US2459186A - Testing and protection of electrical distribution systems - Google Patents

Description

Jan. 18, 1949. w R. SCHEIRMANN 2,459,136

NOW BY CHANGE OF NAME R. SHERMAN TESTING AND PROTECTION OF ELECTRICAL DISTRIBUTION SYSTEMS Filed July 19, 1943 s Sheets-Sheet 1 FIE) ll j Jan. 18, 1949. R, SCHEIRMANN 2,459,186-

NOW BY CHANGE OF NAME R. SHERMAN TESTING AND PROTECTION OF ELECTRICAL DISTRIBUTION SYSTEMS Filed July 19, 1943 3 Sheets-Sheet 2 INVENTOR.

Jan. 18, 1949. 5 NN 2,459,186

NOW BY CHANGE OF NAMER. SHERMAN TESTING AND PROTECTION OF ELECTRICAL DISTRIBUTION SYSTEIS Filed July 19, 1943 3 Sheets-Sheet 3 Patented Jan. 18,1949

TESTING AND PROTECTION OF ELECTRICAL DISTRIBUTION SYSTEMS Raphael Scheirmann, Warren, Ohio, now by change of name Ralph Sherman Application July 19, 1943, Serial No. 495,311

Claims. 1

This invention relates to means for permanently or intermittently supervising, testing, indicating and controlling the condition of contact between electrically conductive parts which are required to maintain intimate contact between them, and this without interruption of the operation of the devices or installations which these parts belong to.

This invention is applicable to contacts between contacting parts of any kind provided they are capableof conducting electrical energy, but

. it is particularly valuable in application to systems traversed by a working current i. e. 9. current doing useful mechanical, electrical or chemical work and liable to be disturbed or injured by fundamentally imperfect or suddenly or gradually deteriorating contact between two contacting current-carrying parts.

It is one of the objects of this invention to provide means whereby the operator of an electrical installation is warned of any imperfection or deterioration of contact in the circuit as soon as it arises, thus enabling him to remedy the defeet before it has resulted in injury to the inwhich they are operating. The oxygen or the moisture in the air will cause the formation of an oxid layer on. contact surface. Dust, oil or grease may form insulating coatings thereon.

Oxidation may also occur through the action of heat. Loosening of mechanical connections may occur through many causes. In all such cases there results a rise of resistance to the passage of current.

According to this invention the variations in this intermediate resistance are rendered visible or audible in indicating or alarm devices, or are utilized to activate switches, permanently connected to the contacting parts to be protected or supervised, while these parts are traversed by the working current. In any case the attendants or operators are thus warned and induced to remedy the defect or, if necessary, to temporarily throttle or cut out the current.

All variations of the intermediate resistance 'between contacting surfaces result in corresponding variations of the voltage drop at these surfaces. This is particularly true in the case of high current installations such as are present in electric melting furnaces operating with currents up to and beyond 50,000 amp. Obviously in such cases any unnoticed rise of the intermediate resistance in a contact may cause very considerable injury to the parts and losses of energy and/or metal. Frequently the electrodes are overheated to the extent of becoming incandescent, arcs may form between the electrodes and their holders, the bus bars may be heated to melting temperature etc. In most cases the defect that is at the root of all this, will not or cannot be detected in time and the installation may be out of working for many hours or even days.

Hitherto no means have been available for preventing with certainty and automatically the damage arising from such causes.

Numerous devices are on the market for measuring the specific resistance of solid conductors, but these devices are not suitable for the purpose here in view, i. e. the detection of imperfect contact between contacting parts traversed by a working current. For, in contrast to the resistance in solid conductors, the intermediate resistance between contacting parts varies owing to circumstances such as set out above and also to the intensity of current. If this resistance is ascertained or measured under current conditions considerably below those of the normal working current, the results may be deceptive. Therefore in these measuring devices, all of which operate with a separate current, frequently as high a current intensity is used as possible, but since they must be portable, the intensity is limited for technical reasons to a few hundred amperes. Obviously now it is impossible, with so low a current, to correctly measure contact resistances in apparatus, for instance electric melting furnaces, traversed by many thousand amperes. But quite apart from this technical limitation, these devices are only used to measure resistances in a de-energized equipment. They do not indicate all conditions arising during actual operation, when vibrations or temperature influences or some other circumstances may altogether change the resistance at the contacts, sometimes followed by arcformatlon and eventual melting down of contacting parts.

