JPH04156223A - Essentially safe system - Google Patents

Abstract

PURPOSE: To realize an essentially safety system inexpensively by connecting a plurality of essentially safety conductors, respectively, with a common power supply, current limit means combined with essentially safety conductors and effective voltage limiting means connected to limit the voltage of the common power supply. CONSTITUTION: A protected feeder line 67 forms a fail-safe power supply of a voltage limited to an essential safety ground so long as it is protected against intrusion of a dangerous energy or power level bypassing a current interruption means 62. When an over current or overvoltage is generated in a power supply 60 by a transistor having short circuit transient, voltage on the line 67 exceeds the threshold level of clover circuits 68a, 68b, 68c. Even if two clover circuit 68 failed or do not function, at least one clover circuit provides a low impedance path to the ground and the voltage on the line 67 is limited to an undangerous level.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to electrical systems, such as equipment systems, operating in environments that may contain flammable materials. More particularly, the invention ensures that electrical energy and power supplied to an environment that may contain flammable materials is limited to minimize the possibility of ignition of the flammable materials. The present invention relates to a method and apparatus for doing so.

BACKGROUND OF THE INVENTION Many locations, such as within and around industrial processes, have become or are becoming "hazardous" areas.

That is, an area where flammable gases may be concentrated or may become an area of explosion or fire hazard. Such hazardous areas often require electrical equipment and/or wiring as depicted herein, such as equipment and wiring for equipment systems that monitor and control industrial processes.

Several attempts have been developed to address the explosion hazard in such areas. Initially, the equipment was housed in a heavy, expensive metal case, and the wiring was routed through heavy, expensive metal culverts, so that any explosion ignited by the fruiting device was confined to the interior of the frame or culvert system. and prevents the system from being able to ignite a hazardous situation around it. Such "explosion proof" systems are very expensive and require the area to be made non-hazardous before opening the explosion proof frame to test, adjust or inspect equipment within the structure. It is inconvenient because there is The advent of semiconductor electronics has made it possible to design instruments and control systems that operate at power levels low enough to render certain hazardous situations essentially incapable of ignition. As far as the energy storage capacity of the circuit given limited energy and power is concerned, the supply is low enough that such equipment cannot release enough energy to ignite a dangerous situation, and the wiring and connections The complete system, including both electrical equipment, is safe for use under hazardous conditions.

Because all real-world systems are prone to component failure, systems are generally designed to react to a certain number of events that could be considered system shortcomings, while keeping energy and power levels low enough that a hazardous situation ignition does not occur. Designed to withstand. -According to acceptable standards for boat fishing, while the energy and power levels are kept below the limits necessary to ignite the hazardous situation, if it A system is considered "intrinsically safe" for a particular hazardous situation if it can withstand even

Intrinsically safe "barriers" have been developed to prevent hazardous voltages and currents from entering inherently safe circuits. Such barriers include input terminals adapted to couple to a power supply, output terminals adapted to couple to inherently safe wiring and circuits, means for voltage limitations provided to the output terminals, and with means for current limiting provided. Such barriers generally include fuses to protect voltage and current limiting components from failure due to high power events applied to the input terminals.

Such barriers are generally characterized by their failure mode, configuration,
And redundancy includes components such that their output terminals are inherently safe. Circuit components and terminals typically include:
It is supplied as an encapsulated assembly in a container that does not allow access to the components, including the fuse. By providing one such barrier to each other's ungrounded wires in a hazardous area, these wires are made inherently safe by virtue of inherently safe wiring practices, and these wires are The combined device has a reasonably low energy storage capacity.

However, such intrinsically safe barriers are expensive items, significantly impacting the price of intrinsically safe circuits and systems. This is especially the case for system J1 with many inherently safe circuits. Additionally, when a failure occurs that blows a fuse in the barrier, it typically must be replaced, which is more costly and inconvenient. Some intrinsically safe barriers or barriers lack the ability to have each limited component individually tested to ensure that it fully functions with the intended redundancy. Even if such testing were to be done within the barrier, testing would require removal of the barrier from service, a procedure that may be quite expensive, time consuming, and inconvenient.

