The following question was posed by one of our customers.
Question:
At work we are having problems with ELI tests on RCD and RCBO we have an AVO tester with 15mA test. We’ve just had it calibrated, it’s a 2 wire lead, and the office sent no instructions. The results are different sometimes on retest and if you bypass the RCBO the results are not the same as if you test on a normal setting 20AMP E.L.I test. Can you help as the bosses are saying we can calculate by adding R1+R2+ze of d.b, I think the live loop test is one of the most important tests?
Answer:
Your question is one that we come across on a regular basis. The problem with performing a non-trip loop test is that as the current is approximately 15mA the readings you get will often differ as noise on the line caused by connected appliances, lighting or PCs etc, can seriously affect the reading you get, as well as the time it takes to perform the test.
What many users don’t look at is the specification for the testers. For instance the accuracy of the LTW325 on the non-trip range is +/- 5%, +/- 0.03 ohm @ 230V, therefore if we connected the unit to a calibrator on a 1 ohm setting, the reading the tester will give can be as low as 0.92 or as high as 1.08. It’s not uncommon to have three or four different non-trip testers test the same circuit and give different readings.
In contrast, the high current loop test will always give a more accurate result as the higher current cancels out any noise, giving a clearer signal for the tester and resulting in a quicker and more accurate result.
On an RCD protected circuit the maximum allowable limit is 1667 ohms, therefore the chances are the readings you’re getting from the non-trip test are well within this limit, and if you have recently had it recalibrated then the tester would appear to be working as expected.
If your still unsure about the readings please feel free to send us the details of the circuit being tested and the readings you have been getting from both the non-trip and high current tests, and we can run them past our calibration engineers.