In contrast to these existing methods of resistance measurement, the present invention enables me to test the conditions of contact in the equipment while it is operating and traversed by the working current, I can provide for a continuous testing and measuring and an automatic signalling of contact conditions and variations, and I may also provide means for automatically throttling or cutting out the current. In every case I utilize the circumstance that any deterioration of contact is accompanied by a corre-,

sponding change of voltage drop at the contact.

In cases where the amperage is subject to considerable variation, I prefer to use a measuring instrument indicating the quotient of the prevailing system current and voltage, for instance a cross-coil instrument, which is known to indicate directly the resistance, for indicating the disturbance.

While as a rule the working current will be used to actuate the indicating instruments, alarms or switches, I may also use a separate current for this purpose without in any way impairing the result, which will be the same as that obtained with the aid of the working current, since I employ the separate current while the working current fiows.

For instance, if a separate direct current is used in an alternating current system to indicate the condition of contacts in accordance with my invention, the direct current voltage drop arising from the intermediate resistance exactly corresponds to the actual intermediate resistance formed at this moment by the entire working current. Thus the separate direct current used for testing and indicating these variations furnishes the same picture of the conditions of operations, as would be obtained with the aid of the full working current of thousands of amperes. This method may be particularly advantageous whenever it is desired to eliminate the induction effect exerted on the test (measuring) results. In all cases the operator, without being required to take action for this purpose, is continuously furnished full information regarding any deterioration of the contacts during operation and is thus enabled to prevent in time injury to, or destruction of, the equipment.

I will now proceed to describe by way of example some characteristic applications of my invention in order to thereby show that it will be useful in widely different fields of utilization of electrical energy. The examples hereinafter described with reference to the drawings should not in any way be considered as limiting the scope of my claims to the individual means or combinations of means nor to the particular uses and applications shown and described. For there exists hardly any form of electrical installation in which my invention could not be applied with advantage. Certain features of my invention disclosed herein but not claimed are claimed in my copending application Serial No. 55,605, filed October 20, 1948.

Throughout the drawings affixed to specification and forming part thereof, similar numerals are intended to designate similar parts. In the drawings Fig. 1 is a diagram of an alternating current system comprising an electrical melting furnace with an electrode mounted in a holder, in combination with means according to this invention for continuously supervising, indicating and controlling the variations of contact between these parts during operation, making use of the work ing current for effecting this indication and control.

Fig.2 is a similar view of a direct current system in combination with means for supervising contact conditions in the bus bars by means of the working current.

Fig. 3 is a diagram of an alternating current system with a bus bar contact controlled by means of a separate direct current.

Fig. 4 is a similar view of a direct current system in combination with means for supervising a bus bar contact by means of a separate alternating current.

Referring to the drawings and first to Fig. 1 this is a diagram of connections of a circuit feeding alternating current of high intensity through the lead I to the electrode holder 2 of the electrode 3 of an electric melting furnace and through it to the body 5 of molten metal. The condition of contact between the electrode and the holder is supervised and controlled continuously, while the furnace is in operation, or while the supply circuit is connected to a useful load, by providing a measuring instrument which may be permanently connected in the circuit and at any time indicates to the operator Whether the contact is good or requires improving. To this end two wires 8 and 9, connected to the holder and the electrode at 5 and l, respectively, lead to the primary winding ii of the voltage transformer id. The potential of the secondary transformer. winding i2 is transmitted by wires 13 and I l! to the bridge rectifier 22, while on the other side a wire it leads to the contact piece I47 of a small controller whose normal position is marked by the letter A, where the current from contact piece It! passes through the contact segment l5, connecting wire it, segment i'i, contact piece I53 and wire 2| to the other side of the rectifier 22 and from one side of the rectifier through wires 23, I43, 25 to the voltage coil 25 of the measuring instrument (cross-coil ohmmeter) 26, from the other side through Wire I63, contact 21 and wire 28 to the series resistance 29 allowing adjustment to different measuring ranges, and from this resistance through the brush I44 and wire 3| to the second end of the voltage coil 25 of the measuring instrument.