(Problems to be Solved by the Invention) The purpose of the present invention is to provide cheap and essentially safe ten-tier coins.

A further object of the invention is to provide an inherently safe means that is adaptable for use in many circuits.

Another object of the invention is to provide an inherently safe means that can be adapted to a wide variety of field equipment.

Another object of the invention is to provide an inherently safe means of using only passive components in an inherently safe line.

Another object of the invention is to provide an inherently safe means that does not degrade the signals passing through it.

Another object of the invention is to provide an inherently safe means of providing a clear distinction between a power source and the hazardous area equipment it supplies.

Another object of the invention is to provide a simple, convenient and qualitatively safe means.

It is another object of the present invention to test the intrinsically safe circuits without removing them from the screws and by inherently safe means without interrupting the operation of the circuits being protected. It is to provide the means.

(Means for Solving the Problem) According to the above-mentioned object, the qualitatively safe means of the present invention includes a plurality of qualitatively safe conductors, each of which is connected to a common power supply,
Voltage limiting means in combination with each other's inherently safe conductors and are connected together to limit the voltage of the common power supply and are connected to effective voltage limiting means. According to a preferred embodiment of the invention, the voltage limiting means consists of a redundant rover circuit.

These and other objects and features of the invention will be understood with reference to the following description, FIG. 5, and the claims.

EXAMPLE FIG. 1 shows a schematic table 9. of a hypothetically safe barrier used in the prior art for protecting a single line intended to be routed within a hazardous area. The barrier connects input terminals 24 and 2 intended to be connected to a power supply.
．． It includes an output terminal 726 that is intended to be connected to wiring or circuitry in the hazardous area. The barrier includes redundant Zener diodes 20 and 22 for limiting the voltage H appearing at output terminal 26 in the event of a fault condition, such as an abnormal voltage and/or current at input terminal 24. Resistor 18 is output from j terminal 2
(provided to limit the maximum current that can pass through Fuse 14, which isolates the output and the circuitry protected by the barrier from the input in high input current conditions), 12
and 16 are provided to limit the current, and thus are consumed in the inner diodes 1, 20 and 22 in the event of a fault condition before the fuse 14 blows. Variation 7 includes redundant connections 28 and 30 adapted to be connected to an intrinsically safe ground 1'-0, the ground of which is 1 ohm or less, depending on the intrinsically safe wiring practice. (by means of redundantly protected wiring with a resistance of Fuse 1
Components of an inherently safe barrier, including the encapsulated or encapsulated. Therefore, input failure conditions generally require complete barrier replacement, an expensive and inconvenient procedure. Furthermore, resistor 18 is required to limit the possible output current when a maximum voltage is presented across Zener diode 22 in a fault condition, so resistor 1
The additional resistors 2 and 16 reduce the resistance that the field wiring will deliver to the field under normal operating conditions. This, for example, unduly limits conductor lengths and physical separation between barriers and connected field equipment.

FIG. 2 depicts a typical prior art system that provides an inherently safe circuit in a hazardous area.

A plurality of field devices 46a, 46b-46n are required to be located in a hazardous area and are connected to power lines 44a, 46b-46n.
44 b-44n are given. Such field equipment may include transmitters, transducers, indicators, and the like. Generally, such field equipment is required to be installed within a hazardous area to monitor or control process or material conditions within the hazardous area. Frequently, such field equipment is a 4-20 mA operating transducer or other equipment in a two-wire loop, with lines 44 reciprocally responding to events in one conductor of such a loop.

Field equipment 46 in the hazardous area is typically powered from circuits in the non-hazardous area. Such a circuit is typically a power supply 40, which is typically an AC input power source with an approximately 24V DC output. Supply 40 typically energizes some circuitry coupled to a conductor or line 44 and what is shown in FIG. 2 as a function block 42.

