US2736012A

US2736012A - Electric fire alarm system

Info

Publication number: US2736012A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

Feb. 21, 1956 T. w. BLAND ET AL ELECTRIC FIRE ALARM SYSTEM 2 Sheets-Sheet l Filed Feb. 6, 1955 INI/EN TORS B4 Awa, 4. 0 use/5M THOMAS z. 41s Tse Wam/@Wwe @man Feb. 21, 1956 T. w. BLAND ET AL 2,736,012

ELECTRIC FIRE ALARM SYSTEM Filed Feb. 6, 1953 2 Sheets-Sheet 2 I N V EN TORJ` THOMAS w. 31m/v0. 535; 715e 4. o 'ae/544 United States Patent O 2,736,012 ELECTRIC FRE ALARM SYSTEM Thomas W. Bland and Lester A. OBrien, San Jose, Calif., assgnors to Fire Spotter Electric Alarm Co., San jose, Calif., a firm Application February 6, 1953, Serial No. 335,446 4 Claims. (Cl. 34a- 227) This invention relates to electric lire alarm systems, and more particularly to a re alarm system having supervisory testing means whereby any defect in the system can be detected, and which is adapted to respond to conditions, such as earthquakes or detonatlons, other than re.

It is among the objects of the present invention to provide an improved electric tire alarm system which can be easily installed in a structure, such as a building or ship, and arranged to indicate the presence of tire in any part of the building by a suitable alarm signal; which utilizes alternating current, such as the commercial electric current supplied to buildings by public utility companies, and an auxiliary source of power, such as a storage battery, and means for automatically switching the system to the auxiliary power source in the event of failure of the normal power supply, and returning the system to the normal power supply when the normal supply is restored; which includes means for testing the circuits or the system for detects without setting oil the alarm; which includes means for indicating the operating condition in which the system is disconnected from the normal power source and connected to the auxiliary power source; which may include alarm means responsive to conditions other than tire and may provide distinctive alarms for different conditions; and which is simple and durable in construction, economical to install, reliable in operation, and easy to test and service.

Other objects and advantages will become apparent from a consideration of the following description and the appended claims in conjunction with the accompanying drawings, wherein:

Figure 1 is a wiring diagram of an alarm system illustrative of the invention;

Figure 2 is a longitudinal, medial cross sectional View of a temperature responsive element constituting an operative component of the system;

Figure 3 is a transverse cross sectional View on the line 3 3 `of Figure 2;

Figure 4 is a top plan View of a shock responsive unit constituting an operative component of the system; and

Figure 5 is a cross sectional view on the line S-5 of Figure 4.

With continued reference to the drawings and particularly to Figure 1, the numeral l@ generally indicates a stepdown transformer for alternating current having a primary winding 1l connected at its opposite ends to power wires 12 and 13 of a house wiring system or similar alternating current supply, and the numeral 14 indicates the secondary winding of the transformer 1t). The numeral 15 generally indicates a multiple terminal board, the numeral 16 generally indicates an auxiliary power source, such as a storage battery, the numeral 17 generally indicates the protective circuit for the lire warning means and the numeral 18 generally indicates the tire alarm signal device which is put into operation when the protective circuit is broken.

The protective circuit includes a plurality of heat responsive elements, as generally indicated at 20 and 21 in Figure l and illustrated in detail in Figures 2 and 3, connected in series with each other by a conductor wire 22.

A high resistance, electrically conductive element 32 is disposed within cuit is maintained through the protective circuit regardless of the opening of the temperature controlled switches as long as the conductor wire 22 remains intact.

Designated at 3S is a relay which controls the substitution of a battery 16 for the transformer power supply responsive to the failure of said transformer power supply.

Designated at 65 is a relay which operates in response to the opening of the thermostatic switches in the detector units Ztl, 21 to energize a signal relay 68, which in turn controls a signal drop relay 105.

Designated at 125 is a supervisory relay which operates in response to a break in the detector circuit 17 to energize a trouble signal 153.

Designated at is a relay which to the opening of the thermostatic tector circuit to energize signal relay 68 and signal drop relay tlS when the battery 16 has been substituted for the transformer power supply by a relay 38.

