant zener diodes 20 and ~2 ~or limiting the ~oltage which may be presented to output terminal ~ upon t~e occurrence o~
fault conditions, ~uch as excessive voltage and/or current, at lnput terminal 24. Re~lstQr 18 i~ prov ded to 2~28~28 limit the maximum current available through output terminal 26. Fuse 14 is provided to disconnect the output and th~ protective circuitry of the barrier from the input upon high input current conditions to the barrier, and resistors 12 and 16 are provided to limit the current and hence the dissipation in zener diodes 20 and 22 under ~ault ~onditions in the time interval before fuse 14 blows. The barrier includes redundant connection~ 28 and 30 adapted to be coupled to an intrinsically safe ground, which in accordance with intrinsic safety wiring practices is requ~red to be connected to a ~acility's central grounding location by redundant protected wiring having resistance of one ohm or less. The components of the intrinsic safety barrier, including ~u~e 14, are gener~lly potted or encapsulated to preclude intentional or accidental actions which may affect the ability of the barrier to maintain its output terminal intrinsically ~afe. Accordingly, an input fault condition generally requires replacement o~ the entire barrier, an ~xpensive and inconvenient procedure. Moreover, because resistor 18 i5 reguired to limit the output current available when the maximum voltage is present acros~ zener diode 22 under fault conditions, the additional r~sistance o resistors 12 and 16 r~duces the resistance which may be allocated to field wiring or ~ield devices und~r normal operating conditions. Thi~ may, for instance, und~lly limit the conductor length and physlcal separation ketwe~n the barrier and 3 connected field device.
Figure 2 ~how~ a typical prior art ~ystem providing intrin~ically ~a~e circuitry in a ha~ardous 2~28~8 area. A plurality of field devices 46a, 46b ... 46n are required to be plac~d in a hazardous area ancl supplied with power over lines 44a, 44b, ... 46n. Such field devices may include transmitters, transducers, indicators, and the like. Typically, such field devices will be required to be placed in a hazardous area in order to monitor or control the ~tatus of processes and materials in the hazardous area. Often such ~ield devices will be transducers or other devices operating in a 4-20 mA two-wire loop, i~ which event the lines 44 would eachcorrespond to one conductor of each such loop. The oth~r co~ductor o such two-wire loops may be coupled to a barrier-protected line or to an intrinsically safe ground.
Field devices 46 in the hazardous area are generally supplied with power from circuitry in a non-hazardous area. Such circuitry will ~enerally include a power supply 40, generally an AC line-powerPd supply of approxi~ately 24 volts DC output. Supply 4Q generally energizes certain circuitry coupled to the conductors or lines 44 and designated in Figure 2 as function block 42.
Function block 42 may comprise signal generating, monitoring, or measuring devices, displ~y device~, multiplexing circuits, and/or a variety of other devices performing disparate ~unctions xelating to t~e ~ield devices. As shown in Eigure 2, a single ~nCtiQn block 42 is illu~trated w~ich is coupled to all field devices and power supply co~mon; it will be under~tood that one or more functional blocks may ~e dedicated to eac~ ld device ~6 and associated line 44.
2 ~ 2 ~
In order to protect lines 44a, 44b, ... 44n and their associated field devic~s 46a, 46b, ... 44n from intrusion of hazardous energy upon fault conditions in the non-hazardous area or in the hazardous area, intrinsic safety barriers 48a, 4~b, ... 48n are provided. Each such barrier 48 may have the structure shown in Figure l; other designs, however, are in use. Barriers are provided in each non-grounded line entering the hazardous area.
Accordingly, conventional two-wire signal 10QPS require two barriers per loop. For circuits in which one conductor is coupled to the intrinsically safe ground, only one barrier per loop is needed. However, this can still result in considerable ~xpense and inconvenience which the present inve~tion is intended to avoid.
Power ~upply 40 may be coupled to other or auxiliary clrcuitry 50 which ls not coupled to or related to the field devic~s. In the system of Figure 2, barriers 48 do not protect or otherwise af~ect such circuitry.
Fi~ure 3 shows an intrinsically ~afe system according to the preerred embodiment o~ the lnvention.
Like the ~ystem o Figur~ 2, the 6ystem of Figure 3 includes a plurality o fi~ld devices 74a, 74~, ... 74n which may be located in a hazardous area and w~ich are to be supplied with electrical energy over conductors or ~5 lines 7~a, 72b ... 72n. As pr~vio~sly described, such ield devices may include indicators, tra~sducer~, tra~smlt~erfi including ~-20 mA tr~nsmitter~J ~nd the like.
Power for the ~ield devices ~4 i8 derived ~rom power ~upply 60 g~nerally loea~ed in the non-hazardous area.
2028~28 Power supply 60, like that of Figur~ 2, is generally an AC
line powered supply having an output on the order of 24 volts DC. Shown coupled to power supply 60 by supply line 67a, 67b is function block 66 which, like function block 42 described above with respect to Figure 2, may comprise signal generating, monitoring, or measuring devices, display devices, multiplexing circuits, or a variety of circuits performing other functions. In general, function block 66 comprises the functional interface between the ~ield devices and the equipment in the non-hazardous ~rea, and the means by which power is coupled to lines 72 and thus to field devices 74. It will be understood that while a ~ingle function block is shown in Figure 3, e~uivalently a plurality of function blocks could be provided each o~ which serves one ~r more lines and field devices. It will further be understGod that systems in accordance with the invention n~ed not include means for providin~ any f~nction in function block 66 other than coupling power to the field circuit.
~0 The supply line coupling current interrupting means 62 to function block 66 and auxiliary circuitry 64 1~ identified in one portion as 67a and in another as 67b, to enable comparison o~ corresponding circuit portions in the d2scrip~.ion of Figure 5. The diff~rentiation is not fiigniicant i~ Figure 3, and both portions will be re~erred to collectively as supply line 67.
In accordance with the invention, at least one crowb~r circuit is pro~ided between he ~upply llne 67 and a conductor coupl~d to intri~sicalLy ~a~e ~round. By 2~28~28 providing such a crowbar circuit, supply line 67 is rendered a voltage limited conductor and the power supply means coupled to function block 66 is rendered a voltage limited power supply means. Preferably, and as shown in Figure 3, a plurality of such crowbar circuits is provided. In Figure 3, three such crowbar circuits 68a, 68b, and 68c are provided, each of which may be identical.
Such crowbar circuits sense the voltage of the protected supply line 67 and, if it exceeds a predetermined threshold, such as 28 volts, provide a low impedance path between the protected supply line and intrinsically safe ground.
In accordance with the preferred embodiment of the invention, a current interrupting means 6~ such as a fuse is provided in the power supply linP between supply 60 and function block 66 to current limit supply line 67.
