US2579042A - Differential relay sensitivity adjuster - Google Patents

Description

Dec. 18, 1951 c. L .HEADLEY 2,579,042

DIFFERENTIAL RELAY SENSITIVITY ADJUSTER Filed July 14, 1950 2 SHEETS-SHEET l l IO ATTORNEYS Dec. 18, 1951 Q L, HEADLEY l 2,579,042

DIFFERENTIAL RELAY SENSITIVITY ADJUSTER 2 SHEETS-SHEET 2 Filed July 14, 1950 l /Nl/ENTOE Chesi'er' L. Head/ey M w m, M

Patented Dec. 18, 1951 DIFFERENTIAL RELAY SENSITIVITY ADJUSTER Chester L. Headley, Baltimore, Md., assignor to Consolidated Gas Electric Light and Power Company of Baltimore, Baltimore, Md., a corporation of Maryland Application July 14, 1950, Serial No. 173,847

24 Claims.

This invention relates to means for automatically changing the sensitivity of a differential protective relay of the type having restraint windings symmetrically connected with respect to an operating winding. Such relays are used to protect electrical apparatus such as power transformers and the invention will be described hereinafter with reference to the use where energy may flow to the transformer from either side. It is to be understood that relays embodying the invention can be used in any transformer protective scheme.

It has been observed that when a transformer is connected to a power supply there may be an inrush of current of great magnitude that lasts for a fraction of a second and then subsides to the normal load value. When the sensitivity adjustment of differential relay protective means associated with the transformer is suitable to protect the transformer under normal operating conditions, the relay may respond to the inrush current with the result that a false tripping occurs, disconnecting the transformer from the supply circuit. One method of overcoming this difficulty is to increase the current setting at which the differential relay operates to such a 'value that inrush current will not cause tripping, but the setting is then excessive for overload currents to be expected, and hence some protection is sacrificed. Accordingly it has been proposed to provide means for automatically decreasing the sensitivity of the differential relay for the short time during which the inrush current flows and then restoring normal sensitivity to the relay for subsequent operation.

One of the objects of the present invention is to provide improved means for automatically reducing the sensitivity of the differential relay upon energization of the transformer and for a short time immediately thereafter during which magnetizing inrush current may flow.

Another object is to provide improved means of the above type that are applied symmetricall with respect to the relay and that produce uniform desensitization regardless of which side of the transformer is first energized.

Another object is to provide improved means of the type characterized in the preceding objects that can be applied readily to an existing differential relay and does not have to be built into the relayA at the time of its manufacture.

A further object is to provide improved means as characterized above and s-uch that faults occurring during the inrush period when the relay is desensitized can still cause the relay to operate.

A still further object is to provide improved means as characterized above that are simple in construction and operation, relatively inexpensive, and yet rugged and efficient in service.

Several embodiments of the invention are illustrated schematically in the accompanying drawings, but it is to be expressly understood that said drawings are for purposes of illustration only and are not to be construed as a definition of the limits of the invention, reference being had to the appended claims for this purpose.

In the drawings,

Fig. l illustrates a single-phase arrangement embodying the invention;

Fig. 2 shows another single-phase arrangement embodying the invention;

Fig. 3 shows a further single-phase arrangement utilizing linear impedance in the desensitizing circuits;

Fig. 4 illustrates a three-phase application of the invention; and

Fig. 5 illustrates another three-phase system embodying the invention.

In Fig. l the transformer to be protected is indicated at l0 and is provided with windings Il and l2 which may supply energy in either direction between two supply circuits l and la. Circuit breakers l 3 and 13a control the energization of transformer I0. Current transformers i4, l5 are associated with the circuits connected to windings I2 and i I, respectively, and have secondary windings I6, I1, respectively, that are connected in series by wire I8; the other terminals of these windings being indicated at i9 and 20, respectively. The differential relay for the protection of transformer I0 against abnormal conditions is shown schematically within the rectangle 2! and includes restraining windings 22, 23 which oppose closing of suitable relay contacts 24. The restraining windings 22, 23 are connected in series by a wire 25. The terminal 26 of restraining winding 22 is connected to the terminal 20 of current transformer l5 by wires 26a; and the terminal 21 of restraining winding 23 is connected to the terminal I9 of current transformer I4 by wires 21a. The differential relay 2l includes an operating winding 2B, one terminal oi which is connected to th-e wire 25 (which is common to restraining windings 22, 23) and the other terminal of which is connected to the wire i8 (which is common to current transformer windings it, il). The diierential relay contacts 24 are normally open, but are closed when the eiiect of current llow through the operating winding 23 overbalances the effect of the current owing in' restraining windings 22, 23. Upon closure of the contacts 24 circuit breakers I3 and i3d are operated to disconnect transf former I from the system as described below.

