LOW ZE READINGS

[Wilko]

But if the client asks how we resolve it were do I go with it

I can’t focus on the graph just now, but perhaps pick a fuse with less let through?

 

[Marvo]

At 0.02 ohms you're way outside the range of accuracy for any normal tester.

Make a 3 separate Zs tests at a socket at least 10 or 15 meters away from the point of supply. Note the average result.

Remeasure the Zs 3 times with a wander lead in series with the tester earth lead and take the average reading. Hopefully it's at least 0.5 ohms, if it's much less try find a socket outlet that's further away from the supply and use that one instead.

Take the first Zs reading from the second Zs reading to get an accurate measurement for the wanderlead.

Use the wanderlead in series with your tester earth probe and take a Ze measurement.

Take your test result and subtract the known wanderlead impedance. This should give you a far more accurate Ze result

Even if the new and hopefully more accurate result is 0.01 or 0.02 ohms higher than your original result it will have a profound effect on the fault current by calculation.

 

[Farmelectrics]

marvo thank you take your thoughts on board. might of been easy just putting max value on saves me a headache.

 

[Dartlec]

Ran into a very similar issue today in a nasty Barratt flat. Ze of 0.08, but L-N 0.03 according to my Megger, though it gave me a calculated pfc of 6.3k which suggests it was calculating to more digits than it was showing me.

Anyway, sub main to the flat 16mm T&E protected by a 63A MCB with 6kA rating. Also a main fuse somewhere, but not in the same building it appears (red fuselink).

Given that the source feed is an MEM enclosure with MEM breaker, and the flat board is LeGrand throughout, I'm assuming I can rely on the type testing for 16kA so no need to consider coding in my case.

As pointed out, the extra 0.01 can make quite a difference.

 

[pc1966]

Sadly MFT don't have the option for 4-lead Kelvin style measurements, and the ones that do cost something like x10 more so not an option for most folk

GN3 points out the impact that the odd 0.01 ohm error can make at high PFC value.

As Julie already mentioned, if there is the typical incomer BS88 fuse of 100A or so it is going to limit the PFC let-through to a value that will be safe. Remember, fuses are your friend!