The electrode 3 is supplied with current through wire I54, the primary winding 33 of the current transformer 32, whose secondary winding 34 is connected on one side to the rectifier 42 by way of wire 35, amperemeter 33 and wire 4!, while the other winding is connected to the rectifier by the wire 43. From this rectifier the wire 41 leads to one end of the adjustable shunt 48, while a Wire 49 connects the rectifier to the brush 50 of this shunt, whose ends are connected on one side through wires I55, to one end of the current coil 53 of the cross-coil instrument, on the other side through wires 5|, 52 to the other end of this coil.

Since the cross-coil ohmmeter directly indicates the ratio of potential and current, the resistance can be read directly on its scales. Adjustable series resistance 29 and adjustable shunt 48 are provided for the purpose of adapting the instrument to various conditions in different installations.

Since this ohmmeter merely indicates the ratio of potential and current, the amperage fed to the furnace is immaterial to the result. As long as the intermediate resistance between the furnace electrode and the holder remains constant, its pointer I5I will remain stationary. Whenever the contact should deteriorate by a loosening of the electrode in the holder or from other causes the pointer will move at once, and if the deterioration of contact reaches a certain limit, the pointer will establish contact between the terminals 54 and 59 of a signalling circuit which comprises the current source 58. one terminal of which is directly connected through wire 51 to the relay 55, while the other terminal is connected to the relay through wire 58, closed contacts 59, 54 and wire I55. Relay 55 will now close the contact 54 and thereby excite, with the aid of current supplied by the current source 55, the optical oracoustical signalling device 55. Thus the operator, unless he had ascertained the deterioration of contact by observing the position of the pointer of the ohmmeter, is warned at once of the danger threatening the furnace and its contents. If he should not heed the signal or if he should be absent, the signalling instrument will, by means of a time-relay, resistance and circuit breaker (not shown) connected to it by the wires 51, 58 first throttle the current supply and thereafter cut it out altogether, An excess voltage which may arise occasionally, is provided for by a switching relay I 38 connected in parallel to the voltage coil 25 of the cross-coil instrument by a wire I3I leading to the regulating brush I44, the other wire I32 being connected between wires I43 and 23. Any excess voltage in the potential coil of the instrument will cause the relay I38 to open the contact 21. Further means serving as a protection against excess voltage will be described farther below.

In the case of extremely low voltage it is advisable to ascertain from time to time whether the contacts in the testing circuit are perfect. To this end a rotatable controller is provided with a separate alternating current source feeding a low tension transformer 18 with a resist ance 13 regulating the current output of the secondary winding 12 which is connected by the wire 14, amperemeter' 15 and wire 15 to the contact piece I55, while on the other side the wires 18 and 19 lead to the contact piece I55. Between the wires 14 and 18 a voltmeter 11 is connected for measuring the voltage output.

When the controller is turned into the first testing position B, current will flow from one terminal of the low tension transformer 18 through contact piece- I55, segment 82, wire 83, segment 84, contact piece I48, wire I49 and wire I3 to one end of the winding I2 of the voltage transformer I8. The other end of winding I2 is connected to the source of current by means of the contact piece I56, segment 81, wire 85, se

ment 85, contact piece I41 and wire I4. Hereby this winding I2 is excited and a voltage is induced in the other winding II, which, if all the contacts are in good order, is short circuited through wire 9, contact 1 of the electrode 3, wire 8 and the contact 5 of the electrode holder 2. The deflection of the ointer of amperemeter 15 at a predetermined deflection of the voltmeter 11 then allows to readily ascertain whether the contacts 1 and 5 at the electrode and its holder v are in order. If they should be oxidized, a mateis exactly the same as in Fig. l.

' (check up device in position B").

