Testing of an earth leakage (EL) is important to ensure that it is functioning correctly and is available to react to potential life-threatening emergencies. However, one needs to fully understand what it does and what it does not do, and how it can be influenced by site factors into incorrect operation.
Of course, we need to first know the basics of what the EL does and its role in the electrical system.
Definitions
Earthleakages are designed to monitor and detect diverted current flow and, if above their set threshold, initiate trip. High sensitivity types set for 10/30 mA are used for people protection while lower sensitivities (50/100/250/375/500/1000 mA) are system protectors, typically for mining in South Africa. They can be all in one type, be similar to circuit breakers in shape and have overload functions too. They can be core balance modules for attaching to separate circuit breakers.
Detection is, in most cases, done by passing the line current via a CT (toroidal core) and checking the return current. Should the current be the same, the CT has zero output and therefore no action is taken. If the set point is exceeded, the EL reacts to trip.
Tripping can be immediate or timed tripping depending on the needs. Having an electrical system with the star point of a transformer connected to earth is acceptable.
Test frequency: One of the most hotly contested subjects is how frequently the EL should be tested. Legally, all ELs require a test button and to have the wording “test regularly/frequently” as a label.
The test button circuitry is in fact an electrical injected current which is detected as an imbalance above the technical set point and, if good, the EL will trip. This test is therefore an electrical trip and not a mechanical one. Many testing persons would rather opt for the separate tester route but there is benefit to using the test button method, especially for persons without test equipment, e.g. the home owner.
There is no confusion with the fact that the unit requires regular testing, simply the frequency thereof.
Testing: On testing equipment, opening the test routine with an EL test and closing it would be highly recommended. This is after all a safety device and testing proves the correct function.
Domestic/commercial: For domestic and commercial installations, somewhere between monthly and quarterly would be advisable. Bear in mind that testing may switch off some important equipment in some situations which may not be very tolerable.
Mining use: In South Africa there is widespread EL use underground which should at least be tested monthly if not daily in risk areas. Proof of this testing is also important when events require proof of compliance, so keep records.
Another legal requirement would be EL tests during the CoC (Certificate of Compliance) and at any replacement event. Official record of this function is required.
Test equipment: The most basic of testers insert a resistor set between live and earth. There is no indication of actual current flow so one cannot see if all is well. Should a failure be detected, it may be wise to retest to confirm. A broken circuit/resistor cannot be seen from the test. It may be more obvious if all ELs are suddenly found inoperative. Remember, no manufacturer would have large numbers of faulty products. These testers use Ohm’s Law, so must be voltage-specific.
More upmarket testers will have a mA meter fitted so that current flow can be confirmed during tests. This is the preferred tester which is offers a more confident test routine.
For safety, only use these types on specified voltage and short duration as the resistor heats up in time. Some testers will have thermal cut-outs fitted too. These testers can determine mA tripping only.
Certain models/brands will require the EL to disconnected from the mains supply before insulation tests can be carried out. Confirm with your manufacturer(s).
Three-phase applications would be required to test on each individual phase initially but confirmed on a single phase thereafter.
Many of the leading instrument manufacturers make very able testers ramping currents until trip as well as the tripping time. The only concern may be that the tester gives a test variation. A rapid ramp may be one of these problem areas. Consult with the EL manufacturer when in doubt.
Another area of known conflict is that described below: 1-phase EL indicated as being 30 mA, tests at 22 mA. The question then, is it a fail? The answer would be no.
Specifications for 30 mA in South Africa are +0% – 50% e.g. 15 – 30 mA under qualified tests. The manufacturer will set at say 26 mA, site may add some local variations.
Early model ELs in South Africa were marked 25 mA with tolerance of +-25%, e.g. 15 – 30 mA, technically the same as 30 mA current. There are still many, many of these in service.
Testing of mining types of EL units will require extensive cooperation from both the EL manufacturer as well as from the EL tester manufacturer before launching plant-wide testing and then possibly erroneously claiming a faulty EL.
Remember, only a working EL can save lives!
By Kevin Flack
Flack has recently retired from industry and believes he still has valuable in-depth knowledge of the South African electrical landscape due to wide experiences gained in his 45+ years working career. He is keen to explore the training arena so that this can be passed on to the current youth so as to benefit their practical ability in the electrical field. In these trying COVID-19 times, Flack can provide online training to keep your staff up to date on all aspects of circuit breaker deployment. Email kevin.flack@outlook.com
