US3060417A - Condition responsive network insensitive to electrical leakage - Google Patents

Description

R. F. BLAKE CONDITION RESPONSIVE NETWORK INSENSITIVE Oct. 23, 1962 TO ELECTRICAL I .EAKAGE 5 Sheets-Sheet 1 Filed Jan. 20, 1959 Oct. 23, 1962 R. F. BLAKE CONDITION RESPONSIVE NETWORK INsENsIIIvE TO ELECTRICAL LEAKAGE 5 Sheets-Sheet 2 Filed Jan. 20, 1959 ALARM mmATOR 5- l vom-AGE Y la' l g oMpARA-roll l5 Oct. 23, 1962 R F BLAKE 3,060,417

CONDITION RESPOSIX'IE NETWORK INSENSITIVE TO ELECTRICAL LEAKAGE Filed Jan. 20. 1959 5 Sheets-Sheet 5 l www# l i l l l. l. i

United States Patent O CONDITION RESPGNSEVE NETWGRK INSENSI- 'UVE T() ELECTRICAL LEAKAGE Richard F. Blake, Mountain Lakes, NJ., assigner to Specialties Development Corporation, Belleville, NJ., a corporation of New Jersey Filed Jan. Ztl, 1959, Ser. No. 787,873

8 Claims. (Cl. S40- 227) The present invention relates to electrical networks which are automatically responsive to a change of a condition, and more particularly, to such networks which are relatively insensitive to the presence of electrical leakage paths in the condition detecting element and electrical connectors attached thereto.

The present invention, although useful for many purposes, is primarily concerned with providing an improved heat and flame detecting system of the type including an elongated fire detecting element comprising two conductors within a metallic sheath and spaced apart by a thermistor material having a negative temperature coetlcient of resistivity, and an indicating circuit for monitoring the resistance of the thermistor material and giving an indication when the resistance of the material indicates the presence of a fire or an overheat condition.

In systems of this type, a connector is provided at the end of the element for connecting the conductors in the element to a pair of conductors in a cable from the indicating circuit. To facilitate installation in cramped locations, through bulkheads, etc., elements are normally made in sections which are joined together by connectors to provide continuous electrical connection between the conductors in each of the sections.

When used in aircraft, the elements are installed within the engine nacelles where the air is normally contaminated with gasoline, oil, and water mists. The mating members of the connectors are made tight titting in an attempt to prevent their contacts from being contaminated or impaired by the impurities in the air, however, a small air space always exists within the connector and variations in the ambient pressure with changes in altitude cause the connectors to breathe, drawing in the contaminated air.

ln the systems previously used, the contamination of the connector contacts and surfaces in many instances created a low resistance current path between the conductors in the element, in parallel with the thermistor material, and the indicating circuit sensing the low resistance gave a false tire alarm.

These systems also gave false alarms when a low resistance current path was established within the element between one of the conductors and the sheath, for instance, when vibration caused the thermistor material to pulverize allowing one of the conductors to move toward the sheath.

The emergency procedure which is adopted on an aircraft when a lire is indicated is in many instances dangerous to both the aircraft and the personnel, therefore, it is highly desirable that false lire indications be eliminated completely. For example, when a fire indication is given during take off when full power is needed, the pilot must, if possible, bring the aircraft to a halt on the runway because the loss of an engine during the later critical period when the aircraft is climbing to avoid obstacles could result in a crash. This creates a very dangerous situation since the pilot must decide in an instant whether there is sufficient runway left to safely halt the aircraft and an error in his judgment could result in disaster. Also, when a re indication is given a few 3,060,417 Patented Oct. 23, 1962 "ice n minutes after take off, the pilot must land immediately even though the aircraft is carrying a full load of fuel which, if the pay load is at maximum, will place the weight of the aircraft in excess of that which is permissible for safe landing. Such premature landings, therefore, can place excessive strain on the aircraft structure resulting in possible damage, and the structure must be thoroughly checked before the aircraft can be placed in operation again.

Aircraft fire warning systems are checked between each ight, therefore, since it is extremely unlikely that an actual tire and an extraneous condition affecting the warning system would both occur during any one iiight, it is preferable that any such extraneous condition render the system inoperative rather than cause a false indication which might result in the destruction of the aircraft.