Circuit breakers iii and Ia Ymay be operated by control power separate from the supply circuits I,l Ia, being for example connected by conductors til, 3l and 30a, Sla to thesame` source of control power which may, in many cases, be a direct current source, The conductors 35, 35a are connected to corresponding circuit breaker terminals S2, Sa and the conductors 3i, 31a are connected to corresponding circuit breaker terminals 33, 33a: by wires 34, 54a. Any suitable circuit closing means, as push buttons 35 and 35a, may be used in series with wires 34 and 34a. When moved to closed position button 35 energiz'es terminals S2, 33 of circuit breaker I3 to eiect closure thereof and when push button 35a is closed terminals 32a and 33a of circuit breaker I3a are energized to effect closure of breaker' lia. Tripping of circuit breaker I3 by diierential relay 2I is accomplished upon energizing a circuit which includes Wire 5i?, terminal 32 and a terminal 35, of circuit breaker I3, wires 3l', differential Vrelay contacts 24, and wires 38, 3|. Tripping of circuit breaker Ia by the differential relay takes place upon energizing a similar circuit 30a, 32a, 36a, Sla, relay contacts 24, and wires 'et and 3l. Relay contacts 24 close under fault conditions existing between the current transformers I4 andk I5 that result in net current through the operating winding E8 suicient Vto operate the differential relay 2l. Push buttons 39 and 39a may be provided as indicated in Fig. 1 to permit manual tripping of circuit breakers I3 and Ita to open position. Y

The apparatus thus far described comprises I; as is relay winding 5U.

4 former 55 is energized when the transformer I0 is energized.

To illustrate the operation of the present invention as shown in Fig. 1, it is assumed that circuit breakers i3 and Ita are open, and transformer l) is therefore deenergized. Under this condition, diiierential relay contacts 24 are open and contacts 48 of sensitivity adjuster 4t are closed. Potential transformer 55 is deenergized, In orderito .energize transformer It, push button 35 or 35ans closed manually to effect closure of circuit breaker I3 or I3a. Upon closure of either circuit breaker Y potential transformer 55 is energized thereby en- 'I ergizing relay winding 5G to cause the then closed f large magnitude may be produced in current transformer secondary windings I5 orV Il. For purposes or" illustration, assume circuit breaker I3 to be closed before'breaker i3d, then current will flow in part through the circuitY including terminal I9, wires 27a and 44a, terminal 44, the portion oi winding l2 to center tap 45, resistor 4l, contacts 48, wires 49 and I8, to the other side of secondary winding I5. Another portion of the current iiows in the circuit which includes terf minal I5, wire 2id, restraining windings 23,y 2 2,

wires 25a and 43a, terminal 43, the portion of winding 42 to center tap 45, 'resistor'il contacts 48, wires 49 and IS, to the otherA side of transformer Winding I6'. These paths will carry a large portion of the currentproduced in ywinding It for the reason that restraining windings 22 23 are of relatively low impedance as compared l to operating winding 28, and also because of the low impedance of reactor Winding 42vbetween terdiierential relay protection means for transiormers which are in common use and to which the present invention can be applied in the foln lowing manner. As indicated above, the present invention provides means for decreasing the sensitivity of differential relay 2l for a shortv period following closure of either circuit breaker I3 or l3nt. These means are shown schematically in Fig. A1 within the rectangle il and include a reactor 4! and a reactor winding i2 having terminals 43, 44 and a center tap terminal 45. Terminal 43 is connected to terminal 25 of relay 2l by Wires 43a and 26a; and terminal 44 is connected to terminal 2i of relay 2i by wires 44a and 2id. Center tap Q5 is connected to wire I8 by a circuit which includes a resistor 4l, normally closed relay contacts 48, and, a wire 4S, said contacts 43 being associated with a relay winding 5@ and this relaybei'ng of any suitable type having delayed operation so as to cause contacts dil to open after a predetermined time interval following energization of electro-magnet winding 55. The terminals 5I, 52 of the relay winding 55 are connected by wires 53 to the secondary winding 54; of a potential transformer 55, the primary winding 5% of which is connected by wiresv 5'! to the transformer i0 so that the transminals 43, 44. The low vreactor impedance results from the low of current in opposite directions through the two portions 43,45 and 44, 45 of reactor winding 42, the magnetizing effect of the current in portion 43, 45, largely neutralizing the magnetizing eiect of current in the portion 44, 45, thereby neutralizing the reactance of winding 42. The eiect of the current flow through restraining coils' 22, 23 is to raise the setting at which differential relay contacts 24 close. There 1s another current path which includes terminal is, wire 27a, restraining coil 23, wire 25, operating coil 2S, and Wire I3 to the other side of current transformer secondary I6. In this circuit the current ilow through operating winding 23 lowers the setting of differential relay 2 I, but the magnitude of this current is relatively small due to the high impedance of operating coil 28 in comparison with the impedance of the circuit including restraining coil 22, reactor winding 43,