The relay 38 comprises a winding 39 and three poles 4l?, 41 and 42. When the winding is energized the pole 4l) 1s held in conductive engagement with relay switch contacts 43 and 44, the pole 41 with contacts 45 and 45 and the pole 42 with contacts 47 and 48. When the winding 3i? is de-energized the poles 4i), 41 and 42 are simultaneously moved by suitable resilient means to move the pole 4Q away from the contacts 43 and 44 and into electrically conductive engagement with contacts 50 and 5l, to move the pole 41 away from contacts 45 and 46 and into engagement with contacts 52 and 53 and to move the pole 42 away from contacts 47 and 48 and into engagement with contacts 54 and 55,

A conductor 56 leads from one end of the secondary winding 14 of the transformer 10 through Wire 91 to the operates in response switches in the deend of the housing series connected temperature' contact 47, said conductor 56 also being connected to one side of the winding 35. A conductor 5.8 leads from the other end of the transformer winding 14 and a conductor 59 connects the conductor 58 to the other side of the winding 39 so that this winding is energized as longV as the transformer 10 is in operation.

One end of the conductor wire 22 ofthe protective circuit 17 is connected to terminal 60 of the terminal board 1,5Y and this terminal 60 is connected by a conductor 61 to the conductor S extending from one end of thetransformer secondary winding 14. The other end of the wire 22 isconnected to terminal 62 and a conductor 63 leads from terminal 62 to one side of thewinding 64 ofthe electromagnetic drop relay 65. A conductor 66 leads from the other side of thewinding 64 to one terminal 67 of the single pole double-throw electromagnetic rela ,generally indicated at 68having a winding 70 and a pole 7i vin electrically conductive engagement with the Contact 67 and a secondA contact 72 while the winding 70 is deenergized, as it is normally when the winding 64 of relay 65 is energized. A conductor 73` leads from the contact 72 to a contact 74 of the electromagnetic relay switch, generally indicated at 7,5, having a winding 76 and a sfingle pole 77 closed simultaneously withk the contact 74 and a contact 78 when the winding 76 is de-energized as it normally is. The winding 76 is deenergized as longV as winding 39 is energized by operation of the transformer 10.

VA conductor 80 leads from the contact 7Std a conductor 90 which leads to the contact 48 of the relay 38. Eole 42 of relay 38 connects contact 48 to contact 47 of this relay vand contact 47 is connected by a conductor 91 to conductor 56 leading tothe end of secondary winding 14` opposite the end of this winding from which conductor 58v extends.

This completes an energizing circuit for drop relay 65 asV long as transformer is in operation and protective circuit 17 remains closed.

One side of the main signal device 18 is connected by a conductor 94 to the terminal 95 and this terminal is connected by a conductor 96 and conductor 61 to the conductor 5S so that one side of the signal device is directly connected to `the secondary transformer coil 14. The other side of the main signal device is connected by a conductor 97 to the terminal 98 and the terminal 98 is connected by a conductor 99 to one contact 100 of a single pole double-throw manual switch 101. The pole of switch 101 is connected by a conductor 102 to one contact 103 of the electromagnetic drop relay, generally indicated at 105, and, when the pole of switch 101 is in position' for operation of the signal 1S, the terminal 98 is connected to relay drop contact 103.

The drop relay 105 is a two pole single-throw relay having avwinding 106 and poles 107 and 108. The winding 106r is connected at one side to the conductor 58 by a conductor 109 and is connected at itsother side by condoctors 110 and 111 to .a contact 11,2 of the electromagnetic relay 68 and a contact 113 of the electromagnetic relay 75. lThe pole 108 of relay 105 connects contact 10,3 of this relay to a contact 114 of the same relay when the winding 106 is energized and contact 114 is connected by

conductors

115 and 116 to conductor 90 leading through relay switch 38 to the other end of transformer winding 14 thus completing an energizing circuit forv signal device 18 when winding106 is energized and the pole of switch 101 engages contact 100.

Winding 106 cannot, however, be energized as long as protective circuit 17 is closed as the protectivecircuit maintains winding 64 energized, as explained above, and this winding, when energized, maintains pole 118 of relay 65 out of contact with contacts 119 and 12o of the same relay Athus maintaining winding 70 Cre-energized and pole 71 out of engagement with contact 112. The winding 76 of relay 75 is maintained dei-energized while the system is operating on transformer current maintaining the pole 77 out of engagement with the contact 113.