Accordingly, an object of the present invention is to provide an improved condition responsive network which is not subject to the foregoing diiculties.

Another object is to provide Such ya network which is relatively insensitive to certain electrical leakages in the condition detecting element and the connectors.

Another object is to provide such a network which will not give an indication in response to certain electrical leakages in the condition detecting element and the connectors.

Another object is to provide such a network which will continue to operate when certain electrical leakages are present in the condition detecting element and the connectors.

A further object is to provide such a network which is simple, inexpensive and reliable.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

In the drawings:

FlG. l is a partially schematic diagram of a network in accordance with the present invention illustrating the detecting element in longitudinal section.

FIG. 2 is a longitudinal sectional view of a portion of the detecting element illustrating the connector construction in detail.

FIG. 3 is a schematic diagram of the network shown in FIG. l in which the detecting element is represented by its equivalent circuit.

FlG. 4 is a schematic diagram of a network including a modified indicating circuit.

FIG. 5 is a schematic diagram of a network including a further modified indicating circuit.

Referring to FIG. l of the drawings in detail, there is shown a condition responsive network in accordance with the present invention which generally comprises an element 10 adapted to change in resistance in response to changes in ambient tempertaure, and circuitry 11 for detecting the resistance changes of the element 10 and for giving an indication when the resistance of the element indicates the presence of fire.

The element l0 generally is of the type shown in United States Patent No. 2,586,252, and includes a plurality of sections l2 each comprising a pair of spaced parallel conductors 14 and 1S ern-bedded in a thermistor material 16 conined within a tubular metallic protective sheath 17. The material 16 has a negative coeflicient of resistance and therefore provides a current path between the conductors 14 and 15 which decreases in resistance as the temperature of the element increases due to an inspec/tir crease in ambient temperature. In aircraft installations, the element sections 12 are normally mounted on a metal wall and in electrical contact therewith, therefore, the sheath 17 of each section is at ground potential as shown schematically by the connections 19.

The element sections 12 have fastened to each end thereof a male connector member 211 comprising a cylir1- drical body of insulating -material 21 fastened to the sheath 17 and having a frusto-conical outer end 22. As shown in FIGS. l and 2, the outer end 22 of each male connector member is provided with three spaced annular grooves 24, Z and 26, each having an electrically conductive metallic bottom surface Z9, 3th and 31, respectively, and three resilient split ring contacts 34, 35 and 36 are positioned respectively in each of the grooves. The conductors 14 and 15 extend through the bodyl 21 and are respectively connected to the surfaces 29` and 31 in the grooves 24 and 26. A third conductor 37 is connected to the surface 3i) in the center groove 215 and extends through the body 21 and is connected to the metallic sheath 17.

The element sections 12 are physically and electrically connected together to form the element 16 by a doubleended female connector member 39 comprising a cylindrical body 4l) formed with a frusto-conical recess 41 at each end thereof. The recesses 41 are dimensioned to mate with the outer ends 22 of the male connector members, and, as shown in detail in FIG. 2, three annular contact strips 43, 44 and 45 are embedded in the body 4@ within each of the recesses 41 with the exposed surfaces flush with the wall of the recess.

An internally threaded sleeve 46 mounted on the body 40 at each end is screwed onto an externally threaded portion 47 on the body 21 of the male connector members 20 to draw the strips 43, 44 and 45 into tight contact with the rings 34, 35 and 36, respectively, thereby forcing the rings against the surfaces 29, 30, 31 to complete an electrical path between the conductors 14, 15 and 37 and the strips 43, 44 and 45, respectively.

Three conductors 48, 49 and 50 embedded in the female connector body 40 connect comparable strips at opposite ends to provide an electrical connection between the conductors 14, the conductors 15, and the conductors 37 in each of the element sections.

