EICR high Ze reading query

The Ze and Zs measurement is used to determine if the main fuse will blow within 5 seconds.

For that 80 amp fuse the required measurement is around 0.575 to 0.6 ohms, yours is 0.65 ohms.

This is not a precise measurement as the test result depends on network conditions and meter accuracy, so it’s fair to say the measurement is on the limit of what is acceptable.

If some connections are cleaned up or cables increased in size before the consumer unit it might be improved upon, but an electrician cannot take the suppliers equipment apart to check it out and there might not be anything that can be done other than reduce the size of the main fuse to 60 amps.

It is something you need to be aware of and monitor, if it gets worse then action needs to be taken.

https://www.tradingdepot.co.uk/info/electrical-2/lawson-fuses/lawson-fuses-application-data/

 

If there’s other remedial work to do ask the electrician to investigate whiles do that, it may just need the main earth tidying up.

I doubt the electric supplier would attend unless it gets worse.

 

Having said that the Prospective Short Circuit Current test result is low, I suspect they have not tested it correctly and have taken it from the Ze test between live and earth rather than between live and neutral.

If the PSC test result is correct then it could be a bad live connection on the DNO side or the property could be a long way from the transformer.

Is the suppliers main fuse in a damp cellar or somewhere similar?

As it's borderline then no further action may be required.

 

It's the joys of testing, you press the button and get a bit of an odd test result then have to work out why.

Basically the Ze test is to ensure the fuse will blow and the PSC test is to ensure the whole consumer unit won't blow off the wall.

 

TN-S means that the Neutral and Earth are Separate.
As such, you would expect there to be different values for PEFC and PSSC.
This is because the Earth cable will be a smaller CSA than the Neutral.
Also because the Earth with TN-S is usually formed by the sheath of the supply cable, there can be corrosion which would reduce the the conductivity.

Maximum resistance allowed for TN-S is 0.8 Ohms, 0.65 is pretty good, and probably means a lot of the network has been converted to PME (TN-C-S).

you can ask the supplier to convert your supply to PME.
Might be free, might be a charge.

 

The note in the front of the IET Onsite Guide says the typical values quoted by the electricity distributors are TN-C-S 0.35 ohms and TN-S 0.8 ohms.

Whilst you could have a TN-S supply with a Ze of 0.8 ohms it would mean you could only have a supply fused at 45 amps if you are relying on the suppliers fuse for earth fault protection without an upfront RCD.

A bit of tinkering tidying up connections may reduce the test result, but so long as the meter tails are short and there's a tidy consumer unit I wouldn't code it anymore than code 3 so as to bring it to your attention.

 

The fuse needs to blow within 5 seconds, it takes Michael Caine around 9 seconds to do that count down in "The Italian Job", 5 seconds is actually quite a long time because as I said in another post recently:

Generally in the UK the suppliers' main fuse is used to provide both overload and fault protection to:

1. The meter tails and metering equipment.
2. Between live parts and the consumer unit metal enclosure.
3. The main switch.
4. The internal tails.
5. The RCDs.
6. The surge protection device and its pig tails.
7. The busbars.

So you don't want a particularly slow blowing fuse.