45, and resistor 4l.

ings flowing under normal operating conditions. Hence, normal sensitivity is restored when contacts 48 open.

- If a fault of suflicient magnitude develops during the short period that contacts 48 of sensitivity adjuster 40 are closed, differential relay 2| will operate and clear transformer |0 from the system by opening circuit breakers I3 and I 3a. However, if the fault is not severe enough to cause the relay to operate (at reduced sensitivity), the transformer will be cleared at the moment relay contacts 48 open and normal sensitivity is restored.

It will be understood that the relay 48, 50 of Fig. 1, in which the contacts 48 are normally closed but are opened and thereafter held open by the relay winding 50, can be replaced by a relay of known type having normally open contacts which are closed immediately upon energization of the relay winding but open again .f

after a predetermined time period. In this case the relay winding need not be continuously energized, and hence can be operated if desired by the control power used to operate the circuit breaker |3 (or 13a) as above described. Fig. 2 shows an apparatus embodying these modications. The differential relay 2| and the sensitivity adjuster 40 are the same as those shown in Fig. 1 and the parts are correspondingly numbered except for the relay contacts 53 and the relay winding 59 which replace the relay 4B, 50 of Fig. 1 and are of the alternative type mentioned above.

The relay winding 59 is energized by control power immediately upon closing either push button or 35a to close the circuit breaker |3 or |3a. To this end two relays 50 and 69a are provided having operating windings 6| and Gla and associated relay contacts 62 and 52a. One end of each of the operating windings 6I and E Ia is connected by a wire 63 or 63a with the control power supply line 30 or 30a, and the other end of each operating winding is connected by a wire 64 or 64a to the terminal 33 or 33a of the circuit breaker I3 or I 3a. One contact of each relay is connected by a wire 65 to the control power supply line 3|, and the other relay contacts are connected by wires 66 and 66a to a common wire 61 leading to one terminal 68 of the relay winding 59. The other terminal 69 of breaker I3 energizes winding 6| of relay 60 by a circuit which includes wires 3|, 34, push button 35, wire 64, winding -6I and wire 63 to wire 35. Energizing of relay winding BI causes contacts 52 to close and energize time-delay relay winding 59 through a circuit including wires 3| and 65, relay contacts 52, wires 66 and 61, terminal 58, relay winding 59, terminal 69 and wires 'Ill and 63 to wire 3|). Closing of push button 35a causes operation of relay 68a and energization of winding 59 in a similar manner. closes immediately upon energization and then opens relay contacts 58 after a time delay. The operation of the sensitivity adjuster 49 is the same as described in connection with Fig. l.

In the arrangements indicated in Figs. l and '5;'

2, non-linear impedances 43, 45 and 44, 45 are arranged symmetrically with respect to relay terminals 26, 2l, but where desired these nonlinear impedances may be replaced by linear impedances as illustrated in Fig. 3 in which like Relay 58, 59 t reference numerals are employed tok designate elements previously described. Instead of the reactor winding 43, of Fig. l, a linear impedance 'II is connected between restraint winding terminal 25 and the terminal of operating winding 28 by wires 26a and 43a, contacts 48 and wire 49. Similarly, a linear impedance I2 is connected between restraint winding terminal 2l and the terminal of operating winding 28 by wires 27a and 44a, contacts 48 and wire 49. Relay 48-50 opens both contact sets 48 after a suitable time delay as described in connection with Fig. 1 and thereupon impedances 1I, 12 are disconnected from the differential relay circuit to restore normal sensitivity after the initial transient is over. Any suitable values of the real and imaginary components of impedances 1|, 12 may be used within the purview of the present invention.

