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High Earth Fault Loop Impedance on TN-S (106ohms)


 
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PhilipKS

from United Kingdom

Joined: 19 Apr 2008
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Location: United Kingdom

PostPosted: Mon Mar 09, 2009 8:01 pm Reply with quote

Hi, any thoughts very welcome:

Domestic Electrical Installation Certificate from March 2004 (full re-wire by NICEIC approved contractor - previous owners) gives measured Zs as 106.6 ohms on TN-S Supply.

Explanation on cert given:

"High ohms on main cable sheath earth 106.6 ohms. All Zs readings high due to this: I have fitted an RCD 100A 100mA to the incomer to compensate"

Circuit Zs vary from 106.0 to 107.6 on the cert.

Have done basic confirmation with new Martindale EZ150 and this indicates earth loop in 10 - 100ohms range.

E-ON Central Networks website indicates 0.8 ohms typical max for TN-S, and I'm near to a town centre and on underground supply.

I assume the RCD on the incomer is to ensure disconnection times achieved, but should this be investigated further? Seems a bit like fixing the symptoms rather than the cause? Should I contact E-ON to see if there is an explanation or known issue in my street?

Thanks
Philip
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flameport

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PostPosted: Mon Mar 09, 2009 8:16 pm Reply with quote

Fitting the RCD was the correct thing to do - they should have also installed an earth rod at the same time.

The problem should be reported to the DNO, as they have a duty to maintain the earth connection.
If it is an old supply cable, it is likely the metallic cable sheath has rotted away underground. If not repaired, it will only get worse.
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RF Lighting

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PostPosted: Mon Mar 09, 2009 8:19 pm Reply with quote

They should have an emergency phone number for reporting dangerous installations.

I recently reported a high Ze to YEDL (my local DNO) at 5.00 pm. They had someone on site to assess the fault within 3 hours of me ringing them, and the fault was repaired the following morning, all free of charge as it is a fault on their equipment.

I have no idea why your previous electrician chose to fit an RCD rather than having the fault repaired. It seems like complete madness to me.

As it is, you have a non compliant installation.

There is nothing to stop the earth degrading further, and unbeknown to you, leaving you with no earth at all.

That would obviously be very dangerous, and also render your RCD next to useless.

I would engage a different electrician to first of all check the rest of the installation is in good working order, and confirm the high Ze.

A decent electrician should contact your DNO for you and arrange the repair.
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securespark

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PostPosted: Mon Mar 09, 2009 11:40 pm Reply with quote

But wait a sec...is it TN-S? Or is it subterranean TT?

If it is TT, the DNO have no responsibility for an earth terminal.
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PhilipKS

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Location: United Kingdom

PostPosted: Tue Mar 10, 2009 3:02 pm Reply with quote

EON have been...(very quick response)

They took Zs reading of 130+ and were able to confirm that there is NO earth connection to my house, the neighbours and very probably the whole street.

The only earthing I'm getting is from the buried lead sheath on the supply cable between the house and road (topped up by the cross bonding on the water / gas!).

So they say, despite there being an earth point soldered into the top of the lead sheath, there may never have been an earth and this may have been soldered on by an electrician at some point...

They have found a PM-E connection point at the end of the street which is not connected up to the house.

So, I either need to apply to EON for a PME connection, or have an earth rod installed etc and convert to a TT system (at my cost). If I go with the former I need to get a certificate from a sparks to say the install is capable of working with a PME connection. I will obviously have to pay for the certificate but then they will make the PME connection FoC.

Does this make sense? I assume I should go PME route?

Interestingly EON don't see it as their responsibility to write to the other properties and inform them that if they also have a wired earth terminal on top of their supply cable it won't be connected to anything! Presumably it will have to wait until some old dear in a house with a 30yr old installation and no RCD comfirms the hard way that she makes a better earth connection than that which appears to be provided!

4 vans in the street and blokes knocking on doors - they'll be talking about this for weeks! (and its all my fault!) icon_lol.gif

Also...makes a bit of a mockery of having an NICEIC certified install if the high Zs readings were effectively ignored...too much trouble to sort out perhaps? icon_rolleyes.gif

Thanks
Philip
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bongos

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PostPosted: Tue Mar 10, 2009 6:00 pm Reply with quote

PhilipKS wrote:
Also...makes a bit of a mockery of having an NICEIC certified install if the high Zs readings were effectively ignored...too much trouble to sort out perhaps? icon_rolleyes.gif

You would think that making a phone call to EON to get them to have a look wouldn't have been too much effort for the original spark. icon_rolleyes.gif

Take the PME option - it's a much better idea and mention the lack of earth to your neighbours. Maybe you'll all get it sorted if they pass it donw the street. icon_lol.gif
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ban-all-sheds

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PostPosted: Tue Mar 10, 2009 7:57 pm Reply with quote

Yes - get the street involved.

PhilipKS wrote:
If I go with the former I need to get a certificate from a sparks to say the install is capable of working with a PME connection.

You might be able to negotiate a good deal for a sparky to whizz round all the properties - it's only the size of the main earth and bonding conductors that has to be verified as OK for PME.
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Click-Sure

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PostPosted: Wed Mar 11, 2009 12:17 am Reply with quote

Interesting that the NICEIC guy who signed it off was not aware that NICEIC stipulate 100ohms as a maximum passable reading for a TT system* and yet he/she has signed this off as OK for a TN-S!

*BS7671 stiulates 200ohms as the maximum for a TT system.

edit: as per JACKC's post, my bad!


Last edited by Click-Sure on Wed Mar 11, 2009 12:55 am, edited 2 times in total
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Click-Sure

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PostPosted: Wed Mar 11, 2009 12:19 am Reply with quote

ban-all-sheds wrote:

You might be able to negotiate a good deal for a sparky to whizz round all the properties - it's only the size of the main earth and bonding conductors that has to be verified as OK for PME.


Try getting a 16mm2 main earth into a wood backed Wylex Standard...
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JACKC

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PostPosted: Wed Mar 11, 2009 12:38 am Reply with quote

Click-Sure wrote:
Interesting that the NICEIC guy who signed it off was not aware that NICEIC stipulate 100ohms as a minimum passable reading for a TT system* and yet he/she has signed this off as OK for a TN-S!

*BS7671 stiulates 200ohms as the minimum for a TT system.


Have you got your mum's mixed up, I thought it was a max?
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Click-Sure

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PostPosted: Wed Mar 11, 2009 12:53 am Reply with quote

Very true JACKC, I have got mixed up, edited post to correspond.
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securespark

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PostPosted: Wed Mar 11, 2009 9:31 pm Reply with quote

Click-Sure wrote:
ban-all-sheds wrote:

You might be able to negotiate a good deal for a sparky to whizz round all the properties - it's only the size of the main earth and bonding conductors that has to be verified as OK for PME.


Try getting a 16mm2 main earth into a wood backed Wylex Standard...


Easy. Split the strands into two terminals.
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