Very low Ze

[rich.250]

Hi Guys,

Have just done a DB change in a Barbers in a high street.

Carried out a Ze and my Megger shows less than 0.01 ohms...!!!
Instantly think that doesn't sound right so try it a few times and it's always no higher than 0.02ohms.
Single phase split concentric TNS supply.
We done the flat above and looking at the cert had a Ze of 0.45ohms.
No transformers around the area as far as I'm aware so don't really know how it can be so low??
The two cables come out of the street in the same place!

Obviously my concern is PFC.
I have installed a small RCBO unit with 6ka breakers.
I didn't have access prior to changing the DB and it is definitely better than the 3036 board that was there lol!
I don't really know of an Rcbo on the market able to handle those sort of fault currents.

I will be returning tomorrow to try again with my my other loop tester aswell.
I need to get this sorted as they are open again on Saturday.

Cheers in advance



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[uksparks]

Are you 100% sure your leads are zeroed?

maybe zero them then check on a calcard

 

[uksparks]

The PFC would probably make the MCBs explode if that's true

 

[rich.250]

Megger don't have the facility to zero on a loop test.
Yes this is my concern and have left the installation off currently.
It's only an 80A supply feeding a very small load.
The PSCC is also very low at 0.02ohms.
I could understand a reading like that if it had an onsite transformer....


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[Simonslimline]

Are you 100% sure your leads are zeroed?

maybe zero them then check on a calcard

On an EFLI test!

 

[rich.250]

On an EFLI test!

I've never seen a live calcard...


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[Richard Burns]

The low reading whilst being far from a local substation does seem odd; there is a low EFLI test procedure whereby you insert a known resistance into the circuit and subtract this from the reading so that the EFLI is in the more accurate range of the tester and will give a more precise figure.
Remember that UK consumer units have to be tested to annex ZB of bs61439-3 that permits a fault current up to 16kA if you have BS88 or similar fuses at the service head.

 

[Simonslimline]

I've never seen a live calcard...


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Nulling for low ohms continuity perhaps, but not a loop test.

I use a megger 1552.

 

[rich.250]

The low reading whilst being far from a local substation does seem odd; there is a low EFLI test procedure whereby you insert a known resistance into the circuit and subtract this from the reading so that the EFLI is in the more accurate range of the tester and will give a more precise figure.
Remember that UK consumer units have to be tested to annex ZB of bs61439-3 that permits a fault current up to 16kA if you have BS88 or similar fuses at the service head.

Thanks for that.
Yes
I'm aware the enclosure should be fine, it's the RCBO's I'm concerned about.
Is there a preferred method of inserting a resistance into the circuit?



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[rich.250]

Nulling for low ohms continuity perhaps, but not a loop test.

I use a megger 1552.

I'm using Megger separates.
I've
Checked my tester on my check box and all appears good.



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[Richard Burns]

The RCBOs must be OK to handle up to 16kA as this is what that test is ensuring.
The low Ze procedure is a sticky in this sub forum http://www.electriciansforums.co.uk...38-higher-accuracy-low-ze-test-procedure.html

 

[Leesparkykent]

The PFC would probably make the MCBs explode if that's true

Doubt it.

 

[rich.250]

You would like to hope so.
Although they have written 6KA on the front.
I will speak to Hager in the morning.


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[davesparks]

The PFC would probably make the MCBs explode if that's true

That will depend on which brand he has used, there's a few interesting videos out there of different MCBs being tested at their rated breaking capacity, some quite scary stuff in the cheaper types.

 

[Leesparkykent]

http://hager.co.uk/files/download/0/14956_1/0/uk_hager_ins_zd0632.pdf

 

[rich.250]

Hi guys,
After returning today and measuring Ze using the method in the link from Richard burns.
I have established a constant 0.04 ohms.
I'm guessing it proves the unreliable nature of testing a low levels using a meter to two decimal places.
So happy days everything is within parameters.
May have to invest in a loop tester to 3 decimal places.
Thanks for all your help.
Cheers


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