When i do EFLI test with my Fluke 1653 i tend to use the three wire test with a probe on each of the three conductors. This also includes a no-trip facility and as most installations that i test have an RCD involved i can only use this method.

I can also test with this meter using a two wire test facility, so one probe on line and another on the CPC. This will trip an RCD if present.

Working this week in a rather ancient industrial facility i did a three wire test on a lighting circuit and go 1.3 ohms. As no RCDs were in use i decided to switch to the two wire test and got less than half of that reading.

Can anyone explain why? This alslo begs the question which to record on the paperwork?

Thanks
john

Did you swap the leads over in the tester (or at the terminals at which you were testing)? If you just switched to the two wire test from the three wire test without swapping the leads then you will have tested between Line and Neutral rather than Line and Earth.

I have a 1651 John and i've always found quite a difference between the readings of 2 wire or 3 wire tests. I used to use 2 wire for pfc/pscc tests, but I find it more reliable to use 3 wires and connect N-E together now,

Dave.

Thanks Primo. I definately tested between line and CPC. Dave i tend to do the two wire test when measuring Ze and PFC. I'll give your method a go.

John

My megger usually adds 0.1 Ohms when using the no-trip 3-wire test. If you're getting 0.65 Ohms difference, I suspect something oxidised or a bit loose somewhere and the higher current (10A, 25A?) of the 2-wire test is breaking it down to give a decent current flow, whereas the <15mA of the 3-wire test is finding the original resistance.
- Andy.