GB2066537A

GB2066537A - Fire, temperature or heat detector

Info

Publication number: GB2066537A
Application number: GB7944474A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-12-28
Filing date: 1979-12-28
Publication date: 1981-07-08

Abstract

A fire, temperature or heat detector comprises a cable having at least two conductors separated by insulation whose electrical characteristics vary with temperature, and means connected to the conductors of the cable for detecting variations in the electrical characteristics of the insulation. The cable is connected in parallel with a padder resistance and forms one arm of a bridge circuit. The insulation is a non-doped polyimide resin. <IMAGE>

Description

SPECIFICATION Line fire, temperature or heat detector This invention relates to a fire, heat or temperature detector, using a Kapton insulated cable as the detecting element. Changes in the electrical characteristics of the cable insulation due to changes in its surrounding temperature, are sensed by a potentiometric type circuit. The output of the bridge provides an analogue signal proportional to the cable temperature and actuates warning a/or protective devices at preset levels.

An undesirable feature, if cable insulation is used as a fire detecting element, is that the insulation temperature vs. impedance vs. length coefficient tends to be linear, i.e. typically the resultant impedance or d.c. resistance of 1 metre at 900C is the same as 100 metres at 300C. This feature, unless rectified, precludes the use of cable insulation as fire detecting elements except in very short lengths. Such a method of fire detection would offer limited advantages over single point/position heat detectors already commercially available.

A second undesirable property of cable insulation, if used as a fire detecting element, is that the insulation resistance between conductors is extremely high, and its absolute value difficult to guarantee, within acceptable limits for instrumentation, during manufacture. Therefore any system or circuitry based upon variations in cable insulation absolute value of resistance would require the design of switches, terminals, electronics, and similar equipment, to much higher insulation values than are commerically available hitherto to avoid errors in the measured signal, and would further require individual input circuitry per cable.

The circuitry of this invention now to be described has been developed to overcome the disadvantages mentioned in the preceding paragraph, and to exploit the technical attraction of using cable insulation as a fire/temperature detecting element. This attraction is that the temperature vs. impedance or d.c. resistance curve of cable insulation has a very steep angle, and although there are variations in absolute values, the angle of the curve is consistent for all standard cable insulations.

In the invention now to be described one end of a cable having two conductors separated by Kapton* insulation to be used as a fire detector is connected to potentiometric type circuit (see Fig.

1). At the other end of the cable, the conductors are connected to a fixed resistor designated the 'padder'. The connection of the padder to the same conductors effectively makes a parallel circuit with the insulation to form the first arm of the bridge.

The resistance value of the padder is chosen to be slightly higher than the equivalent d.c.

resistance of the insulation of an optimum length *or other polyimide resin of the cable, at the chosen monitored/alarm temperature of the proposed fire/temperature monitored/alarm installation. The effect of the padder resistance in parallel with the cable insulation is to: (a) Reduce the resistance variations at the bridge to economically measurable values, since the resultant of the two resistances in parallel will always be lower than the padder value on its own.

(b) 'Swamp' variations in the electrical characteristic absolute values of the cable insulation caused by manufacturing intolerances.

The resistance values of the insulation at the monitored/alarm level, is sufficiently consistent between cables of similar construction, to enable the exact point per cable to be set by a potentiometer in a universal design of bridge.

(c) Introduce desirable non-linearity into the temperature vs. electrical resistance vs. length coefficient of the resultant effective d.c. resistance at the bridge.

The non-linearity introduced by the padder is enhanced by careful selection of the resistance values forming the other three arms of the bridge circuit. A typical set of values developed to give the required knee or'kink' in the characteristic, is shown in Fig. 1 , the result of these values is shown in Fig. 2, other shapes with other values are possible.

The capacitance in parallel with the resistance in the second arm of the bridge, is necessary to balance to the cable insulation impedance. The capacitor is not critical in values but is critical in type, polyester the most acceptable. The value of the resistance in the bridge second arm is varied as a coarse bias for the monitored/alarm setting potentiometer. The capacitance in parallel with the resistance does not have to change its value, for changes in the resistance value.

The invention circuitry enables any Kapton insulated cable to be used as a fire-temperature detector but there are other factors which encourage the use of a preferred configuration.

Repeatability and accuracy of the system is a function of the contact pressure and surface area between the conductors and the insulation.

Theoretically and empirically it has been demonstrated that a coaxial type of construction is the most efficient and stable.

The electrical analogue signal generated by the unbalance of the bridge is fed as the input to a differential amplifier. This amplifier must be individually capable of, or in conjunction with other amplifiers capable of, providing signals suitable to drive standard commercially available equipment, for fire alarm, temperature indication or heat control. Such amplifiers using FET or similar inputs are readily available and do not form part of this invention.