For purposes of illustration, linear impedances I I, I2 have been illustrated in Fig. 3 as applied to the arrangement shown in Fig. l. These impedances may be applied equally well to other circuit arrangements such as that shown in Fig. 2, it only being necessary to include a pair of contacts on the associated time-delay relay in series with each impedance in order to prevent current flow through said impedance at times where normal sensitivity of the differential relay is required.

In polyphase application of the circuit arrangements of Figs. 1 to 3, inclusive, relay 48, or 58, 59 may include a single electromagnet winding 50 or 59 and separate sets of contacts 48 or 58 for each of the phases. Thus, for a threephase circuit, current transformer secondary windings I6, II in a given phase may be isolated from corresponding windings in the other phases and relay 48, 50 or 58, 59 may include three sets of contacts and a single electromagnet winding. A similar relay arrangement may be used where the transformer windings ISA, IBB and IEC (the letters relate the circuit elements, previously described, to their respective phases) are Y connected and the windings HA, I'IB and I'IC are likewise Y connected, the neutral point of the Ysbeing connected together. This arrangement is illustrated in Fig. 4, which for simplification includes only the circuit arrangements on the secondary sides I5, |'I of the current transformers I4, I5 in the three phases. One terminal of each of the transformer secondaries IGA, IBB and ISC are Y connected by a wire 'I3 and one terminal of each of the transformer secondaries I'IA, I'IB and I'IC are Y connected by a wire 14, the wires 'I3 and 14 being connected together and grounded at I5. The other two terminals of each pair of transformer secondaries ISA and I'IA, ISB and ITB, and IBC and I'IC are connected to the restraining coil terminals of a diierential relay and to the sensitivity adjuster corresponding to the differential relay 2| and sensitivity adjuster 40 of Figs. 1 and 2, the differential relay contacts 24 being omitted for the sake of simplifying the illustration. The terminals of the operating coils 28A, 23B and 28C of the diierential relays are connected together by a wire l5, and this wire may also be connected to one of the contacts of three contact sets TIA, l'iB and IIC of a relay having a single electromagnet winding 18. This relay may be of either type shown in Figs. 1-3. The other contacts of the sets 11A, TIB, and 'IIC are connected through resistors 41A, 41B and 41C (compare Figs. 1 and 2) to the midpoints 45A, 45B and 45C of the three sensitivity adjuster reactor windings. Y In effect, when the vrelay contacts are closed, the terminals of the three operating coils 28A, 28B and'28C and the three circuits from thecenter taps 45A, 45B and 45C are all connected to a common point. However, the relay construction may be simplified in the manner illustrated in Fig. 5, if desired. Y

Fig. 5 illustrates a three-phase arrangement in which the secondary windings liA, IBB, IBC are connected in delta by wires 19, as are the secondary windings IIA, IIB, HC by wires B. A-diierential relay 2l and sensitivity adjuster 40 is connected between-each pair of transformer secondaries ISA and HA, IBB and ITB and IBC and IIC, and the terminals of the operating coils 28A, 28B and 28C are connected together by a wire 8l. The center taps 45A, 45B and 45C of the recator windings are also connected together during the period of desensitization of the differential relay by suitable relay means which may be of either type shown in Figs. l-3. However, the relay construction can be simplied as compared with Fig. 4 by omitting the connection of the operating coils 28A, 28B and 28C to the relay contacts (i. e., by omitting the connection thereto of the wire i6 of Fig. 4). In this case only two sets of relay contacts are required. As shown in Fig. 5, the relay comprises a single electromagnet winding 82 and two pairs of contacts 83 and 84. The center tap 55A and resistor 41A are connected in series and to one contact 83, and the center tap 45C and resistor 41C are connected in series and to one contact 84. VThe center tap'45B and resistor 41B are then connected in series and to both of the other contacts 83 and 84 so that whenthe relay is closed, all three center taps are connected to a common point.

