UK Zs Readings seem to be a bit low.

[alexander jackson]

Gents.
I did some earth loop tests on Rolec power pedestals (2x 16 amp outlets) that are harbour side, looped in and out from one another.
Three months ago I got Zs readings from about 1.2 ohms to 0.50 ohms. This time round the readings are all less than 0.01 ohm.
Megger MFT meter.
The RCD in each meter trips off just fine.
I am using a LO 3 wire test as I am plugging into the sockets (as usual) cannot take readings directly from terminals as there are over 300 to do.
What would cause such a drop in values, we are nowhere near a substation etc, anyway I would have seen low readings before if this was the case.
Any ideas ?
Many thanks.

 

[pc1966]

Can you try a Zs test somewhere known (e.g. you own home) to see if it is sane? My suspicion is the MFT is faulty in some way.

Are they the Megger test leads? My own DiLOG seems to be calibrated for its own probes, though that could not account for as much as 0.5 ohm as I think they measure less anyway.

As low as 0.01 really would only be possible at a substation, and not a small one either!

 

[alexander jackson]

Can you try a Zs test somewhere known (e.g. you own home) to see if it is sane? My suspicion is the MFT is faulty in some way.

Are they the Megger test leads? My own DiLOG seems to be calibrated for its own probes, though that could not account for as much as 0.5 ohm as I think they measure less anyway.

As low as 0.01 really would only be possible at a substation, and not a small one either!

Megger just calibrated and I have been doing the tests for a few days now, it is just these few that are odd.

 

[pc1966]

Megger just calibrated and I have been doing the tests for a few days now, it is just these few that are odd.

Are both the PSCC and PEFC results strange, or is it just the Zs/PEFC ones?

 

[alexander jackson]

Are both the PSCC and PEFC results strange, or is it just the Zs/PEFC ones?

Just the Zs ones. I am going to do a Hi 2 lead test on them tomorrow.

 

[timhoward]

The "old way" might be worth a try. Isolate and lock off circuit. Short R1+R2 at the socket. Then 2 wire high current test at the CU between bus bar and outgoing live. It's effectively extending the CPC back to the CU using R1.

Megger's have separate calibration settings for low current and high current. Low current tests can of course vary wildly in accuracy but 0.01 ohms is just daft.

 

[telectrix]

sure you nit got tester on the 500V IR setting?

 

[alexander jackson]

The "old way" might be worth a try. Isolate and lock off circuit. Short R1+R2 at the socket. Then 2 wire high current test at the CU between bus bar and outgoing live. It's effectively extending the CPC back to the CU using R1.

Megger's have separate calibration settings for low current and high current. Low current tests can of course vary wildly in accuracy but 0.01 ohms is just daft.

Many thanks for the input, as it was ----ing down today I will have to check this out tomorrow.