Earth Fault Levels - UK 2

[Prospero]

For the UK . .

I work for a large utility in the Sth of England.

We have a DNO substation located in a compound on our site, this is a sole supply - we are the only consumer on this transformer. The DNO provides us with a 500KVA supply at 415VAC, the LV tails are 25m long and terminate into the back of our main LV switchboard via the electricity supply meter. The supply type ie TN-S or PME, is unknown

The DNO supply and electrical installation is circa 25 yrs old.

Some recent project work has highlighted a high earth loop impedance reading of 0.17 Ohm. We have raised this as a problem with the DNO but thier first response is that as the earth fault loop impedance is below 0.35 or 0.8 Ohms (depending on the type of supply system) there is no problem!

These impedance values quoted by the DNO can't be reasonable or fault clearance times would never be satisfied for supplies greater than circa 40KVA! So who is right who is wrong? And where are, and how are their legal obligations for earth fault levels defined and how can we induce the DNO to investigate this matter at their expense?

PS we do not want RCD protection.

 

[ScottyUK]

The DNO's system is not governed by the requirements of the BS7671 wiring regs. The earthing electrode's primary purpose is to protect their system by providing a return path for fault current which is sufficiently low impedance to clear the fault before their equipment is damaged.

The electricity supply regs allow a value of up to 20 ohms for an LV earthing system resistance, but under normal conditions a much better earth should be achieved. Your company almost certainly IS expected to comply with BS 7671, so you must ensure that YOUR installation and YOUR protection scheme meets the requirement within BS7671 for disconnection within 5 seconds of fixed equipment. It may require some form of earth fault relay to achieve this requirement.

Before you discount 'RCDs', have you considered that there are earth fault relays which operate on a similar principle to an RCD which can provide a time-delayed response and a tripping level of several amps, perhaps as high as 50A? Merlin Gerin's Vigi series of earth fault relays is typical, but most of the manufacturers have something similar. This obviously assumes that your incoming device is a circuit breaker rather than fuses.

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Sometimes I only open my mouth to swap feet...

 

[isquaredr]

the values quoted by your dno are correct 0.3-ohms for pme and 0.8 ohms for tns, unfortunately to comply with BS7671 wiring regs you would have to fuse at 200amps, is a 200amp supply big enough, if not as scottyuk has already said the only way to go would be a cb with apropriate relay.
By the way where was the earth loop impeadance taken ?

 

[wareagle]

Prospero, Scotty and isquaredr
I am in the US and have some questions about your post.

What is a DNO substation and DNO?

Quote"Some recent project work has highlighted a high earth loop impedance reading of 0.17 Ohm. We have raised this as a problem with the DNO but their first response is that as the earth fault loop impedance is below 0.35 or 0.8 Ohms (depending on the type of supply system) there is no problem! "
Are you saying that the impedance of earth is
measured and found to be 0.17 ohms. I have measured
earth impedance in the range of 1500 ohms. How could the
reading be that low?

Quote"These impedance values quoted by the DNO can't be reasonable or fault clearance times would never be satisfied for supplies greater than circa 40KVA! So who is right who is wrong? And where are, and how are their legal obligations for earth fault levels defined and how can we induce the DNO to investigate this matter at
their expense?
In the US you are not allowed to use earth as a fault
return path for low voltage installations. Is that what you are indicating?

Thanks
Bob

 

[ScottyUK]

Hi Bob,

DNO - Distribution Network Operator. Formerly the regional boards and then the Regional Electricity Companies (RECs). The company responsible for operation of the distribution network, which in the UK is 132kV and below (except Scotland where I think it is 66kV and below - the 132kV is classed as a transmission voltage up there). The DNO is the company who all but the very large consumers deal with to connect to the power system. The consumer isn't bound to buy from the local DNO, but the power is physically taken from the network operated by them.

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Sometimes I only open my mouth to swap feet...

 

[Prospero]

ScottyUK + isquaredr
Where does the 20 Ohm and 0.35/0.8 Ohm values come from? - I have read the Electricty Safety, Quality + Continuity Regs and there is nothing so specific in there!?

The loop Z was taken at the incoming side or our LV ACB

Wareagle
A large part of our electrical regulations are taken up by earthing systems - Surely you use such systems in the US??

 

[ScottyUK]

Electricity Supply Regulations 1988 is pretty definitive about 20[Ω].

http://www.opsi.gov.uk/SI/si1988/Uksi_19881057_en_3.htm#mdiv6

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Sometimes I only open my mouth to swap feet...