Function block 42 may consist of signal generators, monitor or measurement devices, display devices, multiplex circuits, and/or other devices for accomplishing disparate functions on the field equipment. As shown in FIG. 2, signal function block 42 is depicted connected to a common power source with all field equipment.

That is, one or more function blocks are connected to lines 44 that are associated with each other's field equipment.

Intrinsically safe barriers 48a are provided to protect the lines 44a, 44b-44n and the field equipment 46a, 46b--46n connected thereto from the ingress of hazardous energy in non-hazardous or hazardous area fault conditions. , 48b.

...48n is given. Mutual such barriers 4
8 may be in the configuration shown in FIG. 1, however, other designs may also be used. Barriers are applied to each ungrounded line in the hazardous area. Therefore, conventional two-wire signal loops require two barriers per loop. For circuits where one conductor is tied to an inherently safe ground, only one per loop is required. However, this also leads to rather expensive and inconvenient consequences, which the present invention seeks to avoid.

The power supply 40 is connected to an auxiliary circuit 50 that is not coupled to or associated with other or field equipment. In the system of FIG. 2, barrier 48 does not protect or otherwise affect such circuitry.

FIG. 3 depicts an inherently secure system according to a preferred embodiment of the present invention. As in the system shown in Figure 2, the third
The system shown is located in a hazardous area and the conductor or line 72
A plurality of field devices 74a to be provided with electrical energy through a, 72b, . . . -72n.

74b, ---74n, as mentioned above,
Such field equipment may include conductors, exchangers, transmitters, including 4-20 mA transmitters, and the like; power for field equipment 74 is typically provided from a power supply 60 located in a non-hazardous area. It will be done.

As in that of FIG. 2, the power supply 60 is typically 24V.
It is an AC input power supply with a DC voltage output.

As connected to the power supply 6o by supply lines 67a, 67b, like the function block 42 previously mentioned with respect to FIG. 2, it is a signal generator, monitor or measuring device, display! , multiple circuits, or various circuits that accomplish other functions.Generally, function block 66 provides a functional interface between field equipment and equipment located in non-hazardous areas, and thereby allows power to be connected to line 72. and means for coupling with field equipment 74. That is, while one function block is shown in Figure 3, equivalently multiple function blocks can be used with each other serving one or more lines or field devices. Understood. It is further understood that the system according to the invention need not include means serving any other function within the funxing lock 66 other than coupling power to field circuitry.

The supply line connecting the current interrupting means 62 to the function block 66 and the auxiliary circuit 64 is connected to the supply line 5, 6, so that a comparison of the corresponding times in the description of the figures can be made.
7a and °C one part, and 67b.

It is divided into other parts. The differences are not significant in FIG. 3, and the portions of the image are collectively referred to as supply line 67.

According to the invention. At least one crowbar circuit is provided between the supply line 67 and a conductor coupled to an intrinsically safe ground. By providing such a rover circuit, the supply line 67 becomes a voltage limiting conductor and the power supply means 6G connected to the function block 66 becomes a voltage limiting power supply means. Preferably, a plurality of such rover circuits are then provided, as shown in FIG. 3. In FIG.
b, and 68c are given, each of which can be the same. Such a low bar circuit monitors the voltage on the protected supply line 67 and, if it exceeds a predetermined threshold such as 28V, indicates that the protected supply line is inherently safe. Provides a low invidance path between ground and ground.

In a preferred embodiment of the invention, a current interrupting means f,')2, such as a fuse 7, interrupts the current (A supply) between the power supply 60 and the fan block 66 to limit the current in the supply line 67. You can get it at the inn.

Other current interrupting means may be used, such as a power supply.