The element is connected to the circuitry 11 by a single ended female connector member 51 comprising a cylindrical body 52 of insulating material having a frustoconical recess 54 in one end thereof. Three contact strips 55, 56' and 57 are embedded in the body 52, and three conductors 59, 60 and 61 are connected respectively to f the strips and extend through the body Afor connection to the circuitry. A threaded sleeve 62 mounted on the body 52 is screwed on the male `connector portion 47 to join the connector members 20 and 51 and place the conductors 14, and 37 of the element in electrical connection with the conductors 59, 66 and 61, respectively.

The circuitry 11 includes a source of direct current 64 having its negative terminal connected to the conductor `60 and having its positive terminal connected to the conductor 61; and an alarm network including a relay having a coil 65 connected between the conductor 59 and the positive terminal of the source 64 and having a spring biased switch 66 under the control of the coil 65 and connected n series with a lamp 67 and an electrical power source 68.

It will be seen that if moisture is present between any of the mating connector members, a path of low resistance cannot be established between the contact ring 34 and the ring 36 except by establishing a current path from each of the rings 34 and 36 to the grounded contact ring 35. Such moisture, therefore, can produce a low resistance between conductors 59 and 60 only by producing a low resistance current path between the conductor 61 and each of the conductors 59` and 60.

The effect of moisture between the connector members may therefore be schematically represented by two resistances, one connected lbetween the conductor 59 and ground, and the other connected lbetween the conductor and ground. In FIG. 3, the network of FIG. l is shown with the element 10 illustrated schematically, the conductors 14 and 15 being represented by the connection points 14 and 15', and the resistance of the material 16 being represented by the variable resistance 16. The leakage resistance due to moisture 'between the rings 34 and 35 is represented by the resistance Rm connected between the point 14 and the grounded conductor 61, and the leakage resistance between the rings 35 and 36 is represented by the resistance RL2 connected between the point 15 and the conductor 61.

The resistance of the material 16 separating each of the conductors 14 and 15 from the sheath 17 is disregarded since the leakage resistances are in parallel therewith and are a problem only when their values are small compared with the resistance of the material 16.

In operation, when the element 10 is exposed to normal ambient tempertaures, the value of the resistor 16 is very high and the current owing in the coil is limited to a very small value. The switch 66, therefore, is open indieating the absence of a dangerous condition. As the temperature of the element increases, the value of the resistor 16 decreasse and when the current through the coil 65 reaches a predetermined value, the switch 66 closes, illuminating the lamp 67 to give a warning.

lf there is no moisture present in the connectors, the resistance Rm and RL2 are infinite in value and the only current path is through the resistor 16' and the coil 65. The current flowing through the coil 65 is then dependent only on the value of the resistance 161.

However, when moisture is present inthe connectors, the resistance Rm and Rm have finite values and two additional current paths are established, one through the resistance Rm shunting the source 64, and the other through the resistance 16 and the resistance RL, shunting the coil 65. It may be seen that the presence of the resistances Rm and RL?A does not provide a current path which is in parallel with the current path through the resistor 16' and therefore moisture in the connectors cannot shunt the resistor 16 and allow increased current to flow through the coil 65 to cause a false alarm. The leakage resistances Rm and Rm aifect the circuit only in that the resistance Rm increases the drain from the source 64, and the resistance Rm draws current away from the coil 65 so that the resistor 16 must drop to -a lower value in order for the coil 65 to receive the current necessary to close the switch 66.

The eifect of the leakage resistance RL, on the trip point of the circuit (the temperature at which the element 10 will cause the circuit to give an excessive heat or tire indication) is minimized by using a relay coil having a low resistance. For example, if the element `1t) is constructed so that the resistance 16 assumes a value of 200 ohms at a desired trip temperature of 500 F., and if the relay coil 65 is chosen so that its resistance is equal Ito the minimum expected leakage resistance Rm, the trip value of the resistance 16 will decrease to 100 ohms increasing the trip temperature only to 565 F. when the leakage resistance Rm is Kat the minimum value. This 657 F. change in the trip temperature does not materially affect the operation of the network since the element will rapidly reach the higher temperature when a tire is present.

It can be seen that current leakage within an element section 12 between the sheath 17 and either or both of the conductors 14 and 15 would have the same effect as leakage at the connectors.