'rially lower current will pass through. In every installation the empirically determined values are ascertained by tests. If the deflection of the amperemeter is found to be too low, the voltage at the regulating resistance 13 is increased until the instruments show that good contact is reestablished.

By setting the small controller to the position C, the other'side of the circuit of the voltage coil is then tested. To this end one pole of the voltage transformer is connected with one side of the rectifier 22 by way of contact piece I55, segment 88, wire 89, segment 98, contact piece I58 and wire 2I. The other pole is connected with the other side of the rectifier 22 by way of contact piece I55, segment 93, wire 92, segment 9I, contact piece I48, wire I49 and wire I4I.

By applying a predetermined voltage and varying it any deflection of the pointer I5I of the cross-coil instrument 25 will be noticed, while the current cell 53 is fed by the working current through the voltage transformer 32.

The check-up device hereabove described can be used also with advantage for testing any other part of the apparatus.

While Figure 1 illustrates an electrode and electrode holder of a furnace, I wish it to be understood that this is merely one example out of the great number of points of contact in an alternating current installation which can be controlled and protected in the manner here described.

Figure 2 illustrates the application of the principle underlying this invention to a direct current system, the testing device being again traversed by the working current. Here the contact between two bus bars 28I, 282 shall be tested.

The greater part of the equipment including.

the wiring from the cross-coil instrument and its voltage coil up to the contact 21 and wire I43 Contact 21 is connected by way of wires 285 and 285 to the contact 283 on bus bar MI. The other side of the voltage coil is connected to the other bus bar 282 through wires I43 and 2I8, contact piece 2I3 (the controller being in the normal operating position A), segment 2I2, wire 2| I, segment 2I8, contact piece 289, wire 281 and contact 284.

This inner connection of the current coil 53 up to the adjusting shunt 48 is also the same as in Fig. 1. 7 One end of the shunt 48 is connected through the wires 233, 232 to one end of the shunt 23I which is inserted in the lead 298 to bus bar 28L The brush 58 is connected through wire 238, contact piece 225, segment 224 (the controller beingin the position A), wire 223, segment 222, contact piece 22I, wire 228, amperemeter 291 and wire 2I9 to the second terminal of the shunt 23 I.

This system functions exactly like the one illustrated in Fig. 1. Here also any excessive rise of the intermediate resistance will cause the pointer of the cross-coil instrument to be deflected so far as to close the contact 54 and 59, whereby the signal device 55 is actuated. The protection against excess voltage is the same as in Fig. 1 and here also two positions of the check- 'up device are shown, in which the contacts are tested. Y

This example comprises a separate source of direct current 234 with a voltage divider 235, One pole is connected to wires 242, 243, contact piece 245, segment 252, wire 253, "segment 254, contact piece 289 and wire 281 to the contact 284 on bus bar 202. The other pole from the separate direct; current source (battery) is connected through contact piece 24!, segment 241, wire 248, segment 249, contact piece 250 and wires 25!, 205 to the contact 203 of bus bar Here, as in the system shown in Fig. 1, the deflections of amperemeter 238 and the voltmeter 29! of the separate source of current will show whether there exists an intimate contact in the tested system. Here also, if necessary, the voltage will be increased until the contacts are reestablished.

In the position C the connection leading to the shunt 23! is tested. Since this is done in exactly the same manner as explained with reference to position B, there is no need for going into the details of this connection.

Obviously here also the check-up device can be used for testing any other contacts in the sys-,

tem

Fig. 3 illustrates an alternating current system comprising bus bars, the contacts between which are placed under supervision of devices supplied with separate direct current.

This direct current circuit comprises a lowtension source of direct current for instance a battery 303. feeding direct current through the regulating resistance 304, wire 305, amperemeter 306 and wire 30'! to the bus bar 30!. From the other pole of the battery current flows through wire 30B, shunt 309, wire 3i0, regulating inductive resistance (choking coil) 3!! and Wire 1H2 to the other bus bar 302.

