High no trip earth loop readings

[Spaloni]

Hi,

I'm buying a new test kit and was wondering if anyone had a problem with non trip earth loop readings?

This was taken on the same installation- new mk sentry split load rccb CU.

I've got a secondhand, calibrated megger 1711 MFT which I've taken readings on a ring = 1.04 ohms
And previously tested with a fluke 1662 = 0.54 ohms

Same with cooker radial which calculated res should be 0.31 (ze + r1 + r2.) And thr megger read 0.72 ohms!

And the readings are consistent like this on all circuits. No good for me if I'm doing lots of EICRs.

Now I've read that some people experience 0.5 ohm uplift with rcds but I've never had anything this high before!

I need a reliable test kit asap! What are your experiences with these testers, and what would people recommend?

Thanks

Dan

 

[telectrix]

megger MFTs are generally good on no trip Zs testing. less prone to noise than flukes. maybe try and compare readings with a mate's megger, of check yours for calibration.

 

[littlespark]

You can lose/gain ohms with badly connecting test leads.
The tester itself could be calibrated fine.

 

[Spaloni]

megger MFTs are generally good on no trip Zs testing. less prone to noise than flukes. maybe try and compare readings with a mate's megger, of check yours for calibration.

Hi,
Thanks for the reply.

You can lose/gain ohms with badly connecting test leads.
The tester itself could be calibrated fine.

That did cross my kind, but I made sure I was on securely and repeated tests 4 times and got all the same readings?

 

[telectrix]

possiblecause... on the megger, you don't have to null the leads for Zs. i think that on the fluke, you do. this could be the cause.

 

[Vortigern]

I think if I got the readings you originally got and the latter ones I would suspect there is a bad connection/s somewhere on the ring and would have to look around a bit to check if that is true. On the basis of your figures you must have a ring circuit that is very large, can't be bothered to work it out. Have you done the calcs prior to testing that would show what they should be based on the length (estimate) of the circuit?

 

[pc1966]

An error of 0.5 ohm is not outside of the "RCD uplift" figure that you see mentioned.

I suspect that any new MFT are OK for doing reasonably accurate no-trip measurements. My DiLOG 9118 seems to give sane results in both high current and no-trip testing, and in keeping with DC measurements of R1+R2. Sadly not earth rod testing (beyond doing a Ze on it).