Therefore, in the circuit of FIG. 3, the protected supply line 6'7 is essential as long as it is protected from the ingress of dangerous energy or power levels bypassing the current interrupt stage 62. φ7c = Creates a Failsafe II source of voltage limited to safe ground)1-. In the event of an overcurrent overvoltage in the source 60, such as caused by a transistor with a short circuit in the power supply 60, the current H- on the line 6l is transferred to the crowbar circuits 68a, 68b and/or 68e. Two crowbar circuits 6 that will shoulder the threshold voltage
In the event that line 67 fails or fails, the provision of a low impedance path, at least one to ground, operates to limit the voltage on line 67 to a non-hazardous level. In most cases, this will open the current interrupt stage 62 and remove power from the crowbar circuit at the function block 66, field wiring and equipment. The short circuit current capacity of the crowbar circuit is the current interrupting means 62
If the current required to open the line is less than
Under manual overvoltage and/or overcurrent fault conditions, it is essentially unsafe even if the 2'2) voltage circuit fails.

By limiting the lines that supply power to field wiring and field equipment to an inherently safe voltage, such wiring and equipment limit the current that may be supplied to them. This would make it inherently safer. This is for each line 70a, 70
b. This can easily be achieved by providing current limiting means at -70n. Such voltage limiters are safe if they fail. 1] is also preferable in this case to fail in an open circuit or high impedance rather than a short circuit or low impedance. Metal-coated or wire-wound resistors are suitable. The use of a passive element f such as a resistor to limit the current may lead to failure.
Nobility, special? This minimizes the possibility of failure on the low impedance side. Furthermore, such a passive element f also minimizes signal degradation within the current limiting stage 1'.

It should be appreciated that any field line or field insulator can be protected by two interleaved columns containing inexpensive resistors. As a result of the power supply connected to the field wiring being properly voltage limited, any field circuit becomes inherently safe simply by including an inexpensive resistor in the line. This is particularly advantageous in systems serving multiple field circuits.

Of course, it will be appreciated that normal intrinsic safety considerations, such as limiting the energy storage capacity of lines 72 and field equipment 74, and the use of intrinsically safe wiring techniques, also apply in the system of FIG.

Since there are no resistors installed to limit the current flowing in the crowbar circuit 68 other than the power supply 60, the wiring, the current interrupting means 62 and the crowbar circuit itself, these resistors as described above with respect to the intrinsically safe barrier resistors 12 and 16 are not installed. The effects associated with resistance are avoided. Thus, the system of the present invention maximizes the resistance allowed in field wiring and equipment, and maximizes the isolation that can be obtained under given conditions between field equipment 74 and circuits located in non-hazardous areas.

FIG. 3 further shows an auxiliary circuit 64 connected to the power supply 60. According to the invention, a funk silane block 66
Other non-hazardous area circuits, including other field equipment, are powered by the power supply 60.

If it is desired that the circuit be protected from overvoltage, the circuit will be driven from the protected line as shown.

A rover circuit useful for incorporation into the present invention is shown in FIG. However, it will be appreciated that many crowbar circuits, including the circuit of FIG. 4, are known per se and can be used in the system of the present invention.

The shorting element of the crowbar circuit of FIG. 4 is a silicon controlled rectifier (SCR) 80 whose main terminal is connected to an inherently safe ground and to the supply line 67 to be protected. The 5CR80 is an overvoltage detection element 82, such as Motorola's Model 3423 (trade name), which is an integrated circuit (IC) specifically designed for use in crowbar circuits.
controlled by IC 82 draws current from bin 8 if the voltage applied to bin 2 exceeds the internal reference voltage. Resistors 84 and 86 thus form a divider to establish a voltage on conductor 67 which, when exceeded, produces a current to bin 8. This current generates enough current to fire the 5CR80 and thus shut off the power supply. Resistor 88 is 5C
To limit the gate current of R80, a resistor 90 is installed to bypass leakage current. Capacitors 92 and 94 are provided for noise cancellation so that short duration noise transients do not cause accurate isolation and render the system inoperable. Zener diode 96 is provided to protect ICB 2 from overvoltage supply conditions and assists in opening the supply fuse in the event of an input overvoltage.

