Engineering Tip – Locating Cables with Earth Faults
The Kentec range of control panels are designed and certified to meet the requirements of EN54-2 and EN54-4.
Clause 8.2.4.c of EN54-2 specifies that a fault indication should be given for any single earth fault which affects a mandatory function and is not otherwise indicated as a fault of a supervised function.
The purpose of reporting the fault is the panel should continue to operate satisfactorily with any single earth fault present. However if two earth faults occur on the system then there will be current paths between the affected circuits, with unpredictable consequences. The worst case is that a positive line is shorted to a negative line via earth – effectively shorting the power supply and rendering the panel inoperative.
If a panel is reporting an earth fault then this should be resolved as soon as possible, to reduce the possible risk of damage arising from further earth faults.
The technique to find earth faults is as follows.
1. Carefully remove all field cabling from the control panel and ensure that there is no earth fault reported. If the panel is reporting an earth fault with all cables removed, this implies either there is a component fault or there is something causing a short circuit between one of the panel circuit boards and the enclosure internally within the panel.
2. Assuming the fault clears with all field cabling removed, connect a Digital Volt Meter (DVM) set to d.c. volts, between the Aux 24V output “+” terminal and the panel earth.
3. Make a note of this d.c voltage.
4. Carefully connect each field cable into the panel one at a time, monitoring the DVM voltage reading as each cable is applied.
5. If there is a significant and permanent change in the DVM reading when a cable is applied, this will suggest that there may be a current path to earth via that cable.
6. If the DVM voltage reduces, this suggests a leakage between the positive core of the cable and earth. If it increases, this suggests a negative to earth leakage current.
Using this technique will allow quick identification of cables with earth leakage currents, without having to wait for the delays associated with the panel monitoring & reporting process.
This technique also will allow indirect earth faults to be determined, for example if a “remote indicator” cable or addressable loop input device switch input has an earth fault – such faults are not easy to find with resistance measurement alone.
However, once the cable that has the fault on has been identified, traditional fault finding techniques will be required to locate the fault itself.