Question

With reference to fire detection unit

Describe a rate of heat rise detector with the aid of a sketch.

Describe the following methods of checking the integrity of a suction of fire detection heads

Loop monitoring.

Line termination monitoring.

Bimetal strips made up from brass and INVAR will deflect when heated due to the high expansion coefficient of the brass and the low expansion coefficient of the INVAR, which is 36% nickel steel Deflection increases with temperature and could in itself be used to complete an electrical circuit and operate an alarm at some specific figure. However, rate of rise can be detected using two bimetal strips (Figure) of different thickness.

Figure

These are set up parallel to each other and arranged to deflect in the same direction. Rapid temperature rise will cause the thinner element to deflect more quickly than the other, so causing contact to be made and the alarm circuit to be completed. A slow rise causes a similar deflection in both elements, so that a gap between the contacts persists, until a certain maximum temperature if reached when contact is made because of other differences in the make up of the strips.

Loop monitoring.

The alarms are activated by the closure of the contacts in the heads through 1 and 2

While continuity is checked through 1 and 4 and 2 & 3.

Line termination monitoring.

The alarms operate when contact across 1 & 2 close. Continuity is checked by monitoring the resistance at the end of the line.

If the resistance of the line measured across 1 and 2 is less than the end resistance the alarm sounds

If the resistance of the line measured across 1 and 2 is greater than the end resistance plus the cable resistance a system fault alarm is activated.