M15.20 Fire protection Flashcards
What type of sensor is used to detect duct leaks?
Bi metallic strips
What are the two types of fire loop detectors?
Semi conductor
Pneumatic loop
What is the pneumatic loop known as?
The systron and donner loop
What are the manufactures of semi conductor loops?
Graviner
Fenwall
Kidde
Edison
How does a semi conductor system work??
Each loop has a metal tube that contains a semiconductor material. This
material decreases its electrical resistance with increasing temperature. An
alarm is triggered when the resistance between the central wire and the tube
decreases below a predetermined threshold
Negative of a bi metallic strip sensor?
This sensor can fail in either the open or closed position, either failing to detect a fire or giving a false alarm. This system cannot be tested to find defective sensors
On a continuous loop sensor, how is a false alarm prevented?
By using an AND gate
How is a semi conductor loop assebled?
Each semiconductor loop has a stainless steel tube, filled with a
semiconductor ceramic material with a wire in the centre.
The ceramic material functions like an endless number of resistors. These
resistors have an NTC characteristic, because of the semiconductor material
What type of electrical connection does the Graviner, fenwal, graviner and edison loops use?
All resistors are connected in parallel on one side by the internal wire that is
connected to the control unit. On the other side they are connected by the
tube that is connected to the ground
What type of electrical conection does the Kidde loop use?
One semiconductor loop type has two internal wires. The upper wire is also
connected to the control unit and the lower wire is connected internally to
ground. This means that there is no difference in the function
What is the resistance for short loops?
10 mega ohm
What is the resitance for long loops?
1 mega ohm
What is the trigger value for semi conductor loops?
500 ohm
How is an open loop situation prevented on a semi conductor loop?
To prevent the situation of a fire which cannot be detected due to an open
loop, the end of the loop is connected to the beginning and a closed loop is
formed
How is the semi conductor loop tested?
So that this double loop failure is almost impossible, the closed loop condition
is tested during the operational test, for example, before each engine start.
Normally the loop is closed via the relaxed contact of the test relay, but during
the test the loop will open at the end and is connected to ground via a test
resistor. An open loop condition is always detected during a fire warning test
On a wheel well fire system with semi conductor loop what can cause a false alarm?
A short circuit as the resistance will drop
How are short circuit false alarms prevented on duel loop semi conductor systems?
By using the duel loop, if one loop detects fire, and the second does not for 15 seconds the warning is cancelled and the loop indicating ire will be shown as faulty
What is the maximum bend of a semi conductor loop?
3.2 MM
What is the minimum bend radius of a semi conductor loop?
25.4 MM
What makes up a modern gas pressure loop?
Two pressure switches and a resistor
What are the two purposes of the pressure switches in a gas loop?
One is a monitor switch, and is typically closed by the gas pressure, typically the pressure in the loop is two times normal atmospheric pressure.
The second is the alarm switch which closes at a higher pressure.
What is the purpose of the resistor at in the gas loop?
The resistor is the responder, the control unit uses the resistor to know is the switch is open or closed.
What hapens to the resistor is the fire switch is triggered?
The fire switch triggers and the resistor is bypassed causing the alarm to be triggered by the control unit
What are the conditions orequied to trigger an alarm in a gas loop?
Either the whole loop has an average temp of 20 degrees, or a small section is heated to 400 degrees.