It will be appreciated that other crowbar circuits, including means of providing a low impedance path other than an SCR, such as a TRIAC or DIAC, are compatible with the system of the present invention.

FIG. 5 shows a modification of a portion of the system of FIG. 3 for testing and verifying the operability of a crowbar circuit. The system of FIG. 5 can be implemented without interrupting the normal operation of field equipment 74, auxiliary circuitry 64 or the elements comprising function block 66 of FIG. 3, and without affecting the inherent safety provided by the system. To provide such test means, including means for testing and verifying, the test circuit comprises a pass element 100, a control circuit 102 and a switch means 11.
6a-c are provided. without opening the current interrupting means 62 which would stop operation of the system (pass element 100 is closed to allow operation of the crowbar circuit during testing);
is inserted in series with the conductor 67. The zero passing element 100 is inserted in series with the conductor 67a connected to the current interrupting means and the control line 12.
6 to control the current between the conductor 106 and the conductor 106 under the control of 0
Pass element 100 may be comprised of a transistor connected to control line 126, and a load, such as function block 66 of FIG. 3 and auxiliary circuitry, is connected to conductor 67b.

Rover circuits useful in the present invention generally include means for comparing a signal associated with the monitored voltage with a threshold, and means for operating a switch in response to the comparison.

This feature allows the crowbar circuit to be tested without the need to bring the voltage to be monitored iW to a level above the normal threshold voltage of the crowbar circuit.

The crowbar circuits are arranged in such a way that when the voltage of the conductor to be monitored is increased very slightly, the crowbar circuit with the lowest threshold voltage operates first and the other crowbar circuits do not operate. If tested, the testing of individual crowbar circuits would be very complex.

The system of Figure 5 avoids this difficulty by simulating overvoltage conditions for each crowbar circuit. This condition is similar to each crowbar circuit.
and can be tested individually. In the system of FIG. 5, each crowbar circuit 68a, b and C
includes inputs 114a, b, and C to which signals are applied to establish, control, and change the thresholds at which the crowbar circuit operates. The test of the crowbar circuit of FIG. 5 is executed under the control of the control circuit 102. The control circuit 102 includes means for starting the test, such as timer means for automatically starting the test. Control circuit B 102 may also initiate a test 17 in response to an input 130 provided by an operator, external circuitry, or the like.

Test equipment beam 1-J-bar circuit 68 input 11 of FIG.
4. In the system shown in Figure 5,
The means for changing the input includes switches 116a, b and C controlled by control circuit 102. In the normal position shown, switch 116 connects input 114 of crowbar circuit 68 and conductor to be monitored 106 J:
A conductor 120 is coupled to the circuit to ensure a conventional threshold for inherently safe operation, such as 28V at 111V. In normal operation, the control
Is 102 also a passing element? 100 is controlled so that the current passing through it does not flow through the element 100 (substantial limit, depending on Landisk's saturation).

In order to carry out the test, Jinji passes through the element f'lOO's electrolyte.■
is the control circuit 102 t: 7, the first stream of frost is blocked! tJ'i
111. The electro-hydraulic which opens IM 6) is controlled in more ways than one. Next 1ti1. Control circuit 102 operates switch means 116 to connect conductor 122 with the corresponding control power 114 for the particular crowbar circuit G to be tested. This establishes a monitoring voltage threshold for operating the crowbar circuit 68 to be tested, which is less than the voltage normally supplied by power supply 60 and appearing on conductor 67 at 106 J-. When the crowbar circuit under test operates, a monitoring voltage higher than its threshold voltage is superimposed between the low impedance path in the crowbar circuit, ie, conductor 106, and an inherently safe ground. This causes a rise in the voltage on conductor 106 which would be detected to indicate that the crowbar circuit is active. The detector 104 of the circuit of FIG.
．． in the embodiment is responsive to changes in the voltage on conductor 106 to be monitored. As shown, detector 104 detects the voltage on conductor 106 that corresponds to the voltage on conductor 67, i.e., is responsive to the voltage drop on ticket Y100. However, the detector 104 is equally responsive to voltages on the conductor 106 corresponding to other potentials 2 or other causes, such as detecting a current or a change in current passing through the crowbar circuit 8. It also detects the operation of the receiver circuit.

