TESTING UNDERGROUND ELECTRIC CIRCUITS
Testing underground circuits involves much more than randomly running around with a multimeter, and improper testing causes more delays in diagnosing cable faults than anything else.
Is It The Cable Or The Device?
This is the first thing that must be determined, is the underground cable bad or is the device itself the problem? If you are working on street lights you could have blown fuses, bad ballasts, or just bad bulbs. Residential electrical services could have bad breakers, faulty interior wiring, or defective appliances. Always test the device and the cable separately.
If the cable is determined to be the problem, is the neutral at fault? Is just one phase in trouble, or do you have multi-phase faults? Is a short circuit on the cable dropping the voltage across the circuit?
These are all things that need to be determined well in advance of using any other equipment to locate a fault.
In order to locate any underground cable fault, two things must be known
You must know, with certainty, the point where the cable is good and the point where the cable is bad
So - how do we do this?
First, all testing should be performed while there is a load on the circuit. Voltage can exist on cable that is faulted, deteriorated, or contains bad splices, but the voltage will drop or disappear completely once a load is applied to the circuit.
Now, with the load applied we can begin testing each section of the circuit. Testing can be started at either end of the circuit. If you begin at the end of the circuit, test each section until you discover proper voltage readings. Inversely, if you begin at the feed and test sections away from the feed point you will be looking for the point where low voltage exists. Once you discover the first point with low voltage and the last point where good voltage exists you are ready to isolate that section from the circuit and begin locating the fault.
Remember to test each phase at every test point, as well as phase voltage to ground and neutral to ground. Be on the lookout for crossed phases and bad connections at each test point. Also be aware that a short circuit towards the end of the circuit can draw the voltage down across the entire circuit. The way to determine if this is happening is simply to isolate the circuit into sections. If the voltage readings return to normal after isolating a section this tells you that a short circuit may exist further along the circuit.
The desired result of circuit testing is to narrow down the faulted section of underground cable as closely as possible and to determine which phase of the cable is faulted. Taking these steps ahead of time greatly increases the accuracy of the underground cable fault locating equipment.
We are now prepared to move on to the next step:
next - CABLE TRACING