Other current interrupting means, ~uch as a circuit breaker, may also be used. Figure 3a shows fuse 63 which is the pre~erred current interrupting means 62 of the present invention.
Accordingly, in the circuit of Figure 3, the protected supply line 67, so long as it is prokected from intrusion of hazardous energy or power levels which bypass current interrupting means 62, forms a fail-safe supply which is voltage limited with respect to intrinsically ~afe ground. In the event of an overvoltage and/or overcurrent co~dition at power supply 60, ~uch as may be cau~ed by a shorted pass transi tor in power ~upply 60, the voltage on line 67 will ri~e to the thresh~ld voltage of crowbar circuits 68a, ~8b, and/or 68c. Even if two crowbar circuits 6~ are ~aulted and non~unckional, at ~828~
least one will be functional to impose a low impedance path to ground and to limit the voltage on line 67 to non-hazardous levels. In most circumstances, this will cause current interrupting means 62 to open, there~y removing power ~rom function block 66, from the ~ield wiring and devices, and from the crowbar circuits. So long as the short circuit current capacity o the crowbar circuit~s) is greater than the current required to open current interrupting means 62 and their short circuit impedance is sufficiently low, and the resistanc~ of the electrical return path to power supply 60 is low, the voltage on line 67 will be maintained at intrinsically safe lsvels even with input overvoltage and/or overcurrent fault conditions and two aulted crowbar circuits.
By providing intrinsically safe voltage limiting to the lines supplying power to the field wirin~ and field devices, such wiring and devices may ~e made intrinsically ~afe by limiting the current which may be introduced into them. Thi5 i8 easily accomplished by provision o~ current limiting means 70a, 70b,...70n, in each line 72a, 7~b,...72n, Such current lim~t~rs are desirably of a construction which, if they fail, will fail safely, i.e. which in this application will ~ail to an open circuit or high impedance condition rathe~ than a short circuit or low impedance condition. Figure 3b shows a resistor 65 which is the preferred current limiting means 70 of the present invention. Metal film and wirewound resistors are suitable. Use of passi~e components such as resistors for current limiting minimizes the likelihood of failure and, in particular, low-impedance failure. Moreover, such passive components minimize any signal degradation in the current limiting means.
~2~02~
It will be noted that any field line and field device may be protected by simply including an inexpensive resistor ~n series with the line. Thus, by en~urlng that the power supplies which may be coupled to the field wiring are appropriately voltage limited, any number o~
field circuits may be rendered intrinsically safe by the ~imple and inexpensive inclusion of a resistor in the line. This is particularly advantageous 1~ systems supplying large numbers of field circuits. Of course, it will be understood that ordinary intrinsic safety considerations will apply to the system of Figure 3, such as limitation of the energy storage capability of the lines 72 and field devices 74 and use of intrinsically safe wiring practices.
Since no resistan~e is provided to limit the current in crowbar circuits 68, other than the internal resistances of the sourc~ 60, wiring, current interrupting means 62, and the crowbar circuits them elves, the parasitic e~fect~ o~ such resi~tance, described above with re3pect to intrinsic ~aety barrier resistances 12 and 16~
are avoided. Thus the system of the ~nvention maximizes the re istance available ~or ~ield wirin~ and devices and thu~ the separation whlc~ may be obtained under given conditions between the field device~ 74 and the non-ha~ardous area circuitry.
Eigure 3 further ~hows auxiliary circuitry S4couplad to power ~upply 60. In ac~ordance with the invantion, other non-ha~ardous-area circuitry be~ides the function block 66 as~ociated with the ield devices may be supplied from power supply 60. In the ev~nt that it is desired that such circuitry be protected from overvoltage conditions, such circuitry may be operated from the protected supply line 67, as shown.
Figure 4 shows crowbar circuitry useful in connection with the present invention. It will be understood, however, that many crowbar circuits including the circuit o~ Figure 4 are known per se and may be used in the system o~ the present invention.
The shorting element of the erowbar circuit of Figure 4 is silicon controlled rectifier (SCR) 80, connected with its main terminals coupled to intrinsically safe ground and to the protected supply line S7. SCR 80 is controlled by o~rvoltage sen~e circuit 82 which may be Motorola type MC 3423, an integrated circuit (IC~
speci~ically designed for use in crowbar circuits. IC 82 sourc~s current from pin 8 when the volta~e applied to pin 2 exceeds an internal reference vol~age. Accordingly, resistors 84 and 86 form a voltage divider to establish the voltage o conductor 67 which, when axceeded, will cause current flow out o~ pin 8. Such current ~low provides gate current e~fective to ~ire SCR 80 and thus to crowbar the ~upply. Resistor 88 i8 provided to limit the gate curr~nt of SCR 80, and resis~or 90 bypasses leakage current. Capacitors g2 and 94 are provided for noise immu~ity so that brief noise transients do not actuat~ th~
rrowbar and diRable the system. Zener diode 96 is provided to protect IC ~2 from supply overvoltage ~2~
conditions, and may assist in causing the supply fuse to open in the event o input overvoltage conditions.
It will be understood that other crowbar circuits may be employed in the system of the invention, which may involve means ~or imposing a low-impedance path other than an SCR, such as a triac or a diac.
Figure 5 shows a modification of a portion of the system of Figure 3 to permit testing and verification of operability of the crowbar circuits. The system of Figure 5 includes means ~or such testing and verification which may take place without interruption of the normal functioning of field devices 74, auxiliary circuitry 64, or devices comprising function block 66 of Figure 3, and without affecting the intrinsic ~afety affordad by the system. In order to provide for such testing, testing circuitry including pass element 100, control circuit 10 and switch means 116a-c i8 provided. To permit ac~uation of the crowbar circuit~ during testing without opening current interrupting means 62, which would disable the sy~tem, pass element 100 is insarted in series with conductor 67. Pass element 100 controls current ~low between conductor 67a, connected to the current interrupting ~e~n~ and conductor 106, under th~ control of control line 126. Pass element 100 may comprise a ~5 transistor coupled to control line 126. Load , such as function block 66 and auxiliary circuitry 6~ of Figure 3, are co~pled to conduct~r 67b.
~28~8 Crowbar circuits useful in t~e present inYention will generally include means ~or comparing a signal related to the monitored voltage with a threshold value, and for actuating a ~witch in response to the comparison.
Such features render the crowbar circuits amenable to testing without the necessity of raising the monitored voltage to a level above the normal threshold voltage of the crowbar circuits. While crowbar circuits could be tested in this fashion, doing 50 greatly oomplicates testing of the individual crowbar circuits; if the voltag~
o~ the monitored conductor were merely rai~ed, the crowbar circuit having the lowest threshold voltage would b actuated first and prevent actuation of the other crowbar circuits.