In FIG. 4, there is shown a network in which the circuit 11 is modified by adding a snap action circuit for controlling the energization of the coil 65 to eliminate the possibility that the switch 66 might be closed prematurely as a result of mechanical vibrations when the value of the resistor 16' approaches that which indicates the presence of a re. This circuit includes a resistor 70 connected between the resistor 16' tand the grounded terminal of the source 64, a pair of resistors 71 and 72 connected in series across the source 64, a voltage comparator 74 connected between the conductor 59 `and the junction 75 of the resistors 71 :and 72, and an alarm initiator 76 connected across Athe source 64 and connected to the voltage comparator 74 and the coil 65. The voltage comparator 74 compares the potential of the conductor 59 (with respect to ground) with the potential of the junction 75 (with respect -to ground) and produces an output only when the potential of the conductor 59 equals or exceeds the potenti-al at the junction 75. The output of the comparator actuates the initiator 76 to energize the relay coil 65K.

The potential of the conductor 59* equals the voltage of the source 64 minus the voltage drop across the resistor 16', therefore, when the value of the resistor 16 is high with respect to the lvalue of the resistor 70, the voltage drop across the resistor 16 is high and the conductor 59 is approximately at ground potential. As the value of resistance 16 decreases, the voltage drop thereacross decreases `and the potential of the conductor 59* increases. When this potential equals the potential at the point 75, the voltage comparator produces an output operating the initiator 76 to energize the coil 65 and eiect illumination of the lamp 67.

As discussed in connection with FIG. 3, the presence of moisture in the connectors of the element 10 has no effect except that the resistance Rm increases the drain of the source 64, and the resistance Rm increases the current ow through the resistance 16 thereby causing the Voltage drop across this resistance to increase and the potential of the conductor 59 to decrease so that the resistor 16 must drop to a lower value before an alarm indication is given. ln this circuit, the effect of the leakage resistance Rm lis minimized by using a resistor 70 having a low Value, preferably equal to the minimum expected value of the leakage resistance Rm.

By way of example, the potential of the source 64 may be l volts, the resistance v-alues of the resistors 70, 71 and 72 may be 10.5 ohms, 105 ohms, and 2000 ohms, respectively, and the resistance value of the resistor 16 may be 200 ohms `at the desired trip temperature. The potential at the point 75 with respect to ground is then 0.5 volt, and the potential of the conductor 59 with respect to ground is approximately zero when the element is at normal ambient temperature and the value of the resistance 16 very high. When the value of the resistance 16 is 200 ohms, the potential of the conductor 59 with respect to ground is 0.5 volt providing there is no leakage. The presence of a finite leakage resistance Rm causes the potential of the conductor 59 to move toward ground potential and when this leakage resistance is 10.5 ohms, the value of this resistance 16 must decrease only to 100 ohms in order to place the conductor 59 at a potential of 0.5 Volt. It may be seen that the leakage resistances Rm and Rm can never cause a false alarm since, as their values decrease, the potential of the conductor 59 moves away from the required trip potential and toward ground potential.

In FIG. 5, 4there is shown a network in which the circuit 11 is further Inoditied by providing a second direct current power source 77 between ground and the junction of resistors 70 and 71 to thereby increase the sensitivity of the network and to decrease the current requirement of the network at the trip temperature.

Perferably, the source 77 has a voltage greater than that of the source 64 and has its positive terminal connected to the resistors 70 and 7l and its negative terminal connected =to the grounded positive terminal of the source 64. The resistor 71 has a Value greater than the resistor 72 and the ratio of the value of the resistor 71 to that of resistor 72 is greater than the ratio of the voltage of the source 77 to that of the source 64, whereby the voltage drop across the resistor 72 is less than the voltage of the source 64 and the point 75 is negative with respect to ground.

It may be seen that when the value of the resistance 16 is high, the potential of the conductor 59 with respect to ground is positive and approximately equal to the voltage of the source '77, and in order for the votlage comparator 7 i to intiate the alarm, the potential of the conductor 59 must change from this high positive value to the negative value existing at the point 75. The large change in Value and the polarity reversal of the potential of the conductor 59 enable the comparator 74 to more accurately `sense the condition of the circuit and thus give a more precise indication. It also will be seen that in order for the conducter 59 to achieve a negative value, the resistor 70 must be large in comparison to the Value of the resistance 16 which will cause the circuit to trip and therefore limits the drain ofthe circuit.