It will be noted that one side of the winding 70 of the relay 68 is connected by a conductor 121 to line 58 and that a conductor 122 leads from the other side of winding 70 to a contact 123 of the electromagnetic supervisory relay, generally indicated at 125, the winding 126 of which is normally energized, as will be later explained, so that one pole 127 of this relay connects the contact 123 to the complementary contact 128. A conductor 129 leads rom contact 128 of relay 125 to contact 43 of relay 38 and contact 43 is connected by pole 40 to contact 44 of relay 38, contact 44 being connected by conductor 130 to contact of relay 65. Contact 120 is connected by pole 118 to contact 119 of relay 65 only when winding 64 is de-energized by opening of the protective circuit 17 and contact 119 is connected by a conductor 131 to line 98 leading through pole 42 to transformer lead 56.

With this arrangement, if the protective circuit 17 is opened by opening of one or more of the temperatureresponsive switches therein, the current inthe energizing circuit for winding 64 will be reduced and pole 118 will close the

contacts

119 and 120 of relay 65. This will establish an energizing circuit for winding 70 of relay 68 moving the pole 71 o this relay into engagementtwith contacts 112 and 132, and as the contact 132 is connected by

conductors

115 and 116 to line 90, this will connect conductor 110 to line 90 and establish an energizing circuit for the winding 106 of relay drop 105. When winding 106 is energized it moves pole 108 into engagement with

contacts

103 and 114 of relay 105 thus completing an energizing circuit for the main signal device 18.

When winding 106 is energized it also moves pole 107 of relay 105 into engagement with

contacts

134 and 135 to establish the following energizing circuit for an auxiliary signal device, generally indicated at 136.

Contact-134 is connected through conductor 116 to line 90 and contact 135 is connected through conductor 137 to the pole of single pole double-throw, manually operated switch 138. Switch 138 has a contact 139 connected by a conductor 140 to terminal 141 and this terminal is connected by a conductor 142 with one side ofthe auxiliary signal device 136. The other side of the auxiliary signal device is connected by a conductor 143 with terminal 144 and this terminal is connected by

conductors

96 and 61 to line 58.

A buzzer, generally indicated at 145, is connected at one side to line 58 by a conductor 146 and at its other side by conductor 147 to alternate Contact 148 of switch 101 and alternate contact 149 of switch 138. When the pole ofk switch 181 is connected to contact 148 instead of contact 100 and pole 108 of relay drop 105'closes with

contactsy

114 and 103 the buzzer 145 will be energized instead ofthe signal device 18 and similarly, lwhen the pole of switch 138 is closed with contact 149 instead of contact 139 andV pole 107 of relay 105 closes with

contacts

134 and 135 the buzzer 145 will be energized instead of the auxiliary signal device 136.

The buzzer will be used principally while the sys-` tem is being installed, repaired or tested so that the operative condition of the system can be indicated without giving a false tire alarm.

It is contemplated that the main signal device 18 will be located in the building in which the system is installed and may be provided in multiple if desired, while the auxiliary signal device 1,36 will be disposed in some other location, such as a superintendents ofi-ice or a nearby iire station. The buzzer 145 will ordinarily be mounted in the cabinet in which the various terminals, relays and switches are housed.

When the winding 64 of relay 65 has been de-energzed by interruption of the protective circuit 17 it will Vnot be re-energized when the protective circuit 17 is restored because the winding 70 of relay 68 is now energized interrupting the energizing circuitl for'winding 6'4 at contact 67 A circuit energizing circuit from of relay 68. In order to re-energize winding 64 reset switch 150, which is a normally open, push button switch, is manually closed. This establishes an energizing circuit for winding 64 from line 58 through conductor 61, terminal 60, protective circuit 17, terminal 62, conductor 63, winding 64, conductor 151, reset switch 150, conductor 152, line 90, contacts 47 and 48 and pole 42, and conductor 57 to transformer lead 56. As soon as winding 64 is reenergized it opens the energizing circuit for winding 70 at contact 120 and this restores the system to its normal operating condition.

The system is constantly supervised by a circuit including the supervisory relay 125 and the trouble alarm signal 153. The winding 126 of relay 125 is constantly energized by a circuit from line or transformer lead 58 through conductor 61, terminal 60, protective circuit 17, terminal 62,

conductors

63 and 154, resistance 155, conductor 156, pole 41 of relay 38, conductor 157, adjustable rheostat 158, conductor 159, winding 126 and

conductors

160 and 152 to line 90. While winding 126 is energized,

pole 127 is held out of engagement with contacts 161 and 162.