The system of Figure 5 avoids such difficulties by simulating oYervoltage conditions at each crowbar circuit. Such conditions may be simulated separately for each of the crowbar circuits present, en~bling tham to be individually tested. In the ~ystem of Fisure 5, each of the crowbar circuits 6Ra~ b, and c includ~s an input 114a, b, and c to which a ~ignal may be applied to establi~h, control, or vary the threshold at which the crowbar circuit will ~e actuated. Testing of the crowbar circuits in Eigure S in carried out under the control of control circuit 102. Control c~rcuit 10~ may include means ~or initiat~ng a test, such a~ timer means ~or automatically initiating a test. Co~trol circuit 102 also may lnitiate a te~t i~ respon~ to an input 130 such as may b~ supplied ~rom an operator, external ~ircuitry, or- the like.
g ~2~8 The testing sy~tem of Figure 5 includes means for varying the inputs 114 of the crowbar circuits 68. In the system of Figure 5, means for varying the imputs comprises switches 116a, b, and c, which are controlled by control circuit 102. In the normal position, as shown, switches 116 couple the inputs 114 of crowbar circuits 68 to conductor 120, which establishes ~or the crowbar circuits the normal threshold voltage to provide intrinsically safe operation, such as 28 volts on monitored conductor 106. In normal operation, control circuit 102 al60 controls pass element 100 so that current through it i8 ~ssentially ~nrestricted, such as by causing saturation of a pa~s transistor in pass element 100.
To perform a test, the current capacity of pass element 100 is controlled by control circuit 102 to be less than the current which would open circuit interruption ~eans 62. Control circuit 102 then causes actuation of the switch means 116 coupled to the particular crowbar circuit to be tested, thereby coupling the corresponding control input 114 to co~ductor 122.
Thi~ e~tablishes a monitored voltage threshold for actuation of the crowbar circuit 6~ to be tested which is less than the voltage noxmally ~upplied by supply 60 or present on conductors 67 or 106. If the erowbar circuit under test is unctional, the ~resence of a monitored voltage highar than itB threshold voltage will cause it to impose a low-impedanca path across the crowbar circuit, 1.e. between conductor 106 and intrinsically Ba~e ground.
This in turn will cause a drop in the voltage of conductor 105 which may be detected to indicate that the crowbar 202~02~
circuit has been actuated. Detector 104 in the circuit of Figure 5 is a means for providing a signal to control circuit 102 which is responsive to actuation of a crowbar circuit, in the embodiment shown by responding to the change in voltage of monitored conductor 106. As shown, detector 104 detects the voltage of conductor 106 with respect to the voltage of conductor 67, i.e., it is responsive to the voltage across pass element 100.
However, it will be understood that detector 104 may equivalently be responsive to the voltage o conductor 106 with respect to the other potentials, or may even s~nse the actuation of a crowbar circuit by other effects, such as by detecting the current or change in current through a crowbar circuit.
Actuation of crowbar circuit under the testing conditions described above will generally cause conductor 106 to be effectively grounded, or at least unable to support the load imposed by field devices, auxiliary ~ircuitry, and the like coupled to conductor 67b. Diode 108 ~nd c~pacitor 112 provide a means for continuing to supply power to such connected loads during te~ting of the crowbar circuits. Capacitor ll~ will be charged during normal system operation, and if it~ capacitance is ~ufficiently large considering the load and the duration o the testing procedure, it will maintain the voltage o~
~onductox 67b ~ufficiently hi~h to permit continued ~ystem opera~ion durin~ ~estins. Diode 108 i~ a means for preventing di~charge o~ c~pacitor 112 by ~he crowbar circuits. Diode 108 and capacitor 11~ in efect ~orm a backup power supply which act~ au~omat~cally to power the ~go2~
load during testing of the crowbar circuits. Accordingly, it will be understood that other means may be provided to perform this ~unction, such as a separate power supply to which the load is switched during testing of the crowbar circu~ts.
Crowbar circuits useful in the present invention include circuits in which the low impedance path imposed upon actuation may per ist after the condition which actuated them. The crowbar circuit of Figure 4 operates in this fashion; once triggered, ~CR 80 will remain on until the current through it is reduced substantially to zero. For ~uch circuits, control circuit 102 may include means for controlling pass element 100 so as to decrease its curre~t conduction below the holding current of a crowbar circuit 60 and p~rmit it to reset. Such means may be employed upon rec~ipt by control circuit 102 of a si~nal indicating that a crowbar circuit has been actuated in response to a test. Thereafter, control circuit 102 causes pass element 100 to assume its normal conduction ~0 state, allowing power supply 60 to recharge capacitor 112 and support the connected load on conductor 67b. It will be understood that other ~eans for resetting latched crowbar circuits may be provided which are appropriate for the particular crowbar circuit6 employed.
2S In the event that detector 104 does not indicate to control circuit 102 that 9 crowbar circuit has been actuated within a predetermined ~ime after a crowbar circult test has been initiated, the crowbar circuit under 2~2802~
test may be deemed to be faulted and the test may be texminated.
Test circuit 102 may comprise means for generating an output 132 in response to the results of testing. Such an output may inclu~e a visual display, a signal ~or communication to other circuitry, or the like.
The foregoing testing procedure may be performed ~eparately for each o the crowbar circuits pr~sent in the ~ystem, enabling separate testing and identification of any faulted crowbar circuits. Testing may be performed automatically and repetitively to provide continued assurance of intrinsically ~afe conditions, or whenever a verification of such conditions is desired.
Accordingly, systems have been disclosed which are capable o~ ~imply, reliably, and inexpensively rendering a plurallty o circuits intrin~ically safe.
While particular methods and apparatus have been described, it will be understood that variations will no doubt occur to those skilled in the art without departing from the spirit of the invention.
Field Of The Inventio~
This invention relates to electrical systems, such as instrument systems, which may operate in an environment which may contain combustible materials. More particularly, this invention relates to methods and apparatus for ensuring that electrical energy and power supplied to an environment which may contain combustible materials are limited so as to minimize tne likelihood of i~nition of comhustible materials.
BacXaround of the_Invention Many loc~tions, such as in and around industrial processes, ar2 or are subject to becoming "hazardous"
areas, i.e. areas in which the concentration of flammable gases i8 or may become in the range which will present an explosion and~or ~re hazard. Such hazardous areas often require electrical apparatus and/or wiring to be present, such as apparatus and wiring for instrumentation systems to monitor and con~rol an lndustrial process.
~0 Several approaches have been developed to cope with the explo6ion hazard in ~uch Areas. Initially, apparatl~s was placed in heavy and expensive metal casings and wiring was run in heavy and expensive metal conduit, so that any axplosion ignited by ~he apparatus would be ~5 con1ned to the interlor of the hou~ing and conduit system 2~28~28 and unable to ignite hazardous atmospheres surrounding the ~ystem. Such "explosion proof" systems are extremely expensive, and are inconvenient because they require that the area be made non-hazardous prior to opening an explosion proo~ housing such as to test, calibrate, or inspect apparatus in the housing.