For the purpose of illustration, the voltages of the sources 64 and 77 may be 2 and 20 volts, respectively, and the values of the resistors 70, 71 and 72 may be 2050 ohms, 20,500 ohms, and 2000 ohms, respectively. The potential at the point 75 with respect to ground will then equal 1.95 volts (voltage drop across the resistor 72) minus 2.0 volts (the voltage of the source 64) or -005 volts, and when no moisture is present, an indication will be given when the Value of the resistance 16 assumes a value of 200 ohms at the desired trip temperature of 500 F.

When the value of the resistance 16 is such as to cause the conductor 59 to be negative, the voltage drop across the resistor 70 is greater than the Voltage of the source 77 and current cannot iiow in the loop including the source 77, the resistor 70, the resistance Rm and the conductor 61. Therefore, current ows through the resistance Rm only in the loop also including the source 64, the conductor 61, and the resistance 16', and the current flow through the resistance 16' is equal to the sum of the currents owing through the resistor 70 and the resistance Rm.

lt will be seen therefore that the trip value of the resistance 16 is equal to the voltage drop across the resistance at the trip point, divided by the sum of the currents iiowing through the resistor 70 and the resistance 16 at the trip point. The current flowing through the resistance Rm equals the potential of the conductor S9 divided by the value of the resistance Rm, therefore,

Rtz-EEM-'gg 59 Ivo-i- R L1 where Rt is the value of the resistance 16 necessary to cause the circuit to trip, E64, is the Voltage of the source 64, E59 is the potential of the conductor 59 with respect to ground at trip, and is the current flowing through the resistor 70.

Since a large percentage variation in the Value of the resistance 16 has little eiect on the current flowing through the resistor 70, L10 can be assumed to be constant` Under the above conditions Lm has a value of 9.75 milliamperes.

From the -above equation it will be seen that the presence of a 10 ohm leakage resistance Rm lowers the trip Value of the resistance 16 only to 100 ohms. As previously discussed, such a change in the trip value of the resistance 16 increases the required trip temperature by only 65 F. and does not materially affect the operation of the network. In this circuit, therefore, the effect of the leakage resistance is minimized while the value of the resistor 70 is sufiiciently large to limit the drain of the circuit when the value of the resistance lo' is low.

It will also be noted that the presence of leakage can never cause a false alarm because a zero resistance short between the point 14' and ground will place the conductor 59 at ground potential while the conductor 59 must be negative for the circuit to trip.

From the foregoing description, it will be seen that the present invention provides a simple, reliable and inexpensive network which is relatively insensitive to certain electrical leakages, will not give a false indication in response to these electrical leakages, and will continue to operate when certain low resistance electrical leakages are present.

As various changes may be made in the form, construction and arrangement of the parts herein, without departing from the spirit and scope of the invention and without sacriiicing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in any limiting sense.

i claim:

il. In a condition responsive network, the combination of an element having a pair of adjacent spaced terminals and constructed and arranged to cause the resistance between said terminals to change in response to a condition land positioned with said terminal in an area subject to contamination by electrically conductive tiuids so that both of said terminals are subject to electrical contact with an electrically conductive Huid present in said area, an electrically co-nductive member at ground potential positioned adjacent to and spaced from said terminals, a source of unidirectional current connected between one of said terminals and said conductive Imember, and element resistance indicating means connected between the other of said terminals and said conductive member, said conductive member being positioned so that an electrically conductive Huid in said area establishes current paths only between said conductive member and each of said terminals whereby current flow between said terminals through the conductive iiuid is prevented.