The operation of the crowbar circuit under the test conditions described above is generally determined by ensuring that conductor 106 is effectively grounded or that it is at least partially grounded by field equipment, auxiliary circuits, and objects connected to conductor 67. Diode LO8 and capacitor 112 are installed to power the connected load such as A during testing of the crowbar circuit.

The capacitor 112 is charged during normal system operation, and its capacity is approximately 90%, considering the period of test operation.
), maintain the voltage on conductor 67h at a voltage sufficient to permit continuous system operation during the test. This is a means for preventing the capacitor 112 from being discharged by the diode 108 beam 117-bar circuit. Die:1-)'108 and capacitor 112 form a banker knob power supply that operates automatically to power the load during circuit testing. It is therefore understood that other means may be provided to perform the above function, such as, for example, a separate power supply to which the load is connected during testing of the crowbar circuit.

Rover circuits useful in the present invention include circuits in which there is a low impedance path resulting from operation after they have been activated. The circuit of FIG. 4 operates as follows. That is, once fired, the 5CR80 remains on until the current passing therethrough is reduced to substantially zero. For such a circuit, the control circuit 10
2 is a pass element 1 to reduce the current below the holding current of the crowbar circuit 60 and allow it to be reset.
00 may be included. Such means may be driven by receipt by control circuit 102 of a signal indicating that the crowbar circuit has operated in response to a test. Control circuit 102 then puts pass element 100 into its normal state, recharging capacitor 112 and providing power to the load connected to conductor 67b.

It will be appreciated that other reset means suitable for a particular rover circuit may be used to reset a latched crowbar circuit.

If the detector 104 does not indicate to the control circuit 102 that the crowbar circuit has operated within a predetermined time after the crowbar circuit test has started, the crowbar circuit under test
The circuit may be determined to be faulty and the test may be terminated.

The test circuit 102 may include means for generating outputs 132 responsive to the results of the test; such outputs may include visual indications, signals for communicating with other circuits, and the like.

The above test method may be performed separately for each of the crowbar circuits present in the system, allowing for individual testing and identification of faulty crowbar circuits. Testing may be performed automatically and repeatedly to ensure that it is inherently safe, or may be performed at any time as necessary to confirm such conditions.

〔Effect of the invention〕

Thus, a system has been disclosed that is simple, highly reliable, and inexpensive and can make a plurality of circuits essentially safe. Although specific methods and apparatus have been disclosed, those skilled in the art will recognize that modifications may undoubtedly be made without departing from the spirit of the invention.

[Brief explanation of the drawing]

FIG. 1 is an intrinsically safe barrier diagram typical of the prior art. FIG. 2 is a block diagram of a typical intrinsically secure system according to the prior art. FIG. 3 is a block diagram of an inherently secure system according to the present invention. FIG. 4 is a schematic diagram of a rover circuit useful in the system of the present invention. FIG. 5 is a block diagram of an embodiment of the invention including testing means for certain protected components. 14... Fuse, 20, 22.96... Zener diode, 42.6
6...Function block, 50.64...Auxiliary circuit, 48a-48n...Barrier, 68a-6
8c... Clover circuit, 40.60...'@B supply, 80... Thyristor.