With the advent of solid state electronics, it became possible to design instrumentation and control systems operable at power levels low enough to be intrinsically unable to ignite specified hazardous atmospheres. So long as the energy storage capacity of circuits supplied from energy and power limited supplies is sufficiently low, such apparatus will be incapable of releasing sufficient energy to ignite hazardous atmospheres and the entire system, including both wiring and connect~d electrical apparatus, will be safe for u~e in the hazardous atmosphere.
Since all real~world 3ystems are s~bject to component ailure, systems are generally desi~ned to tolerate B certain number of events which are considered to be faults in the system, while still maintai~ing energy and power lavals su~ficiently low as to preclude ignition of a hazardous atmosphere. According to generally accepted standards, a ~ystem i~ deemed "intrinsically safeN for a ~peciied hazardous atmosphPre if it can withstand any combination of two ~aults in tha ~y~tem while maintaining en~rgy and power levels below the limits necessary to ignita that ha~ardous atmosphere.
Intrinsic ~afety "barriers" have been developed to prevent intrusion of hazardous voltages and currents into circuitry which is intended to be intrinsically safe.
Such barriers have an input terminal adapted to be coupled to a power ~ource, an output terminal adapted to be coupled to intrinsically safe wiring and circuity, means for limiting the voltage which may be applied to the output terminal, and means for limiting the current which may be supplied to the output terminal. Such barr~ers typically include a fuse to protect the voltage an~
current limiting components from ~ailure in the event of high power being supplied to the input terminal.
Such barriers generally c~ntain components whose failure modes, arrangement, and redundancy are such that the output terminal is considered intrinsically safe. The circuit components and terminals are typically supplied as a potted or encapsulated assembly in which the components, including the fuse, are inaccessibl~. By supplying one such barrier in eac~ non-grounded line entering a 20 hazardous area, those lines will be rendered intrinsically ~afe provided that intrinsically ~afe wiring practices are followed and devices coupled to those lines have appropriately low ener~y storage capacity.
However, ~uch intrinsic ~afety barriers are expen~ive items, and can considerably afect the cost of an intrinsically safe circuit or system. This i5 particularly the case with systems involving a large number of intrinsically safe circuits. Moreover, when a fault cccur8 which blows a fuse in a barrier, it typically must be replaced, leading to additional expense and inconvenience. Some intrinsic saf2ty barriers also laçk the ability to have each limiting component tested individually to verify that the barri~r is fully unctional with t~e intended degree of redundancy. Even in barriers whera quch tests may be made, testing typically requires removal of the barrier from service, a procedure which may involve considerable expense, time, and inconvenienc~.
S mmarv Of The Invention It it therefore an object of the invention to provide inexpensive intrinsic safety means.
It is a further object of the invention to provide intrinsic sa~ety means suitable for use with a lar~e number of circuits.
It is another ob;ect of the invention to provid~
intrinsic sa~ety means usable with a wide variety of ~ield devices.
It i3 another object o~ the invention to provide intrinsic sa~ety means using only passive components in the ~ntrinsically sa~e lines.
It ~s ~nother object o~ the invention to provide intrinsic safety means which does not degrade a ~ignal ~asslng through ~t.
- 2~28~28 It is another object of the invention to provide intrinsic safety means which permits great separation between the electrical supply and hazardous area equipment it supplies.
It is anoth~r object sf the invention to provide intrinsic safety means which is simple and convenient.
It is another object of the invention to provide intrinsic safety means which may be tested without removal from service and without interruption in operation of the circuits protected by the intrinsic safety means.
In accordance with the foregoing objects, the intrinsic &afety means of the present invention includes a plurality of lntrinsically afe conductors each of which is coupled to a common power supply, current limiting me~ns associated with each intrinsically safe conductor, and voltage limiting means coupled to and effecti~e to limit the voltage o~ the common ~upply. In accordance with the pr~ferred embodiment o~ the invention, the voltage limiting means comprises redundant crowbar circuits.
These and other objects and features of the inv~ntion will b~ understood wi~h re~erence ~o the ~ollowing description, the drawings, and th~ claims.
2~2~2~
Briaf Description_of the Drawinqs Figure l is a schematic diagram of an intrinsic ~afety barrier typical of the prior art.
Figure 2 is a block diagram o a typical intrinsically sae system according to the prior art.
Figure 3 is a block diagram of an intrinsically safe system in accordance with the present invention.
Figures 3a and 3b show preferred current interrupting means and current limiting means, respectively, for use in the present invention.
Figure 4 is a schematic diagram of a crowbar circuit useul in the system o~ the present invention.
Figure 5 is a ~lock diagram of an Pmbodiment of 2~ the invention which includes means for testing certain protective components.
Detailed~DescriptiQ~ of the Preferred Embodiment Figure 1 shows a schematic representation of an intrinsic ~afety barrier used in accordance with the prior art to protect a ~lngle line lntended to ~e run in a hazardous area. The barrier includes an input terminal 24 intended t~ be coupled to a source of power and an output terminal 26 intended to be coupled to ~azardous area wirin~ ~nd circuitry. The barrler includes redundant zener diodes 20 and ~2 ~or limiting the ~oltage which may be presented to output terminal ~ upon t~e occurrence o~
fault conditions, ~uch as excessive voltage and/or current, at lnput terminal 24. Re~lstQr 18 i~ prov ded to 2~28~28 limit the maximum current available through output terminal 26. Fuse 14 is provided to disconnect the output and th~ protective circuitry of the barrier from the input upon high input current conditions to the barrier, and resistors 12 and 16 are provided to limit the current and hence the dissipation in zener diodes 20 and 22 under ~ault ~onditions in the time interval before fuse 14 blows. The barrier includes redundant connection~ 28 and 30 adapted to be coupled to an intrinsically safe ground, which in accordance with intrinsic safety wiring practices is requ~red to be connected to a ~acility's central grounding location by redundant protected wiring having resistance of one ohm or less. The components of the intrinsic safety barrier, including ~u~e 14, are gener~lly potted or encapsulated to preclude intentional or accidental actions which may affect the ability of the barrier to maintain its output terminal intrinsically ~afe. Accordingly, an input fault condition generally requires replacement o~ the entire barrier, an ~xpensive and inconvenient procedure. Moreover, because resistor 18 i5 reguired to limit the output current available when the maximum voltage is present acros~ zener diode 22 under fault conditions, the additional r~sistance o resistors 12 and 16 r~duces the resistance which may be allocated to field wiring or ~ield devices und~r normal operating conditions. Thi~ may, for instance, und~lly limit the conductor length and physlcal separation ketwe~n the barrier and 3 connected field device.
