PME Ze too high

[safesinbad]

on a job a last week in the middle of nowhere, tested Ze on a PME before commencing work, reading was 0.42ohms. Phoned scottish power to report. They came out a few days later(after promising a response within 4 hours) and informed the customer that there was nothing wrong.

What do i do now, change it to a TT to comply with regs or note down on cert that the Ze is a fraction too high

 

[plugwash]

I'd say complain in writing to the DNO.

 

[BSBS]

Having a problem trying to find the actual "maximum Values" for ZE, or im just going blind.. Whats the reg number for them ? Are they actually in the Regs ?

 

[ricicle]

Is it a PME located at the end of a rural overhead or far from a substation ?
have you tried a L to N impedance test - this will give you a good idea as it should be the same as the PME Ze (All parallel paths disconnected).
As long as your Zs's comply then I wouldn't worry too much and tell the customer you will pop back in a year or so just to check that it is not getting worse.
If there is a bad connection further along the network it might possibly show up as excessive lighting dip on larger loads, this could apply to L or N.

 

[RF Lighting]

Were you there when the DNO attended?

About a year ago I reported a high Ze on a TN-S (about 5&#937

The DNO chap cane out and did a Ze test at the incommer and said "it's all fine that mate"

I went and had a look and he had done his Ze reading with the bonding still connected

He retested, and 2 days later there was a new head with PME earth fitted.

 

[1john]

BSBS - tncs 0.35, tns 0.8

 

[ericmark]

There is no maximum see here but they have go to tell you what it is.

Statutory Instrument 2002 No. 2665
The Electricity Safety, Quality and Continuity Regulations 2002 states:-

Information to be provided on request
28. A distributor shall provide, in respect of any existing or proposed consumer's installation which is connected or is to be connected to his network, to any person who can show a reasonable cause for requiring the information, a written statement of -

(a) the maximum prospective short circuit current at the supply terminals;
(b) for low voltage connections, the maximum earth loop impedance of the earth fault path outside the installation;
(c) the type and rating of the distributor's protective device or devices nearest to the supply terminals;
(d) the type of earthing system applicable to the connection; and
(e) the information specified in regulation 27(1),

which apply, or will apply, to that installation.

Single of 3 phase supply up to 100 amps - Cable sheath earth terminal 0.8 ohms
Single phase supply up to 100 amps - PME or PNB earth 0.35 ohms

Is the normal figures given to designers of new builds before a supply is given. To be honest many heads have no real way to work out if TN-S or TN-C-S since the combining and splitting of neutral and earth does not necessary need to be done at the head. The Prospective fault current Ipt should be less than 16 kA ( 1 phase ) see here which is not really helpful but to maintain a supply within the +10% and -6% limits for a 100A supply with TN-C-S the impedance will need to be 0.368 ohms and with a 60A supply 0.613 ohms so that no load does not exceed 266.8 volt or full load not below 216.2 volt simple maths. Now 0.368 ohms is very close to the 0.35 ohms normally considered as max for TN-C-S and I would not think any supplier would want to be running right on the limit so it would seem a reasonable figure to expect. I would expect the Ipt to exceed 625A (with 100A fuse) if the supply is to keep within volt drop limits what did you measure?