Claims (1)

Claims: 1. A system for supplying electrical energy to a plurality of circuits at an inherently safe level, the system comprising: a ground connected to an inherently safe ground potential; conductor for supplying power to a conductor at a voltage within a limited range with respect to the grounding conductor, and at least one conductor coupled between the voltage limiting conductor and the grounding conductor; a voltage-limiting power supply means comprising a crowbar circuit; and a voltage-limiting power supply means coupled between each of the plurality of conductors and the voltage-limiting conductor, the means for supplying current from the voltage-limiting conductor to each of the plurality of conductors. A power supply system comprising: current limiting means for limiting inflow; 2. The power supply system of claim 1, wherein said voltage limited power supply means comprises at least two crowbar circuits. 3. The power supply system according to claim 1, wherein said voltage limited power supply means comprises three crowbar circuits. 4. The above crowbar circuit is a silicon controlled rectifier (SCR).
) and an overvoltage detection circuit, each of which is coupled between the voltage control conductor and the grounding conductor. 5. Claim 1, wherein the current limiting means comprises a resistor between each of the plurality of circuits and the voltage limiting conductor.
Power supply system as described. 6. The power supply system according to claim 5, wherein the current limiting means comprises a plurality of resistors, one resistor being coupled between each of the circuits and the voltage limiting conductor. 7. The power supply system according to claim 6, wherein the resistor has a structure that is unlikely to fail even in the event of a short circuit or a low resistance condition. 8. The power supply system according to claim 1, wherein the resistor is a wound type or metal film type resistor. 9. The voltage limiting power supply means includes a power supply means and a current cutoff means, and the current cutoff means is coupled between the power supply means and the voltage control conductor;
The power supply system according to claim 1, having the purpose of interrupting the flow of current through the current interrupting means if the current passing through the current interrupting means exceeds a preset value. 10. The power supply system according to claim 9, wherein the current interrupting means includes a fuse. 11. In a method of essentially making a plurality of circuits used to supply electrical energy to remote locations, the step of: coupling each of the circuits to a power source; limiting the voltage of the power supply with respect to an inherently safe ground potential by providing at least one crowbar circuit; and individually limiting the current that may flow from the power supply into each of the circuits; A method for making multiple circuits essentially safe. 12. The method of claim 11, wherein said step of voltage limiting comprises the step of providing a plurality of said crowbar circuits coupled between said power source and said ground. 13. The step of limiting the current includes providing a resistance between each of the plurality of circuits and the power source,
The method according to claim 11. 14. The method of claim 11, wherein said step of coupling comprises the step of coupling via means for current interruption. 15. The method according to claim 11, wherein the current interrupting means comprises a fuse. 16. An inherently safe system comprising: a plurality of circuits suitable for connection to field equipment in a hazardous area; voltage-limited power supply conductors; a plurality of resistors connected to the power supply conductor and connected at their other terminals to one of the plurality of circuits; at least two crowbar circuits, each of which is essentially connected to the voltage limited power supply conductor; conductors used so as to be connected to a safe ground potential, each having means for sensing the voltage across the crowbar circuit, and a means for detecting the voltage across the crowbar circuit to a predetermined value. at least two crowbar circuits; and means for providing a low impedance path across the crowbar circuits when the crowbar circuits are exceeded. 17, comprising at least three of the above crowbar circuits;
17. The system of claim 16. 18. Claim 18, further comprising current interrupting means coupled to the voltage limited power supply conductor for interrupting current flow into the power supply conductor when the current exceeds a preset amount. 17. The system according to 17. 19. The system of claim 18, wherein the current interrupting means comprises a fuse. 20. A crowbar circuit coupled to an electrical supply line between a power source and a load, the crowbar circuit providing in operation a low impedance path coupled to the line when the input to the crowbar circuit exceeds a threshold. A method of testing the crowbar circuit comprising: applying the input to cause the threshold to be exceeded; determining whether the crowbar circuit is activated; and while the crowbar circuit is activated. A method for testing a crowbar circuit, comprising: supplying power to the load. 21. The power source is provided with means for interrupting the inflow of current into the line when the inflow of the current exceeds a current threshold, and the method includes: 21. The method of claim 20, further comprising the step of: limiting the current to an amount less than the threshold current during testing of the crowbar circuit.