Figure 2 ~how~ a typical prior art ~ystem providing intrin~ically ~a~e circuitry in a ha~ardous 2~28~8 area. A plurality of field devices 46a, 46b ... 46n are required to be plac~d in a hazardous area ancl supplied with power over lines 44a, 44b, ... 46n. Such field devices may include transmitters, transducers, indicators, and the like. Typically, such field devices will be required to be placed in a hazardous area in order to monitor or control the ~tatus of processes and materials in the hazardous area. Often such ~ield devices will be transducers or other devices operating in a 4-20 mA two-wire loop, i~ which event the lines 44 would eachcorrespond to one conductor of each such loop. The oth~r co~ductor o such two-wire loops may be coupled to a barrier-protected line or to an intrinsically safe ground.
Field devices 46 in the hazardous area are generally supplied with power from circuitry in a non-hazardous area. Such circuitry will ~enerally include a power supply 40, generally an AC line-powerPd supply of approxi~ately 24 volts DC output. Supply 4Q generally energizes certain circuitry coupled to the conductors or lines 44 and designated in Figure 2 as function block 42.
Function block 42 may comprise signal generating, monitoring, or measuring devices, displ~y device~, multiplexing circuits, and/or a variety of other devices performing disparate ~unctions xelating to t~e ~ield devices. As shown in Eigure 2, a single ~nCtiQn block 42 is illu~trated w~ich is coupled to all field devices and power supply co~mon; it will be under~tood that one or more functional blocks may ~e dedicated to eac~ ld device ~6 and associated line 44.
2 ~ 2 ~
In order to protect lines 44a, 44b, ... 44n and their associated field devic~s 46a, 46b, ... 44n from intrusion of hazardous energy upon fault conditions in the non-hazardous area or in the hazardous area, intrinsic safety barriers 48a, 4~b, ... 48n are provided. Each such barrier 48 may have the structure shown in Figure l; other designs, however, are in use. Barriers are provided in each non-grounded line entering the hazardous area.
Accordingly, conventional two-wire signal 10QPS require two barriers per loop. For circuits in which one conductor is coupled to the intrinsically safe ground, only one barrier per loop is needed. However, this can still result in considerable ~xpense and inconvenience which the present inve~tion is intended to avoid.
Power ~upply 40 may be coupled to other or auxiliary clrcuitry 50 which ls not coupled to or related to the field devic~s. In the system of Figure 2, barriers 48 do not protect or otherwise af~ect such circuitry.
Fi~ure 3 shows an intrinsically ~afe system according to the preerred embodiment o~ the lnvention.
Like the ~ystem o Figur~ 2, the 6ystem of Figure 3 includes a plurality o fi~ld devices 74a, 74~, ... 74n which may be located in a hazardous area and w~ich are to be supplied with electrical energy over conductors or ~5 lines 7~a, 72b ... 72n. As pr~vio~sly described, such ield devices may include indicators, tra~sducer~, tra~smlt~erfi including ~-20 mA tr~nsmitter~J ~nd the like.
Power for the ~ield devices ~4 i8 derived ~rom power ~upply 60 g~nerally loea~ed in the non-hazardous area.
2028~28 Power supply 60, like that of Figur~ 2, is generally an AC
line powered supply having an output on the order of 24 volts DC. Shown coupled to power supply 60 by supply line 67a, 67b is function block 66 which, like function block 42 described above with respect to Figure 2, may comprise signal generating, monitoring, or measuring devices, display devices, multiplexing circuits, or a variety of circuits performing other functions. In general, function block 66 comprises the functional interface between the ~ield devices and the equipment in the non-hazardous ~rea, and the means by which power is coupled to lines 72 and thus to field devices 74. It will be understood that while a ~ingle function block is shown in Figure 3, e~uivalently a plurality of function blocks could be provided each o~ which serves one ~r more lines and field devices. It will further be understGod that systems in accordance with the invention n~ed not include means for providin~ any f~nction in function block 66 other than coupling power to the field circuit.
~0 The supply line coupling current interrupting means 62 to function block 66 and auxiliary circuitry 64 1~ identified in one portion as 67a and in another as 67b, to enable comparison o~ corresponding circuit portions in the d2scrip~.ion of Figure 5. The diff~rentiation is not fiigniicant i~ Figure 3, and both portions will be re~erred to collectively as supply line 67.
In accordance with the invention, at least one crowb~r circuit is pro~ided between he ~upply llne 67 and a conductor coupl~d to intri~sicalLy ~a~e ~round. By 2~28~28 providing such a crowbar circuit, supply line 67 is rendered a voltage limited conductor and the power supply means coupled to function block 66 is rendered a voltage limited power supply means. Preferably, and as shown in Figure 3, a plurality of such crowbar circuits is provided. In Figure 3, three such crowbar circuits 68a, 68b, and 68c are provided, each of which may be identical.
Such crowbar circuits sense the voltage of the protected supply line 67 and, if it exceeds a predetermined threshold, such as 28 volts, provide a low impedance path between the protected supply line and intrinsically safe ground.
In accordance with the preferred embodiment of the invention, a current interrupting means 6~ such as a fuse is provided in the power supply linP between supply 60 and function block 66 to current limit supply line 67.
Other current interrupting means, ~uch as a circuit breaker, may also be used. Figure 3a shows fuse 63 which is the pre~erred current interrupting means 62 of the present invention.
Accordingly, in the circuit of Figure 3, the protected supply line 67, so long as it is prokected from intrusion of hazardous energy or power levels which bypass current interrupting means 62, forms a fail-safe supply which is voltage limited with respect to intrinsically ~afe ground. In the event of an overvoltage and/or overcurrent co~dition at power supply 60, ~uch as may be cau~ed by a shorted pass transi tor in power ~upply 60, the voltage on line 67 will ri~e to the thresh~ld voltage of crowbar circuits 68a, ~8b, and/or 68c. Even if two crowbar circuits 6~ are ~aulted and non~unckional, at ~828~
least one will be functional to impose a low impedance path to ground and to limit the voltage on line 67 to non-hazardous levels. In most circumstances, this will cause current interrupting means 62 to open, there~y removing power ~rom function block 66, from the ~ield wiring and devices, and from the crowbar circuits. So long as the short circuit current capacity o the crowbar circuit~s) is greater than the current required to open current interrupting means 62 and their short circuit impedance is sufficiently low, and the resistanc~ of the electrical return path to power supply 60 is low, the voltage on line 67 will be maintained at intrinsically safe lsvels even with input overvoltage and/or overcurrent fault conditions and two aulted crowbar circuits.
