When an earth fault warning appears on a fire alarm control panel, it is not always definite that there is an earth fault on an external cable. A fault could have appeared in the circuitry and is just manifesting itself as an earth fault condition. The first step is always to temporarily disconnect the 'incoming' earth (normally the earth of the mains input supply) to the control panel because this is used as a reference to monitor for earth faults. If the fault condition doesn't disappear when you disconnect the 'incoming' earth then it is likely that the internal circuitry is at fault. (please note however that there may be other 'incoming' earths other than that of the mains supply - so check for this) Remember to reconnect the incoming earth after testing - this is required for safety reasons!!!!!
Once the liklihood of a panel circuitry fault is discounted. it is necessary to find out exactly which loop/zone/sounder circuit the earth fault is on since the earth warning indication is normally a general warning and not identified by circuit. To do this, a multimeter can be used to measure the resistance from external circuits to earths to locate the problem circuit. alternatively, all external circuit (loop/zone/sounder etc) earths should be disconnected from the earth terminals and the earth fault warning indication should disappear. Reconnect earth wires one at a time until the fault warning condition reappears. Once it does the last circuit connected is that with the earth fault!
To locate an earth fault on an external circuit, connect just the faulty circuit earth and disconnect the earth wire at various points along that circuit until the fault warning disappears again thereby locating the exact section of cable with the earth fault. The logical way to start breaking the earth on a circuit, is to start at a point approx. half way along and keeping breaking one half or the other (the faulty half) of cable until the fault is precisely located.
Finally, watch out for earth faults on more than one circuit at once. It is possible to have 'slight' earth faults on two or more circuits, that when combined are enough to trigger the earth fault warning condition, but not enough to trigger with just one 'faulty' circuit at a time. These can be a real **** to find!
Main causes of earth faults suddenly appearing on circuits that had previously been clear:
1) Water leaking into detector, sounder, joint box etc. etc.
2) Any recent work that involved drilling or inserting fixings into walls. The customer always tells you that no works have recently been carried out and when you find the fault, it turns out to be the BT man has run new cables, or the hotel owner has fixed a new picture/sign on the wall somewhere and in both cases their fixings has damaged the fire aalrm cables!!!!!