From shunt 309 testing wires 3!'!, 3l8 lead to the adjustable shunt 48 and its brush 50. The shunt is further connected, in exactly the same manner as in the examples described above, to the current coil 53 of the cross-coil instrument 26. The voltage coil of this instrument is connected, exactly as in the previous examples, up to the contact 2'! and wire I43. This wire is connected through wire 3H5 to bus bar 302, while contact 2'! is connected through the regulating choking coil (inductive resistance) 3!4 and wire 3 l 3 to the other bus bar The regulating resistance 304 regulates the direct current used for testing. Its amperage is indicated by the amperemeter 305. Shunt 309 feeds current to the current coil 53 of the crosscoil instrument.

The drop of potential, caused by this direct current, at the point of contact of the two bus bars is fed through the wires 3!5, 3!B to the voltage coil 25 of the cross-coil instrument.

The entire system operates exactly as described before with reference to Figs. 1 and 2.

Obviously the working current (alternating current) passing through the two bus bars 'at the contact point will suiler a drop of potential also. In order to prevent this drop of potential of the alternating current from influencing the testing apparatus, the two choke coils 3H and 3!4 mentioned above are provided. They offer a high inductive resistance to the passage of the alternating current, while allowing the direct current to pass through freely, and therefore exact tests can be obtained during operation with the separate direct current without paying regard to the presence of alternating current. The choke coils 3!! and 3!4, thus serve as iso-, lating units for in effect isolating or substantially separating the measuring circuit from the load circuit, viz., the direct-current circ it from the alternating-current circuit.

The drop of potential of the separate ir current is exactly proportional to the intermediate ohmic resistance ofiered to the entire alterhating working current. Even in those cases where the direct current intensity only amounts to a small percentage of the alternating working current intensity, a correct picture of the intermediate resistance is obtained.

This mode of testing this resistance offers particular advantages in all cases where the effects of an inductive resistance at the contact point to be controlled shall be eliminated. When using alternating current for the tests, this inductive resistance might make the drop of potential appear much higher than would correspond to the purely ohmic resistance. The use of separate direct current eliminates this phenomenon and also offers the advantage of allowing to ascertain the presence of fairly good contact even before operations are started, while any variation of the intermediate resistance during operation is indicated also.

The separate source of direct current, such as the battery, can be charged up from the alternating current system with the aid of a corresponding low-tension transformer and a rectifier.

In Fig. 4 a direct current system is illustrated in which the testing is done at a pair of bus bars during operation with .the aid of separate alternating current. An alternating current source supplies current through a low-tension transformer 404, whose secondary winding 406 is connected at one end to the regulating resistance 401, wire 408, amperemeter 409, and through wire 4!!) to the bus bar 40!. From the other end of the secondary transformer winding 406 current flows through wire 4!! primary winding 33 of the current transformer 32 and wire M2 to the second bus bar 402.

The secondary winding of the current transformer 32 is connected to the current coil of the cross-coil instrument through the rectifier in the same manner as has been explained with reference to Fig. 1, as can easily be ascertained from Fig. l, where the same parts are marked with the reference numerals already used in V Fig. 1.

The drop of potential at the point of contact caused by the alternating current is passed on to the primary winding of the voltage transformer l0 through wires M3, 4 I 4. The secondary winding !2 carries this voltage through the rectifier 22 to the voltage coil of the cross-coil instrument in exactly the same manner as in Fig. 1.

For the sake of simplicity the check-up device has been omitted in Figs. 3 and 4.

The amperemeter 409 indicates the amperage used in this testing.

The drop of potential of the working direct current at the bus bar contact occurs parallel to that of the alternating current, but this drop of potential of the direct current cannot influence the test results. The transformers I0 and 32 transform and thereby transfer the drop of potential of the alternating current and the intensity of this current onto their secondary windings and further to the cross-coil instrument. In contradistinction thereto the transformers prevent the direct-current drop of potential in the network from reaching the secondary windings. Thus here, as in Fi 3, t e tests with separate alternating current are not disturbed by the working current. In this instance the transformers I0 and 32 serve as the isolating units for separating the measuring circuit from the load circuit. If precise preservation of the instrument transformer ratio, and highly' ac-v curate indications are desired, suitable blockin devices such as condensers or rectifiers connected in opposition to direct-current flow may be interposed in the circuits of the input windings II and 33 of the instrument transformers, as will be well understood by those skilled in the art.

