Fault Finding

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How to find a fire alarm fault

Today’s fire alarm panels have come a long way and now provide monitoring of all circuits leaving the control panel which is usually the detection and sounder circuits, power supply, batteries and mains supply.

Conventional and Addressable fire alarm systems monitor the detection and sounder circuits in different ways.

Fault Finding on a Conventional Fire Alarm System

The conventional system has radial circuits for the detection and Sounders. These circuits are monitored by having a device, which can be an EOL (End of Line) resistor or a module, located at the furthest part of each radial circuit.

The control panel continuously looks for the EOL device, should the cable be cut (open circuit) or shorted (Closed circuit) the panel can no longer see the EOL and the panel will initiate a fault with a light and sounder indication. Along with common fault light, you may have an additional light under a zone LED (Light Emitting Diode) or Sounder LED inside the panel. To locate this fault, the engineer will remove the faulty pair of cables from the panel and by using his electrical meter and breaking down the circuit in difference places, will trace the reason for the open or short circuit. When the issue has been cleared the engineer should be able to measure the EOL on his meter and once he can read the unit, he can insert the pair of cables back into the panel and the fault should clear.

Fault Finding on an Addressable Fire Alarm System

On the Addressable systems the circuits are not redials, but start from the fire control panel and return to the panel forming a loop or ring circuit. Due to the system being intelligent, it allows for both detection and Sounders to be wired into the same circuit. (This can also be done on conventional Sav or 2 wire system). However, it is more commonly found on Addressable systems. T

Open Circuit Fault

The loop is continuously monitored by the onboard processor send data to all the devices on the circuit and receiving a reply from each device in a fixed period of time. Should a device not respond within the allocated time period, the panel will register a fault on the LCD display identifying the missing device in the form of a text location message. However, should an open circuit occur, the panel display will show “Open circuit” fault stating which loop it is affected. It is good to note, that all the devices will still operate as the panel can still see all the devices with one open circuit fault.

To locate an open circuit on the detection loop, you need to remove one end of the circuit from the loop terminals, leaving only one end remaining. The control panel will now list as a fault all the devices it can no longer communicate with, the list should proceed right up to the break within the detection loop. If you are fortunate enough to have a drawing showing the device numbers, then this will give you a pretty good idea to the location of the open circuit. You can do the same thing again but this time use an electrical voltmeter to trace the voltage around the circuit.

If you are chasing a short circuit on an addressable circuit, you will have to remove both ends from the loop terminals and use a meter that can read resistance and trace the fault measuring the resistance as you go around the circuit. The closer you get to the short circuit, the lower the resistance.

However, tracing faults can be difficult if there are isolators fitted on the detection loop. If this is the case, it is best to bypass them by linking them out so that the voltage or resistance can be measured by the relevant test equipment.

Earth Faults

Earth faults can be difficult to trace, a starting point, is you have to establish whether the earth is on the fire alarm control panel or the external wiring circuits. To establish this, you have to remove all the external wiring from the control panel, then using an electrical test meter to check to see if the earth fault on the panel has cleared. If it has, you know the fault is on one of the external circuits. However, should it still be on the panel, then by using a process of elimination, you should be able to trace the fault to one of the circuits boards or the power supply.

If the earth is on the external wiring, then first you have to establish which circuit has the fault. You can use a test meter or by placing a circuit back onto the panel one at a time and observe when the earth returns, will tell you which circuit has the problem.

Once you know the circuit, you then have to use your test meter and break the circuit down, into sections until you locate the affected piece of cable.

Intermittent Faults

With intermittent faults, it depends on how frequent the fault occurs, if it is every ten mins or less then you can work on the system until you clear the fault. However, should the fault occur every hour or more, like once a day or week, they are usually very little you can do, except make a change to the affected part of the system. Then by using a process of elimination, return after a suitable period of time and see whether the fault has stopped or moved to another part of the system as per your expectations. On locating the fault, you can make the necessary repair.

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