1. A fire, temperature or heat detection system, comprising a cable having at least two conductors separated by insulation whose electrical characteristics vary with temperature, and means

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Line fire, temperature or heat detector This invention relates to a fire, heat or temperature detector, using a Kapton insulated cable as the detecting element. Changes in the electrical characteristics of the cable insulation due to changes in its surrounding temperature, are sensed by a potentiometric type circuit. The output of the bridge provides an analogue signal proportional to the cable temperature and actuates warning a/or protective devices at preset levels. An undesirable feature, if cable insulation is used as a fire detecting element, is that the insulation temperature vs. impedance vs. length coefficient tends to be linear, i.e. typically the resultant impedance or d.c. resistance of 1 metre at 900C is the same as 100 metres at 300C. This feature, unless rectified, precludes the use of cable insulation as fire detecting elements except in very short lengths. Such a method of fire detection would offer limited advantages over single point/position heat detectors already commercially available. A second undesirable property of cable insulation, if used as a fire detecting element, is that the insulation resistance between conductors is extremely high, and its absolute value difficult to guarantee, within acceptable limits for instrumentation, during manufacture. Therefore any system or circuitry based upon variations in cable insulation absolute value of resistance would require the design of switches, terminals, electronics, and similar equipment, to much higher insulation values than are commerically available hitherto to avoid errors in the measured signal, and would further require individual input circuitry per cable. The circuitry of this invention now to be described has been developed to overcome the disadvantages mentioned in the preceding paragraph, and to exploit the technical attraction of using cable insulation as a fire/temperature detecting element. This attraction is that the temperature vs. impedance or d.c. resistance curve of cable insulation has a very steep angle, and although there are variations in absolute values, the angle of the curve is consistent for all standard cable insulations. In the invention now to be described one end of a cable having two conductors separated by Kapton* insulation to be used as a fire detector is connected to potentiometric type circuit (see Fig. 1). At the other end of the cable, the conductors are connected to a fixed resistor designated the 'padder'. The connection of the padder to the same conductors effectively makes a parallel circuit with the insulation to form the first arm of the bridge. The resistance value of the padder is chosen to be slightly higher than the equivalent d.c. resistance of the insulation of an optimum length *or other polyimide resin of the cable, at the chosen monitored/alarm temperature of the proposed fire/temperature monitored/alarm installation. The effect of the padder resistance in parallel with the cable insulation is to: (a) Reduce the resistance variations at the bridge to economically measurable values, since the resultant of the two resistances in parallel will always be lower than the padder value on its own. (b) 'Swamp' variations in the electrical characteristic absolute values of the cable insulation caused by manufacturing intolerances. The resistance values of the insulation at the monitored/alarm level, is sufficiently consistent between cables of similar construction, to enable the exact point per cable to be set by a potentiometer in a universal design of bridge. (c) Introduce desirable non-linearity into the temperature vs. electrical resistance vs. length coefficient of the resultant effective d.c. resistance at the bridge. The non-linearity introduced by the padder is enhanced by careful selection of the resistance values forming the other three arms of the bridge circuit. A typical set of values developed to give the required knee or'kink' in the characteristic, is shown in Fig. 1 , the result of these values is shown in Fig. 2, other shapes with other values are possible. The capacitance in parallel with the resistance in the second arm of the bridge, is necessary to balance to the cable insulation impedance. The capacitor is not critical in values but is critical in type, polyester the most acceptable. The value of the resistance in the bridge second arm is varied as a coarse bias for the monitored/alarm setting potentiometer. The capacitance in parallel with the resistance does not have to change its value, for changes in the resistance value. The invention circuitry enables any Kapton insulated cable to be used as a fire-temperature detector but there are other factors which encourage the use of a preferred configuration. Repeatability and accuracy of the system is a function of the contact pressure and surface area between the conductors and the insulation. Theoretically and empirically it has been demonstrated that a coaxial type of construction is the most efficient and stable. The electrical analogue signal generated by the unbalance of the bridge is fed as the input to a differential amplifier. This amplifier must be individually capable of, or in conjunction with other amplifiers capable of, providing signals suitable to drive standard commercially available equipment, for fire alarm, temperature indication or heat control. Such amplifiers using FET or similar inputs are readily available and do not form part of this invention. CLAIMS

1. A fire, temperature or heat detection system, comprising a cable having at least two conductors separated by insulation whose electrical characteristics vary with temperature, and means connected to the conductors of the cable for detecting variations in the electrical characteristics for the insulation.

2. A detection system as claimed in Claim 1, wherein the insulation is Kapton, or other polimide resin.

3. A detection system as claimed in Claim 1, wherein a bridge circuit is used to provide an analogue signal proportional to the cable temperature.

4. A detection system as claimed in Claim 3, wherein the bridge circuit is a Wheatstone bridge circuit.

5. A detection system as claimed in Claim 1, wherein by use of a cable parallel resistor referred to as the padder and by use of a Wheatstone bridge type circuit with suitable values, the electrical characteristics of the system are modified from a natural iinear function to a more desirable sharp curve.

6. A fire, temperature or heat detection system as claimed in Claim 1, which provides an analogue signal proportional to the cable temperature.

7. A fire, temperature or heat detection system as claimed in Claim 1 and substantially as hereinbefore described.

7. A detection system as claimed in Claim 1, characterised in that an indication or alarm is produced which is proportional to the temperature in the region of the cable.

8. A fire, temperature or heat detection system substantially as hereinbefore described.

New claims or amendments to claims filed on tilth November 1980 Superseded claims: 1-8 New or amended claims: 1-7 1. A fire, temperature or heat detection system, comprising a cable having at least two conductors separated by insulation formed of a non-doped polyimide resin whose electrical characteristics vary with temperature, and means connected to the conductors of the cable for detecting variations in the electrical characteristics for the insulation.

2. A detection system as claimed in Claim 1, wherein the insulation is Kapton.

3. A detection system as claimed in Claim 1 or 2, wherein a bridge ciruit is used to provide an analogue signal proportional to the cable temperature.

5. A detection system as claimed in Claim 4, wherein a padder resistor is connected in parallel with the said cable.

6. A detection system as claimed in Claim 1, characterised in that an indication or alarm is produced which is proportional to the temperature in the region of the cable.