This mode of testing the contacts will be particularly advantageous in cases where the contacts are liable to frequently deteriorate materially or to even open altogether, since instead of the full network voltage for instance of 110 or 220 volt, only a few volts from the separate source of current can reach the test instrument and it is easier to protect the instruments from injury by this low voltage.

To also illustrate the use of separate alternating cm'rent for testing the contacts in an alternating current system does not appear tobe warranted since the combination of apparatus will be the same as shown in Fig. 4.

For instance in a system of this kind operating with many thousands of periods, separate alternating current of 25 or of 60 periods may be used for measuring and indicating the variations of intermediate resistance. The inductive resistance of the measuring transformers which are designed for a low number of periods, oppose so high an inductive resistance to the high frequency alternating current that any influencev methods, the present invention is not primarily,

concerned with the actual magnitude of the ohmic resistance of a contact, the main purpose being to let the operator know when a deterioration has taken place which renders it necessary for him to take preventive action.

If a protective equipment according to this invention is used in connection with an alternating current system operating with very high current intensities, for instance in a circuit comprising an electric melting furnace or a high-duty transformer, it may happen that the testing and signalling lines must be located in the vicinity of very strong magnetic fields whereby they will be exposed to a high induction effect.

In order to paralyze this effect, the wires will be arranged as closely together as possible in order to keep the induction effect as uniform as possible. It may nevertheless happen that the induction 'will not be exactly the same in two wires leading to a protective device, and in such a case an additional voltage may be induced in the measuring wires, which may even amount to a multiple of the normally measured voltage drop. I prefer connecting with each other the ends of each pair of measuring wires and to connect to their opposite ends leading to the switchboard a sensitive alternating current voltmeter or a moving coil voltmeter with a rectifier with or without a voltage transformer, or some other sufficiently sensitive indicating instruments. If in a In order to eliminate this induction effect normal test the two wires show a difference of inductive potentials, one of them is reduced or increased in length until its induction potential is equal to that of the other wire and'is thus compensated. The indicating instrument is then replaced by the regular testing device.

Obviously the cross-coil instruments shown and described throughout the specification may be replaced by other suitable measuring or indicating instruments, for instance a wattmeter or an amperemeter in combination with, or without, a series resistance.

The examples of applications of this invention in the protection of electrical systems show that this protection can be obtained with the aid of the working current in direct and in alternating current systems. The invention is further applicable to conductors and to entire parts of a network. The devices to be used for protection may be stationary or portable and of a merely indicating (signalling) or recording type and may be provided with amplifying devices. The term "electrical system is employed in the description and claims to designate an electrical installation under load or energized. For example, the term refers to electrical installations where the current from the network, generator, battery, etc., or any .other source of power is taken with the main purpose of producing work or converting electrical energy to another form of energy, for instance, conversion into mechanical work or heat, or another form of electrical magnetic or electromagnetic energy as in high frequency induction heating. The term electrical system as herein used, excludes purely measuring circuits where the only source of current'applied is a separate source used with the purpose of making measurements without performing work in some manner as explained above.

01 the equipments to be protected by means of this invention the following are particularly noteworthy: bus bars, connections in motors, generators, transformers, fuse connections, cable connections within machines and apparatus, electric melting furnaces etc. and in a general way power stations, transformer stations, steelworks, chemical works, electric railway systems, electrochemical installations for the recovery of metals such as copper, nickel, silver etc., but also electrical cooking, heating and illuminating installations. I

While the principal and most important field of application of this invention is the protection of electrical systems, i. e. systems traversed by a working current, it will be apparent that the invention can be used also with similar advantage in the protection of devices, apparatus and machinery of non-electrical character in which two separate electrically-conductive parts are jointed together and held in contact with each other by mechanical means such as rivets, screw bolts or the like or by soldering, brazing or welding. In most types of working machines, prime movers etc. screw bolts jointing two contacting parts may become loose during operation under the influence of vibrations or the like. In aircraft certain parts such as bracing members may be loosened through the impact on the ground when landing.