If winding 64 is de-energized by opening of one or more temperature-responsive switches in the protective circuit 17, causing pole 163 of relay 65 to bridge contacts 164 and 165, short circuiting resistance 155, the shunting resistance 155 is compensated for by the insertion of resistance elements 32 in the protective circuit, as balanced against the resistance 155 by the proper adjustment of rheostat 158 so that winding 126 is energized by current regardless of Whether winding 64 is energized or de-energized.

When winding 126 is de-energized by a complete break in the protective circuit, pole 127 of relay 125 moves into engagement with contacts 161 and 162 establishing a circuit from contact 161 through conductor 166, terminal 167, conductor 168, trouble signal battery 169, conductor 170, terminals 167', conductor 172, trouble signal device 153, conductor 173, terminal 174 and conductor 175 to terminal 162.

When pole 127 of relay 125 moves into engagement with contacts 161 and 162 to complete the trouble alarm circuit, it moves away from contacts 128 and 123 breaking the energizing circuit to winding 70 of relay 68 so that this winding cannot be energized to establish the circuit for energizing signal device 18 thus precluding the giving of a false alarm by an accidental break in the protective circuit. The trouble signal will remain in operation until the protective circuit is repaired or the energy of battery 169 depleted.

Current supplied to winding 126 through resistance 155 or resistances 32 and rheostat 158 is suflicient to energize relay 125 but not suicient to energize relay 65.

In the event of power failure to the transformer 10,

winding 39 of relay 38 is tie-energized and this automatically disconnects the system from the transformer and connects it to theauXiliary power source 16. De-energization of solenoid 39 frees the spring means of relay 38 to move pole of this relay away from contacts 43 and 44 and into engagement with contacts and 51, pole 41 away from contacts 45 and 46 into engagement with contacts 52 and 53 and pole 42 away from contacts 47 and 48 and into engagement with contacts 54 and 55. Movement of pole 42 away from contact 47 completely disconnects the transformer winding 1.4 from the system and precludes any re-energization of winding 39 until power is restored to the transformer.

With winding 39 of relay 38 tie-energized, a protective the auxiliary power source 16 is established as follows:

Beginning with terminal at one end of the protective circuit, conductor wire 22, through conductor 61 to line 58, through a portion of line 58, which is now otherwise de-energized, to conductor 176 through interconnected substantially the same 6 terminals 177 to conductor 179 to one side of battery 16. From the other side of the battery through conductor 180 to terminal 181, thence through conductor 182 including fuse 183, contact 54 of relay 38 and through pole 42 of this relay to contact 55. From contact 55 through a portion of conductor 90 and conductor 80 to contact 78 of relay 75. From contact 78 through pole 77, contact 74, conductor 73,

contacts

72 and 67 and pole 71 of relay 68 and conductor 185 to conductor 66 and one side of wind ing 64 of relay 65. From the other side of winding 64 through conductor 63 to terminal 62 and the other end of conductor wire 22 thus maintaining the relay Winding 64 energized as long as the protective circuit 17 remains closed.

The relay 65 is slow acting so that it will not move its pole 118 into engagement with

contacts

119 and 120 while the relay 38 is changing from the normal power connection to the auxiliary power connection.

if the winding 64 now becomes de-energized by opening of one or more of the temperature-responsive elements in the protective circuit 17, the pole 118 of this relay closes with the

contacts

119 and 120. A circuit is now established from one side of battery 16 through conductor 180, terminal 181, conductor 182, pole 42 of relay 38, conductor 98 to conductor 131, conductor 131 to pole 118 of relay 65, conductor 130, pole 40 of relay 38, conductor 187, contacts 188 and 189 and pole 190 of relay 125, conductor 191, winding 76 of relay 75, conductor 192, conductor 58, conductor 176, terminals 177, and conductor 179 to the other side of battery 16 thereby energizing winding 76 of relay 75 and closing pole 77 of this relay with

contacts

193 and 113 of this rela f.

When Winding 76 is energized, a circuit is established from battery 16 through conductor 180, terminal 181, C011- ductor 182, contacts 54 and 55 and pole 42 of relay 38, conductor 98, conductor 80,

contacts

193 and 113 and pole 77 of relay 75, conductor 110, winding 186 of signal drop 105, conductor 109, a portion of conductor 58, conductor 176, terminals 177 and conductor 179 to the other side of battery 16 thereby energizing the signal drop relay 105.