As an example of the operation of the present invention, the following test results are cited. f

The circuit arrangements of Fig. 5 were` used in a test on a 40,000 kva., 33e/110 kv. three-phase transformer. This transformer was protected by a General Electric type IJD differential relay.

The normal sensitivity adjustment of the relay was Such that operation Vwould occur at approximately 400 amperes at 33 kv. The sensitivity adjuster was set to raise the adjustment of the differential relay during the magnetizing inrush current period by the use of a suitable resistance value for resistors 47A, 47B, 41C. The differential relay used with the sensitivity adjuster introduced a time delay of approximately 0.1 second before it closed its contacts. In a series of tests that were made with the sensitivity adjuster connected, inrush currents between 1140 and 120) amperes were measured on specific phases but operation of the diierential relay was prevented by the action of the sensitivity adjuster. When the sensitivity adjuster was made inoperative as by holding the circuit open at contacts 83, 84, inrush currents in the range of 1160 to 1200 amperes were measured on specific phases and in each case the differential relay connected to these phases closed its contacts to trip the transformer from the system.

It will beV seen that the invention provides a simple and relatively inexpensive means which effectively desensitizes the differential relay and prevents itsv operation during the period when magnetizing inrush currents may flow.' Sensitivity adjusters embodying the invention can be built into differential relays at the time of manufacture, if desired, butfone of their advantages resides in their easy application tc any existing CFI differential relay of the type having restraint windings symmetrically arranged with respect to an operating winding. Hence such sensitivity adjusters can be'built as separate units for installation and use wherever desired. The extent of desensitization can be adjusted simply by varying the constants of the adjuster circuits. Because of the symmetrical arrangementof the sensitivity adjuster, moreover, uniform desensitization is obtained regardless of which side of the transformer is rst energized. On the other hand, even during the period Iof desensitization the diiierential relay will be operated by a fault ci sufficient magnitude occurring on either side of the transformer.

It is tc be understood that the embodiments of the invention described above and illustrated in the drawings are by way of example only and that the invention is not restricted thereto as various changes can be made in the form, details and arrangement of the parts without departing from its spirit. Reference therefore should be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. In a diierential protective arrangement for an electrical apparatus, current transformers responsve respectively to current entering and leaving said apparatus, a differential protective means connected to said transformers and comprising relay means operable to interrupt the iiow of current through said apparatus and relay operating means including series-connected restraint windings and an operating winding connected to a point between said restraint windings, and rsensitivity adjusting means comprising a circuit in shunt with said restraint windings having an intermediate Apoint connected to said transfori-ners, said circuit diverting current from said operating winding to decrease the sensitivity of said differential protective means, and'time-delayed relay means Connected for energization simultaneously with said apparatus for interrupting said circuit after a predetermined time interval and restoring normal sensitivity to said dierential protective means. t

2. A differential protective arrangement as dened in claim l, said circuit including impedances in shunt with said restraint windings on both sides of said intermediate point, the impedance on each side or said intermediate point being small as compared with the impedance of said operating windlng. Y I

3. A differential protective arrangement as dened in claim 2, said impedances being linear and said relay means interrupting said circuit between said impedances.

4. A diierential protective arrangement as dened in claim 2, said impedances being non-linear and said relay means interrupting said circuit between said intermediate point and said transformers.

5. A differential protective arrangement as defined in claim l, said time-delayed relay means being connected to said apparatus for energiza- 9 4 operating means for opening said contacts after a predetermined time interval. y

1V8. A diierential protective arrangement as dened in claim l, said time-delayed relay means comprising normally open contacts, operating means for closing said contacts upon energization thereof, and means for re-opening said contacts after a predetermined time interval.

9. In a differential protective arrangement for an electrical apparatus, current transformers responsive respectively to current entering and leaving said apparatus, a differential protective means connected to said transformers and comprising relay means for interrupting the flow of current through said apparatus and relay operating` means including series-connected restraint windings and an operating winding connected to a point between said restraint windings, and sensitivity adjusting means comprising impedance means in shunt with said restraint windings and a connection from an intermediate point of said impedance means to said transformers, said impedance means having a relatively high impedance as compared With said operating winding under normal operating conditions and diverting current from said operating winding to decrease the sensitivity of said differential protective means during the initial period of energization of said apparatus, and means including a timedelayed relay connected for energization simultaneously with said apparatus and adapted to interrupt the now of current in said connection after a predetermined time interval thereby restoring normal sensitivity to said differential protective means.