The safe operation of electrical installations on board of ships, more especially warships may depend on the automatic supervision of the contacts by means of this invention. In all these cases a low tension current traversing the joint or contact will be aifected by the rise of the intermediate resistance resulting from such loosening, and this rise of resistance can be rendered visible or audible during operation with the aid of my protection devices.

The control efiected according to this invention includes, besides the changes in the ohmic resistance, also the changes in the inductive (apparent) resistance in alternating current systems.

The term every essential magnitude" is intended to include all resistance changes occurring between the normal state of the contacts and a degree of deterioration which involves danger to the functioning of the system.

I wish it to be understood that I do not desire to be limited to the exact details disclosed in the specification and drawings, for obvious modifications will occur to persons skilled in the art.

I claim:

1. A protective arrangement for rendering perceptible predetermined degrees of changes of contact conditions in an electrical system having a circuit traversed by load current including a pair of conductors in contact, said protective arrangement comprising in combination with such contact and connections for connecting said contact to a useful load and for causing passage of load current through such contact, a device responsive to ratio of voltage to current having a movable element, a translating device actuated by movement of said movable element, said ratio responsive device including a voltage winding and a current winding, connections bridging the voltage winding across said contact andadapted to be connected permanently and connections having a serial relation to said contact for energizing the current winding in proportion to the current traversing said circuit and adapted to be permanently connected.

2. A protective arrangement for rendering perceptible predetermined degrees of changes of contact conditions in an electrical system having a circuit traversed by load current including a pair of conductors in contact while connected to a useful load, said protective arrangement comprising in combination with such contacting conductors and connections for causing passage of load current through such contact between the contacting conductors, an instrument responsive to voltage-current ratio having a movable element, said ratio responsive instrument including a voltage winding and a current winding, connections vfrom the voltage winding across said contact, and connections having a serial relation to said contact for energizing the current winding in proportion to the current traversingsaid circuit.

3. An electrical system comprising in combination a current-carrying circuit and a protective arrangement therefor, said circuit including a pair of conductors in contact, the contact impedance of WhiChlS subject to variation, and a useful load supplied by said circuit, said protective arrangement comprising apparatus for indicating the magnitude of such contact impedance, said apparatus comprising in combination with such contact and connections for causing the passage of alternating-current through such contact a voltage-current ratio responsive instrument including a voltage winding and a current winding, connections including a rectifier operatively connecting the voltage winding across said contact, and connections including a second rectifier having a serial relation to said. contact for energizing the current winding in proportion to the alternating current traversing said contact.

4. In an electrical system including a circuit traversed by a load current and containing an impedance element, the impedance of which is subject to variation, an apparatus for indicating the magnitude of such impedance, said apparatus comprising an electrical instrument responsive to ratio of voltage to current including a voltage winding and a current winding, an auxiliary source of current having connections to said impedance for causing measuring current-to traverse said impedance, connections from the voltage winding across said impedance element, and connections having a serial relation to said impedance element for energizing the current winding in proportion to the current traversing said impedance element, each of said instrument connections including isolating units for substantially eliminating load current from the windings of said ratio instrument.

5. In an electrical system including a directcurrent circuit traversed by a load current and including a resistance the magnitude of which is subject to variation, apparatus responsive to the magnitude of such resistance, said apparatus comprising a ratio responsive instrument including a voltage winding and a current winding, a local source of alternating current, connections for causing said alternating current to traverse said resistance, connections from the voltage winding across said resistance, and connections having a serial relation to said resistance for energizing the current Winding in proportion to the current traversing said resistance, said instrument connections each including electrical isolating units for substantially eliminating direct load current from said windings.