When the signal drop relay is energized a main signal energizing circuit is established from battery 16 through conductor 180, terminal 181, conductor 182, contacts 54 and 55 and pole 42, conductor 90, conductor 116,

contacts

114 and 103 and pole 108 of signal drop 105, conductor 102, switch 101, conductor 99, terminal 98, conductor 97, main signal device 18, conductor 94, terminal 95,

conductors

96 and 61,

conductors

58 and 176, terminals 177 and conductor 179 back to the other side of battery 16.

At the same time the auxiliary signal circuit is established from conductor 116 through

contacts

134 and 135 and pole 107 of signal drop 105, conductor 137, switch 138, conductor 140, terminal 141, conductor 142, auxiliary signal device 136, conductor 143, terminal 144 and

conductors

96 and 61 to line 58.

After the winding 64 has been de-energized, it is reenergized by closing the reset switch 150 establishing a re-energizing circuit from the line 58 through the protective circuit and winding 64 to the line 90 as described above.

The supervisory circuit maintaining the winding 126 of relay 125 energized is also the same as that described above except that conductor 157 is now dead and is substituted by conductor 195 leading from contact 52 of relay 38 through adjustable rheostat 196 and

conductors

197 and 159 to winding 126.

The shock alarm apparatus comprises a normally held open microswitch 208 having a terminal 201 connected by conductor 202, terminal 203 and conductor 204 to line 58 and a pole 205 connected by conductor 286 to one side of a signal device 287. The other side of signal device 207 is connected by a conductor 208. terminal 209, and

conductor 239 to line 9@ and with this arrangement signal device 267 will be energized when microswitch G closes regardless of whether the system is operating on normal or auxiliary power.

As shown in Figures 4 and 5, the microswitch 200 is pivotally mounted at one end by a hinge connection 22 on a horizontally disposed base T13 and has `its operating plunger 214 projecting from its side remote from the base and adjacent the free end thereof. Three internally screw threaded sleeves 2id, 2id and 217 extend through the base and are located at the apices of an equilateral triangle. Screws M8, 2id and 226 are threaded through the sleeves 215, 2l6 and 27 respectively and the center lines of these screws converge upwardly to a common point disposed above the base 2id. The switch plunger 2id has its center line substantially coincident with a line extending through the point ot' convergence of the center lines of the screws 2id, 2ll9 and 229. A sphere 22l of heavy material, such as metal rests on the upper ends of the screws which provide a tripod support for the sphere or ball, and the ball presses downwardly on switch plunger 2id maintaining switch 26S open until the ball is dislodged from its support on the upper ends of the screws by a shock, such as an earthquake tremor.

The screws can he moved upwardly or downwardly relative to the base 213 to adjust the sensitivity of the device, the device becoming more sensitive as the screws are moved upwardly bringing their Lipper ends closer together.

A fourth screw threaded sleeve 222 extends through the base 213 at the center of the triangle formed by the sleeves 215, 2id and 217 and a screw 223 threaded through sleeve 222 bears at its upper end against the underside of the housing of microswitch Zilli to maintain the proper pressure of sphere on switch button or plunger 21d as the -screws 213, 2l9 and 22@ are moved relative to the base 2l3 to raise or lower the sphere Timing relays, as generally indicated at 224 and 225 a -e connected between the switch 2%@ and the earthquake alarm signal device Zit-i7. rhe relay 224% is a slow-acting relay and has a winding 27.5 connected between conductors 2% and 2% by conductors 227 and 22d. A relay oV-and-on control switc. 229 is connected into the conductor 227 and a relay-operated switch 234) is connected into the conductor 2% to provide intermittent operation of the signal 207.

The timing relay 225 is a fast-operating relay and includes a winding 231 connected between conductors 206 and 29%,? by conductors 232 and 233, an. oi-and-on control switch 234.1 in the conductor 252 and a relay-operated switch 235 in the conductor 206 to provide an intermittent operation of the signal device 2&7.

The relays 224 and 22S are to be used alternatively and a manually operated switch 236 is connected in shunt of the microswitch Zitti so that the earthquake alarm device can be manually operated if desired.