I2. In a condition responsive network, the combination of a resistance element having a pair of adjacent spaced terminals 4and constructed and arranged to cause the resistance between said terminals to change in response to a condition, said terminals being positioned in an area subject to contamination by electrically conductive uids so that both of said terminals are subject to electrical contact with an electrically conductive uid present in said area, a conductive lmember at ground potential positioned between said terminals and spaced therefrom so that an electrically conductive lilluid in said area can pro- U duce a low resistance between said terminals only by producing low resistances between said conductive member and each of said terminals, a source of unidirectional current connected between one of said terminals and said conductive member, and element resistance indicating means connected between the other of said terminals and said conductive member, whereby an electrically conductive fluid in said area establishes curren-t paths only between said conductive inember and each of said terminals and current liow between said terminals through the iluid is prevented.

3. In a condition responsive network, the combination of a resistance element having a pair of adjacent spaced terminals and constructed and arranged to cause the resistance between 4said terminals to change in response to a condition, said terminals being positioned in an area subject to contamination by electrically conductive fluids so that both of said terminals are subjected to electrical contact with an electrically conductive Huid present in said area; a conductive member positioned between said terminals and spaced therefrom so that an electrically conductive fluid in said area can pro-duce a low resistance between said Aterminals only by producing low resistances between said conductive member and each of said terminals; first and second conductors each in electrical contact with a different one of said terminals; a source of unidirectional current having iirst and second terminals of different polarity, said iirst terminal of said source being connected to said iirst conductor and said second terminal of said source being connected to said conductive member; and element resistance indicating means including a resistance connected between said second conductor and said second terminal oi said source, and voltage sensitive means for giving an indication when the potential on said second conducto-r is of a predetermined magnitude and polarity, whereby an electrically conductive liuid in said area establishes current paths only between said conductive member and each of said terminals and current fiow between said terminals through said fluid is prevented.

4. A. condition responsive network according t0 claim 3 wherein said resistance element decreases in resistance in response to a lcondition and said voltage sensitive means is constructed and arranged to give an alarm when the potential on said second conductor is at a predetermined magnitude land of the same polarity as said first terminal of said source.

5. A condition responsive network according t-o claim 4, wherein said resistance indicating means includes a second source of unidirectional current for placing on said second conductor a potential having the same polarity as said second terminal of said yfirst source when the resistance of said ele-ment is high.

6. A condition responsive network according to claim 5, wherein said second source has a trst terminal connected to said conductive member and a second terminal of opposite polarity connected to said resistance, said rst terminal of said second source being of a polarity opposite `to that of said second terminal of said first source.

7. In a condition responsive network the combination of an element having a pair of conductors and constructed and arranged to cause the resistance between said conductors to change in response to `a condition; a connector subject to contamination by electrically conductive liuids including iirst and second members, means for detachably connecting said connector members, a pair of spaced contacts mounted on said irst member in electrical contact with said conductors, a second pair of spaced contacts mounted on said second member and engaging said rst pair of contacts, each of said contacts being subject to electrical Contact with an electrically conductive iluid contaminating said connector, a conductive element mounted on said second member and positioned between the spaced contacts of said second pair oi contacts so that an electrically conductive fluid contaminating said connector can produce a low resistance between said spaced contacts of either of said pairs of contacts only by producing low resistances between said conductive member and each of said contacts, iirst and second lead conductors each connected to a different one of said contacts of o-ne of said pairs of contacts, a third lead conducu tor adapted to be in electrical contact with said conductive element, a source of unidirectional current connected between said iirst and third lead conductors, and element resistance indicating means connected between said second and third lead conductors, whereby an electrically conductive fluid contaminating said connector establishes current paths only between said conductive member and said contacts and current liow between said spaced contacts of either of said pairs of ycontacts through said fluid is prevented.

8. A condition responsive network according to claim 7, wherein said conductors of said element are positioned within an electrically conductive sheath and `a second conductive element electrically connected to said sheath is mounted on said first connector member to engage said iirst conductive element.

(References on foiiowing page) References Cited in the file of this patent UNITED STATES PATENTS Montgomery Oct. 29, 1918 Woodside Nov. 12, 1940 5 Spooner Ian. 1, 1952 Peters Feb. 19, 1952 10 Petersen June 5, 1956 Lindberg Aug. 14, 1956 Grant Feb. 12, 1957 Curtis Aug. 27, 1957 Postal Sept. 3, 1957 Cutsogeorge Aug. 25, 1959

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