6. In an electrical system including a directcurrent circuit traversed by a load current and including a resistance the magnitude of which is subject to variation, apparatus responsive to the magnitude of such resistance, said apparatus comprising a ratio responsive instrument including a voltage winding and a current Winding, a local source of alternating current, connections for causing said alternating current to traverse said resistance, connections from the voltage winding across said resistance, and connections having a serial relation to said resistance for energizing the current winding in proportion to the current traversing said resistance, said instrument connections each including a rectifier and electrical isolating units for substantially eliminating direct load current from said windmgs.

7. In an electrical system including an alter nating-current circuit traversed by a load current and including an impedance of magnitude which is subject to variation, apparatus responsive to the magnitude of said impedance, said apparatus comprising in combination with such impedance a ratio responsive instrument including a voltage winding and a current winding, connections from the voltage Winding across said impedance, and connections having a serial relation to said impedance for energizing the current winding in proportion to the current traversing said impedance, a local source of direct measuring current and connections for causing said direct measuring current to traverse said impedance, each of said instrument connections including an isolating unit for minimizing the efiect of alternating load current in said instrument.

8, An electric arc furnace system comprising in combination a supply circuit, an arc furnace including an electrode, and an electrode holder connected in the supply circuit, and a protective arrangement responsive to predetermined changes in contact conditions between said electrode and said electrode holder, said protective arrangement comprising in combination with such an electrode and its holder a voltage-current ratio responsive instrument including a voltage winding with a pair of input terminals and a current winding, connections from one of said voltage winding terminals to said electrode and from the other of said terminals to said electrode holder, and connections having a serial relation to said electrode holder supply circuit for energizing the current winding in proportion to the current traversing said circuit.

9. In an electrical system including conductors in contact with connections for causing passage of alternating current, a protective arrangement responsive to predetermined changes in contact conditions, said arrangement comprising in combination with such contact an instrument responsive to ratio of voltage and current including a voltage winding and a current winding, connections including a transformer operatively connecting the voltage Winding across the said contact, connections including.a rectifier having a serial relation to said contact and said altemating-current connections for energizing the current winding in proportion to the alternating current traversing said contact, and a second rectifier interposed between said transformer and said voltage winding.

10. An electrical system comprising in combination a current-carrying circuit and a protective arrangement therefor, said circuit including a pair of conductors in contact and a useful load supplied by said circuit, said protective arrangement comprising in combination with such contact and connections for causing passage of load current through such contact, a device responsive to ratio of voltage to current having a movable element, said ratio responsive device including a voltage winding and a current winding, connections bridging the voltage winding across said 14 contact, and connections having a serial relation to said contact for energizing the current winding in proportion to the current traversing said circuit.

RAPHAEL SCHEIRMANN.

REFERENCES CITED 'Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 292,761 Olmsted Jan, 29, 1884 528,268 Armen Oct, 30, 1894 1,089,814 Beach et a1 Mar. 10, 1914 1,214,763 Dixon Feb. 6, 1917 1,615,648 Pierce Jan. 25, 1927 1,697,188 Keller et al. Jan. 1, 1929 1,779,347 Vawter Oct. 21, 1930 1,814,869 Traver July 14, 1931 1,825,476 Gaarz et al. Sept. 22, 1931 1,839,148 Green Dec. 29, 1931 1,919,079 St. Clair et al. July 18, 1933 1,923,565 Austin Aug. 22, 1933 1,931,862 Felton Oct. 24, 1933 2,044,546 Ryan June 16, 1936 2,057,845 Pattee Oct. 20, 1936 2,125,050 Josephs Jul 26, 1938 2,149,756 Aremberg et al. Mar. 7, 1939 2,218,629 Swart Oct. 22, 1940 2,221,556 Roemisch Nov. 12, 1940 2,232,715 Matthews Feb, 25, 1941 2,261,686 Kesselring Nov. 4, 1941 2,307,499 Frakes Jan. 5, 1943 2,316,170 Kesselring Apr. 13, 1943 2,324,458 Peters et a1 July 13, 1943 FOREIGN PATENTS Number Country Date 426,238 Great Britain Mar. 29, 1935 OTHER REFERENCES Laws, Electrical Measurements, first edition, 1917, pages 94-97 inclusive, Q 535 L3.

Radio World, Dec. 1936, pages 12 and 13.

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