The invention may be embodied in other specic forms without departing from the spirit or essential characteristics thereof. The present embodiment is, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all 'changes which come within the meaning and range ot` equivalency of the claims are intended to be embraced therein.

What is claimed is:

l. An electric re alarm system comprising a protective circuit including normally closed temperature-responsive switches connected together in series, a resistance element connected across each temperature-responsive switch in shunt thereof, a source of electrical energy, a re alarm signaly device, a circuit trouble signal device, a control relay connected in series with said energy source and said protective circuit and having switch means and a winding normally energized as long as all of said temperatureresponsive switches are closed, said temperature-respon;

winding connected in series with said energy source and the switch means of said control relay and energized when the winding of said control relay is de-energized, said signal drop relay having switch means connected in series with said energy source and said lire alarm signal device eiective to establish an energizing circuit for said re alarm signal device when the coil of said signal drop relay is energized, a supervisory relay having a winding conected in series with said energy source and said protective circuit means arranged to energize said supervisory relay only as long as said protective circuit is continuous through the resistance elements connected across said temperature-responsive switches, and switch means formed and arranged to establish an energizing circuit for said circuit trouble signal device when the winding of said supervisory relay is deenergized.

2. An electric alarm signal system comprising a source of electrical energy, a signal device, a protective circuit, a rst electromagnetic relay having a winding and a switch, an energizing circuit for the winding or" saidrst relay including said source of electrical energy, said protective circuit and the winding of said rst relay and formed and arranged to place the switch of said irst rclay in one condition 'when said protective circuit is closed and in a second condition when said protective circuit is open, a second circuit including said source of electrical energy, the switch of said first relay and said signal device formed and arranged to energize said signal device when the switch of said iirst relay is in said second condition, a circuit of high electrical resistance extending along said normally closed protective circuit and connected and parallel therewith, means to maintain said high resistance circuit closed when said protective circuit is opened, a second electromagnetic relay having a winding and a switch connected into said Second circuit, a third circuit including said source of electrical energy, said circuit of high electrical resistance and the winding of said second relay, a trouble alarm device, a source of electrical energy for said trouble alarm device, and a fourth circuit including the switch of said second relay, said trouble alarm device and the source of electrical energy for said trouble device, means to maintain said fourth circuit closed to energize said trouble alarm device and to open said second circuit to preclude giving a false alarm when said third circuit is opened by breakage of said circuit of high electrical resistance.

3. An electric re alarm system comprising a protective circuit including normally closed temperatureresponsive switches connected together in series, and resistance elements connected across each temperature-responsive switch in shunt therewith, a source of electrical energy, a tire alarm signal device, a circuit trouble signal device, a control relay connected in series with said energy source and said protective circuit and having switch means and a winding normally energized as long as all of said temperature-responsive switches are closed, said temperature-responsive switches being arranged to open upon abnormal rise in temperature, whereby the resistance elements are substituted for the temperatureresponsive switches, said resistance elements being sumciently large in value to reduce the current in said pro tective circuit sutlciently to de-energize said control relay, means to energize said re alarm device when the winding of said control relay is deenergizeda supervisory relay having a winding connected in series with said energy source and said protective circuit, means arranged to energize said supervisory relay only as long as said protective circuit is continuous through the resistance elements connected across said temperaturereis tie-energized.

4. In an electric tire alarm system, a protective circuit including a normally closed temperature-responsive switch arranged to open responsive to a predetermined rise in temperature, a resistance element connected across said switch in shunt therewith, a source of electrical energy, a fire alarm signal device, a circuit trouble signal device, a control relay connected in series with said energy source and said protective circuit and having a winding normally energized as long as said temperatureresponsive switch is closed, means to energize said fire alarm device when the winding of said control relay is deenergi2ed, a supervisory relay having a winding connected in series with said energy source and said protective circuit, means arranged to energize said super visory relay only as long as said protective circuit is continuous through the resistance element connected across said temperature-responsive switch, said temperature-responsive switch being arranged to open upon abnormal rise in temperature, whereby the resistance element is substituted for the temperature-responsive switch, said resistance element being suciently large in value to reduce the current in said protective circuit suiciently to de-energize said control relay, and means formed and arranged to establish an energizing circuit for said circuit trouble signal device when the winding of said supervisory relay is de-energized.

References Cited inthe le of this patent UNITED STATES PATENTS