By providing intrinsically safe voltage limiting to the lines supplying power to the field wirin~ and field devices, such wiring and devices may ~e made intrinsically ~afe by limiting the current which may be introduced into them. Thi5 i8 easily accomplished by provision o~ current limiting means 70a, 70b,...70n, in each line 72a, 7~b,...72n, Such current lim~t~rs are desirably of a construction which, if they fail, will fail safely, i.e. which in this application will ~ail to an open circuit or high impedance condition rathe~ than a short circuit or low impedance condition. Figure 3b shows a resistor 65 which is the preferred current limiting means 70 of the present invention. Metal film and wirewound resistors are suitable. Use of passi~e components such as resistors for current limiting minimizes the likelihood of failure and, in particular, low-impedance failure. Moreover, such passive components minimize any signal degradation in the current limiting means.
~2~02~
It will be noted that any field line and field device may be protected by simply including an inexpensive resistor ~n series with the line. Thus, by en~urlng that the power supplies which may be coupled to the field wiring are appropriately voltage limited, any number o~
field circuits may be rendered intrinsically safe by the ~imple and inexpensive inclusion of a resistor in the line. This is particularly advantageous 1~ systems supplying large numbers of field circuits. Of course, it will be understood that ordinary intrinsic safety considerations will apply to the system of Figure 3, such as limitation of the energy storage capability of the lines 72 and field devices 74 and use of intrinsically safe wiring practices.
Since no resistan~e is provided to limit the current in crowbar circuits 68, other than the internal resistances of the sourc~ 60, wiring, current interrupting means 62, and the crowbar circuits them elves, the parasitic e~fect~ o~ such resi~tance, described above with re3pect to intrinsic ~aety barrier resistances 12 and 16~
are avoided. Thus the system of the ~nvention maximizes the re istance available ~or ~ield wirin~ and devices and thu~ the separation whlc~ may be obtained under given conditions between the field device~ 74 and the non-ha~ardous area circuitry.
Eigure 3 further ~hows auxiliary circuitry S4couplad to power ~upply 60. In ac~ordance with the invantion, other non-ha~ardous-area circuitry be~ides the function block 66 as~ociated with the ield devices may be supplied from power supply 60. In the ev~nt that it is desired that such circuitry be protected from overvoltage conditions, such circuitry may be operated from the protected supply line 67, as shown.
Figure 4 shows crowbar circuitry useful in connection with the present invention. It will be understood, however, that many crowbar circuits including the circuit o~ Figure 4 are known per se and may be used in the system o~ the present invention.
The shorting element of the erowbar circuit of Figure 4 is silicon controlled rectifier (SCR) 80, connected with its main terminals coupled to intrinsically safe ground and to the protected supply line S7. SCR 80 is controlled by o~rvoltage sen~e circuit 82 which may be Motorola type MC 3423, an integrated circuit (IC~
speci~ically designed for use in crowbar circuits. IC 82 sourc~s current from pin 8 when the volta~e applied to pin 2 exceeds an internal reference vol~age. Accordingly, resistors 84 and 86 form a voltage divider to establish the voltage o conductor 67 which, when axceeded, will cause current flow out o~ pin 8. Such current ~low provides gate current e~fective to ~ire SCR 80 and thus to crowbar the ~upply. Resistor 88 i8 provided to limit the gate curr~nt of SCR 80, and resis~or 90 bypasses leakage current. Capacitors g2 and 94 are provided for noise immu~ity so that brief noise transients do not actuat~ th~
rrowbar and diRable the system. Zener diode 96 is provided to protect IC ~2 from supply overvoltage ~2~
conditions, and may assist in causing the supply fuse to open in the event o input overvoltage conditions.
It will be understood that other crowbar circuits may be employed in the system of the invention, which may involve means ~or imposing a low-impedance path other than an SCR, such as a triac or a diac.
Figure 5 shows a modification of a portion of the system of Figure 3 to permit testing and verification of operability of the crowbar circuits. The system of Figure 5 includes means ~or such testing and verification which may take place without interruption of the normal functioning of field devices 74, auxiliary circuitry 64, or devices comprising function block 66 of Figure 3, and without affecting the intrinsic ~afety affordad by the system. In order to provide for such testing, testing circuitry including pass element 100, control circuit 10 and switch means 116a-c i8 provided. To permit ac~uation of the crowbar circuit~ during testing without opening current interrupting means 62, which would disable the sy~tem, pass element 100 is insarted in series with conductor 67. Pass element 100 controls current ~low between conductor 67a, connected to the current interrupting ~e~n~ and conductor 106, under th~ control of control line 126. Pass element 100 may comprise a ~5 transistor coupled to control line 126. Load , such as function block 66 and auxiliary circuitry 6~ of Figure 3, are co~pled to conduct~r 67b.
~28~8 Crowbar circuits useful in t~e present inYention will generally include means ~or comparing a signal related to the monitored voltage with a threshold value, and for actuating a ~witch in response to the comparison.
Such features render the crowbar circuits amenable to testing without the necessity of raising the monitored voltage to a level above the normal threshold voltage of the crowbar circuits. While crowbar circuits could be tested in this fashion, doing 50 greatly oomplicates testing of the individual crowbar circuits; if the voltag~
o~ the monitored conductor were merely rai~ed, the crowbar circuit having the lowest threshold voltage would b actuated first and prevent actuation of the other crowbar circuits.
The system of Figure 5 avoids such difficulties by simulating oYervoltage conditions at each crowbar circuit. Such conditions may be simulated separately for each of the crowbar circuits present, en~bling tham to be individually tested. In the ~ystem of Fisure 5, each of the crowbar circuits 6Ra~ b, and c includ~s an input 114a, b, and c to which a ~ignal may be applied to establi~h, control, or vary the threshold at which the crowbar circuit will ~e actuated. Testing of the crowbar circuits in Eigure S in carried out under the control of control circuit 102. Control c~rcuit 10~ may include means ~or initiat~ng a test, such a~ timer means ~or automatically initiating a test. Co~trol circuit 102 also may lnitiate a te~t i~ respon~ to an input 130 such as may b~ supplied ~rom an operator, external ~ircuitry, or- the like.
g ~2~8 The testing sy~tem of Figure 5 includes means for varying the inputs 114 of the crowbar circuits 68. In the system of Figure 5, means for varying the imputs comprises switches 116a, b, and c, which are controlled by control circuit 102. In the normal position, as shown, switches 116 couple the inputs 114 of crowbar circuits 68 to conductor 120, which establishes ~or the crowbar circuits the normal threshold voltage to provide intrinsically safe operation, such as 28 volts on monitored conductor 106. In normal operation, control circuit 102 al60 controls pass element 100 so that current through it i8 ~ssentially ~nrestricted, such as by causing saturation of a pa~s transistor in pass element 100.
To perform a test, the current capacity of pass element 100 is controlled by control circuit 102 to be less than the current which would open circuit interruption ~eans 62. Control circuit 102 then causes actuation of the switch means 116 coupled to the particular crowbar circuit to be tested, thereby coupling the corresponding control input 114 to co~ductor 122.