10. In a differential protective arrangement for an electrical apparatus, current transformers responsive respectively to current entering and leaving said apparatus, a dilferential protective means connecte-d to said transformers and comprising relay means operable to interrupt the flow of current through said apparatus and relay operating means including series-connected restraint windings and an operating winding connected to a point between said restraint windings, and sensitivity adjusting means comprising a circuit connected in shunt with said restraint windings and containing series-connected impedances, a connection from a point intermediate said impedances to said transformers, and time-delayed relay means connected for energization simultaneously with said apparatus for interrupting said connection after a predetermined time interval.

11. A differential protective system as dened in claim 10, said impedances being linear and said time-delayed relay means interrupting also the series connection between said impedances.

l2. A differential protective system as defined in claim 10, said impedances being non-linear and arranged so that they cumulatively oppose flow of current in series therethrough when said connection is interrupted.

13. A dierential protective system as defined in claim l2, said impedances comprising windings on a common core of magnetic material.

14. A diierential protective arrangement for electrical transformers and like electrical apparatus comprising current transformers respectively responsive to current entering and leaving the apparatus to be protected, a differential protective relay comprising restraint windings connected in series with said transformers and an operating winding connected between a point intermediate said transformers and a point intermediate said restraint windings, a circuit in tial protective relay of shunt with said restraint windings and containing series-connected impedances, a connection from a point intermediate said impedances to a point intermediate said transformers, and timedelayed relay means connected for energization simultaneously with the apparatus to be protected for interrupting said connection after a predetermined time interval, said shunt circuit diverting current from said operating winding and decreasing the sensitivity of said diierential relay until said connection is interrupted.

15. A dinerential protective arrangement as defined in claim 14, said time-delayed relay means having two operating coils connected with opposite sides of said apparatus to be protected.

16. A differential protective arrangement for three-phase electrical transformers and like apparatus comprising three pairs of current transformers, the transforrn'rs of each pair being responsive respectively to urrent entering and leaving the apparatus to be protected in a single phase, the three transformers responsive to entering current being connected together in threephase relation and the three transformers responsive to leaving current being connected together in three-phase relation, diierential protective relays one connected between the transformers of each pair and comprising series-connected restraint windings and an operating winding having one end connected to a point between said restraint windings, the other ends of the three operating windings being connected together, each of said differential relays also including sensitivity adjusting means comprising a circuit in shunt with said restraint windings and containing impedances and a conductor connected to a point intermediate said impedances, and timedelayed relay means connected for simultaneous energization with said apparatus to be protected for connecting said three conductors together for a predetermined time interval following energization of said apparatus to be protected and thereafter disconnecting said conductors.

17. Sensitivity adjusting means for a differenthe type having seriesconnected restraint windings and an operating winding having one end connected to a point between said restraint windings, said sensitivity adjusting means comprising a circuit adapted to be connected in shunt with said restraint Windings and containing series-connected impedances, a circuit connected to said rst circuit at a point intermediate said impedances and adapted to be connected to the other end of said operating winding, and time-delayed relay means for interrupting said second circuit.

18. Sensitivity adjusting means as deiined in claim 17, said impedances being linear and said time-delayed relay means interrupting also the series connection between said impedances.

19. Sensitivity adjusting means as defined in claim 17, said impedances being non-linear and arranged sc that they cumulatively oppose flow of current in series therethrough when said second circuit is interrupted.

20. Sensitivity adjusting means as defined in claim 19, said impedances comprising windings on a common core of magnetic material.

2l. A dierential protective arrangement as defined in claim l, including a resistance in the connection between said intermediate point and said transformers, the value of said resistance determining the amount of desensitization of said differential protective means.

22. A differential protective arrangement as 11 dened in claim 9, includingl a resistance in said connection for controlling the amount of desensitization of said diierential protective means.

23. A diierentiai protective arrangement as dened in claim 14, said connection including a series resistance adjustable to vary the amount of current diverted from said operating winding and thereby to control the degree of desensitzation of said differential protective relay.

24. Sensitivity adjusting means as defined in claim 17, said second circuit including a resistance the value of Which determines the amount of desensitization of a differential relay to which said sensitivity adjusting means is connected.

CHESTER L. HEADLEY.

i2 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Bateman Sept. 27, 1949

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