Thi~ e~tablishes a monitored voltage threshold for actuation of the crowbar circuit 6~ to be tested which is less than the voltage noxmally ~upplied by supply 60 or present on conductors 67 or 106. If the erowbar circuit under test is unctional, the ~resence of a monitored voltage highar than itB threshold voltage will cause it to impose a low-impedanca path across the crowbar circuit, 1.e. between conductor 106 and intrinsically Ba~e ground.
This in turn will cause a drop in the voltage of conductor 105 which may be detected to indicate that the crowbar 202~02~
circuit has been actuated. Detector 104 in the circuit of Figure 5 is a means for providing a signal to control circuit 102 which is responsive to actuation of a crowbar circuit, in the embodiment shown by responding to the change in voltage of monitored conductor 106. As shown, detector 104 detects the voltage of conductor 106 with respect to the voltage of conductor 67, i.e., it is responsive to the voltage across pass element 100.
However, it will be understood that detector 104 may equivalently be responsive to the voltage o conductor 106 with respect to the other potentials, or may even s~nse the actuation of a crowbar circuit by other effects, such as by detecting the current or change in current through a crowbar circuit.
Actuation of crowbar circuit under the testing conditions described above will generally cause conductor 106 to be effectively grounded, or at least unable to support the load imposed by field devices, auxiliary ~ircuitry, and the like coupled to conductor 67b. Diode 108 ~nd c~pacitor 112 provide a means for continuing to supply power to such connected loads during te~ting of the crowbar circuits. Capacitor ll~ will be charged during normal system operation, and if it~ capacitance is ~ufficiently large considering the load and the duration o the testing procedure, it will maintain the voltage o~
~onductox 67b ~ufficiently hi~h to permit continued ~ystem opera~ion durin~ ~estins. Diode 108 i~ a means for preventing di~charge o~ c~pacitor 112 by ~he crowbar circuits. Diode 108 and capacitor 11~ in efect ~orm a backup power supply which act~ au~omat~cally to power the ~go2~
load during testing of the crowbar circuits. Accordingly, it will be understood that other means may be provided to perform this ~unction, such as a separate power supply to which the load is switched during testing of the crowbar circu~ts.
Crowbar circuits useful in the present invention include circuits in which the low impedance path imposed upon actuation may per ist after the condition which actuated them. The crowbar circuit of Figure 4 operates in this fashion; once triggered, ~CR 80 will remain on until the current through it is reduced substantially to zero. For ~uch circuits, control circuit 102 may include means for controlling pass element 100 so as to decrease its curre~t conduction below the holding current of a crowbar circuit 60 and p~rmit it to reset. Such means may be employed upon rec~ipt by control circuit 102 of a si~nal indicating that a crowbar circuit has been actuated in response to a test. Thereafter, control circuit 102 causes pass element 100 to assume its normal conduction ~0 state, allowing power supply 60 to recharge capacitor 112 and support the connected load on conductor 67b. It will be understood that other ~eans for resetting latched crowbar circuits may be provided which are appropriate for the particular crowbar circuit6 employed.
2S In the event that detector 104 does not indicate to control circuit 102 that 9 crowbar circuit has been actuated within a predetermined ~ime after a crowbar circult test has been initiated, the crowbar circuit under 2~2802~
test may be deemed to be faulted and the test may be texminated.
Test circuit 102 may comprise means for generating an output 132 in response to the results of testing. Such an output may inclu~e a visual display, a signal ~or communication to other circuitry, or the like.
The foregoing testing procedure may be performed ~eparately for each o the crowbar circuits pr~sent in the ~ystem, enabling separate testing and identification of any faulted crowbar circuits. Testing may be performed automatically and repetitively to provide continued assurance of intrinsically ~afe conditions, or whenever a verification of such conditions is desired.
Accordingly, systems have been disclosed which are capable o~ ~imply, reliably, and inexpensively rendering a plurallty o circuits intrin~ically safe.
While particular methods and apparatus have been described, it will be understood that variations will no doubt occur to those skilled in the art without departing from the spirit of the invention.
Claims (19)
1. A system for supplying electrical energy at intrinsically safe levels to a plurality of circuits comprising:
a ground conductor adapted to be connected to an intrinsically safe ground potential;
voltage limited power supply means for supplying electrical energy to a conductor at a voltage which is limited with respect to said ground conductor conductor, said voltage limited power supply means including at least one crowbar circuit coupled between said voltage limited conductor and said ground conductor;
and current limiting means, coupled between each of said plurality of circuits and said voltage limited conductor, for limiting the current flow from said voltage limited conductor into each of said plurality of circuits.
a ground conductor adapted to be connected to an intrinsically safe ground potential;
voltage limited power supply means for supplying electrical energy to a conductor at a voltage which is limited with respect to said ground conductor conductor, said voltage limited power supply means including at least one crowbar circuit coupled between said voltage limited conductor and said ground conductor;
and current limiting means, coupled between each of said plurality of circuits and said voltage limited conductor, for limiting the current flow from said voltage limited conductor into each of said plurality of circuits.
2. The system of claim 1, where in said voltage limited power supply means includes at least two crowbar circuits.
3. The system of claim 1, wherein said voltage limited power supply means includes three crowbar circuits.
4. The system of claim 1, wherein said crowbar circuit includes an SCR and an overvoltage sensing circuit, each of which is coupled between said voltage limited conductor and said ground conductor.
5. The system of claim 1, wherein said current limiting means includes resistance between each of said plurality of circuits and said voltage limited conductor.
6. The system of claim 5, wherein said current limiting means consists of a plurality of resistors, one resistor being coupled between each of said circuits and said voltage limited conductor.
7. The system of claim 6, wherein said resistors have a structure which is unlikely to fail to a short circuit or low-resistance condition.
8. The system of claim 7, wherein said resistors are wirewound or metal film resistors.
9. The system of claim 1, wherein said voltage limited power supply means includes a power supply means and current interrupting means, coupled between said power supply means and said voltage limited conductor, for interrupting the current flow through said current interrupting means when said current exceeds a predetermined value.
10. The system of claim 9, wherein said current interrupting means includes a fuse.
11. A method of rendering intrinsically safe a plurality of circuits supplying electrical energy to a remote location comprising the steps of:
coupling each of said circuits to one power supply;
limiting the voltage of said power supply with respect to an intrinsically safe ground potential by providing at least one crowbar circuit coupled between said power supply and said ground; and separately limiting the current which can flow into each of said circuits from said supply.
coupling each of said circuits to one power supply;
limiting the voltage of said power supply with respect to an intrinsically safe ground potential by providing at least one crowbar circuit coupled between said power supply and said ground; and separately limiting the current which can flow into each of said circuits from said supply.
12. The method of claim 11, wherein said voltage limiting step includes providing a plurality of said crowbar circuits coupled between said supply and said ground.
13. The method of claim 11, wherein said current limiting step includes providing resistance between each of said circuits and said supply.
14. The method of claim 11, wherein said coupling step includes coupling through current interrupting means.
15. The method of claim 14, wherein said current interrupting means includes a fuse.
16. An intrinsically safe system comprising a plurality of circuits adapted to be coupled to field devices in a hazardous area, a voltage-limited power supply conductor, a plurality of resistors each of which is coupled at one terminal to said voltage-limited power supply conductor and at its other terminal to one of said plurality of circuits at least two crowbar circuits each of which is coupled between said voltage-limited power supply conductor and a conductor adapted to be coupled to an intrinsically safe ground potential and each of which includes means for sensing the voltage across said crowbar circuit and means for providing a low impedance path across said crowbar circuit when said voltage across said crowbar circuit exceeds a predetermined value.
17. The system of claim 16, comprising at least three said crowbar circuits.
18. The system of claim 17, further including current interrupting means coupled to said voltage-limited power supply conductor for interrupting current flow into said power supply conductor when said current exceeds a predetermined amount.
19. The system of claim 18, wherein said current interrupting means includes a fuse.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US42413189A | 1989-10-19 | 1989-10-19 | |
| US424,131 | 1989-10-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2028028A1 true CA2028028A1 (en) | 1991-04-20 |
Family
ID=23681571
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2028028 Abandoned CA2028028A1 (en) | 1989-10-19 | 1990-10-19 | Intrinsically safe system |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA2028028A1 (en) |
| DE (1) | DE4033313A1 (en) |
| GB (1) | GB2238191A (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE4403961C2 (en) * | 1994-02-04 | 1997-07-03 | Hartmann & Braun Ag | Feeding system for an intrinsically safe fieldbus |
| CA2209725A1 (en) * | 1996-08-14 | 1998-02-14 | M-I Drilling Fluids L.L.C. | Control system and method for electrically operated equipment |
| US6154679A (en) * | 1997-06-06 | 2000-11-28 | Rockwell Technologies, Inc. | Bus isolator for industrial control system providing intrinsically safe operation |
| DE29819235U1 (en) * | 1998-10-28 | 1999-12-09 | Siemens Ag | DC power supply unit |
| DE10124683B4 (en) * | 2001-05-18 | 2006-06-14 | R. Stahl Schaltgeräte GmbH | Power limiting circuit |
| DE10335203A1 (en) * | 2003-07-30 | 2005-03-10 | Flowtec Ag | Service interface for connection to field devices of process automation |
| DE102004063708B4 (en) * | 2004-12-28 | 2010-12-16 | Endress + Hauser Flowtec Ag | Circuit for voltage limiting |
| US7372397B2 (en) | 2005-06-03 | 2008-05-13 | Rosemount Tank Radar Ab | Energy storage in a radar level gauge |
| GB201001867D0 (en) | 2010-02-05 | 2010-03-24 | Kitchener Renato | Real time, non-disruptive, IEC61158-2 spur short circuit testing method for active device couplers |
| PT106221A (en) * | 2012-03-21 | 2014-04-11 | Portugal Telecom Inovacao S A | DEVICE WITH TRIPOLAR PROTECTION TO BE INSERTED AT THE POINT OF FRONTIER, AND THAT SIMULTANEOUSLY ADDES THE RC MESH |
| US10680433B2 (en) * | 2017-08-01 | 2020-06-09 | Fisher-Rosemount Systems, Inc. | Methods and apparatus for surge protection of a distributed control system component |
| DE102018100627B4 (en) | 2018-01-12 | 2019-10-10 | Krohne Messtechnik Gmbh | Electrical device with a fused and an unsecured functional device |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1412036A (en) * | 1972-04-06 | 1975-10-29 | Foxboro Co | Interface assembly in a process control system for use where there is a fire hazard |
| US3878434A (en) * | 1973-12-10 | 1975-04-15 | Leeds & Northrup Co | Power-limiting electrical barrier device |
| DE2724176B2 (en) * | 1977-05-27 | 1979-06-13 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | DC power system |
| US4412265A (en) * | 1981-11-27 | 1983-10-25 | Tokheim Corporation | Intrinsic barrier |
| GB2139830B (en) * | 1983-04-26 | 1987-01-07 | Manufacturing Electrical Desig | Power supply for use in inflammable atmospheres |
| DE3906621C2 (en) * | 1989-03-02 | 1995-05-04 | Hemscheidt Maschf Hermann | Intrinsically safe power supply unit |
-
1990
- 1990-10-19 DE DE19904033313 patent/DE4033313A1/en not_active Withdrawn
- 1990-10-19 CA CA 2028028 patent/CA2028028A1/en not_active Abandoned
- 1990-10-19 GB GB9022831A patent/GB2238191A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| DE4033313A1 (en) | 1991-06-13 |
| GB9022831D0 (en) | 1990-12-05 |
| GB2238191A (en) | 1991-05-22 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4228475A (en) | Ground monitoring system | |
| US4321643A (en) | Ground monitoring system | |
| US9197055B2 (en) | Ground monitor current sensing | |
| CA2028028A1 (en) | Intrinsically safe system | |
| US5144517A (en) | Intrinsically safe barrier device | |
| US7616420B2 (en) | Excessive surge protection method and apparatus | |
| KR101911417B1 (en) | Continuous Uninterruptable AC Grounding System for Power System Protection | |
| US6313642B1 (en) | Apparatus and method for testing an arcing fault detection system | |
| US6141194A (en) | Aircraft fuel tank protective barrier and method | |
| EP0513344A1 (en) | Fail-resistant solid state interruption system | |
| US20080100976A1 (en) | Limiting energy in wiring faults combined upstream and downstream protection | |
| WO1998058434A1 (en) | Overvoltage protection circuit for a generating system | |
| US4044395A (en) | Ground fault protection system | |
| CA1185651A (en) | Ground isolation monitoring apparatus having a protective circuit | |
| US6751076B2 (en) | Protection of intrinsically safe circuits | |
| US4160282A (en) | Overcurrent protection apparatus | |
| JP4756991B2 (en) | Surge protection device with automatic closing mechanism | |
| US3771020A (en) | Ground wire monitoring circuit | |
| US4242713A (en) | Triac output asymmetry detector | |
| US3581152A (en) | Round protection and detecting for high voltage dc transmission system | |
| US4223306A (en) | Device for detection of short circuits in industrial uses | |
| EP0310280B1 (en) | Electrical safety barriers | |
| US5432667A (en) | Neutral-to-ground fault sensing circuit | |
| US3308346A (en) | Ground cable continuity check circuit | |
| US4195325A (en) | Means for protecting hazardous areas against electrical faults |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |