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Topic Title: TNC-S Earth voltage above true earth
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Created On: 15 March 2013 07:05 PM
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 15 March 2013 07:05 PM
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AMN

Posts: 644
Joined: 29 June 2007

I was called out today about an installation where there has been reports of "tingling" from people touching the taps when using the showers. This is a sports club fed by a overhead PME supply single phase and appears to be at the end of the line. All bonding is in place and most of the circuits are 30mA RCd'd and certainly all the ones within the shower room are. Using the good old screw driver in the turf outside there is a voltage present between the MET and the "true" earth. With only a small load on there is only a few volts 3-4 V. When the load was increased to 20-30A this rose to over 15 volts, with the possibility of a much higher load increasing this significantly further in theory.
As a temporary measure I fitted a 30mA RCD after the meter and an earth stake and disconnected from the PME earth. No voltage was detectable between the temporary rod and the taps after doing this.
My question, is it likely to be a problem with the supply (dodgy neutral joint) somewhere or a problem within the installation. I was unable to do any insulation testing at the time,but going back tomorrow to get to the bottom of it.

Many thanks for your help.

AMN
 15 March 2013 07:16 PM
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alancapon

Posts: 5698
Joined: 27 December 2005

It will be fault on the suppliers network. In theory with showers, there should be an earthed grid underneath the shower area bonded to the MET. This will ensure that when people use the showers, everything they can reach will be at the same potential. This is also the reason for the endless discussions on an insulating section of pipework for the connection of an outside tap. .

Regards,

Alan.
 15 March 2013 07:42 PM
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slittle

Posts: 3372
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Sounds like my outside tap from last year.

One for the DNO. I suspect if it's overhead fed, the "travelling folk" have stolen the cables from the earthing on the poles.


Stu
 16 March 2013 01:29 PM
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UKPN

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"one for the DNO"

for goodness sake!

Regards
 16 March 2013 02:19 PM
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slittle

Posts: 3372
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Originally posted by: UKPN

"one for the DNO"



for goodness sake!



Regards



We've got to keep you in work otherwise the chinese will be looking to sell more of you off....


Stu
 16 March 2013 02:38 PM
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Legh

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Lol, Was that a teddy bear that just came out of the pram ?


Legh

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 16 March 2013 05:19 PM
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AMN

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Poor old DNO been out in the rain all day trying to locate the fault. He made me cringe when he pulled the cutout fuse with an installation running at about 80 amps!! How lazy is that.? I get the feeling I will get the call to say they can't find anything. This being the case and the fact that I have now TT'd and RCD'd it what is the safety implications if the neutral should be lost altogether ? Can I insist the fault is found and dealt with ?

AMN
 16 March 2013 07:11 PM
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slittle

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It depends where the fault is.

If it's on your service cable then the result will be the lights go out.

If it's further back up the main then a lost neutral can result in single phase supplies seeing voltages heading towards 400 volts between L-N depending on loads and network conditions at the time.

I don't see you can insist, but if they have been working hard all day in the rain that suggests to me that they know there is an issue and are trying to find it.

Stu
 16 March 2013 07:48 PM
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jcm256

Posts: 1792
Joined: 01 April 2006

Well this one say:


3.15.4 Sports Pavilions
Remote sports pavilions having PME supplies may present problems due to out of balance
conditions, particularly if the service is of an appreciable length. Low values of voltage may
appear on the neutral due to out of balance conditions. No problems will exist if concrete
floors have an earth grid bonded to the earthing terminal. However, in the majority of cases
the earth grid will not be installed and low levels of voltage could occur between the
metalwork which is bonded to the neutral conductor and the general mass of earth
(concrete floor) causing unpleasant sensations.
If an earth grid cannot be installed the customer should be advised to install a residual
current device and the PME earthing facility should not be provided.
Where a sports pavilion is of timber construction, including the floor, then the difference in
potential problem will not exist and a PME earthing terminal can be provided.
 16 March 2013 08:27 PM
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UKPN

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"how lazy is that"

so says the voice of experience!

Regards
 16 March 2013 08:31 PM
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UKPN

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"can I insist?"

we can disconnect the supply fella, this installation is a danger to the public. Its not the network at fault here, its the installation not meeting
our regulations.

Regards
 16 March 2013 08:40 PM
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AMN

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Joined: 29 June 2007

Originally posted by: UKPN

"how lazy is that"



so says the voice of experience!


Sorry don't understand your implication there. My primary concern is the safety of the customer and their members. I was simply stating that to pull a fuse under load is plain daft, when you can easily isolate the load and do it safely.

AMN
 16 March 2013 08:46 PM
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AMN

Posts: 644
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Originally posted by: UKPN

"can I insist?"

we can disconnect the supply fella, this installation is a danger to the public. Its not the network at fault here, its the installation not meeting

our regulations.



Yes hence my reason for asking. I have done all I can to make it safe as the post above reinforces what I have done by tt' ing it and providing RCD protection. Or am I still be lazy ?

Clearly the network is at fault and the DNO know it.

AMN
 16 March 2013 08:50 PM
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paulskyrme

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Is it really?

Please remember that HSE have a note at the front of BS7671 that ys along the lines of an install to earlier editions of the regulations is not necessarily dangerous, just because the new version has been published.
So, can you prove from where you are with the information you have that the consumers install is dangerous?
If so on what grounds please.
 16 March 2013 08:59 PM
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slittle

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Originally posted by: UKPN

"can I insist?"



we can disconnect the supply fella, this installation is a danger to the public. Its not the network at fault here, its the installation not meeting

our regulations.



Regards



Can't see how you can say that it doesn't meet regulations. We don't know there is or isn't an earthed grid in the floor of the shower room. OK section 26 gives you the right to disconnect but I fail to see on what grounds at the moment.

When the supply to this building was connected by the supplier, their metering operator must have been happy that it meet the requirements for a PME connection otherwise it wouldn't of been connected. If it was connected back in the day when the electricity board done it all, I'd be pretty certain it would of had to be 100% right.

As to pulling the fuse on load, why not. Always adds excitement to the day


.

Stu
 16 March 2013 10:15 PM
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AMN

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Joined: 29 June 2007

Well just had an update from the DNO engineer. They haven't managed to locate the fault yet and are returning tomorrow. He did however suspect a local PV set up is contributing to the problem. Meeting him on site tomorrow so hopefully he can elaborate.

AMN
 17 March 2013 02:48 PM
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prophet

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I had a similar situation on a converted farm house in the country, with a potential between the taps and the drain (i seem to remember, it was a few years ago). After hours of searching and resorting to asking the DNO guys who happend to be working down the road, we resorted to turning next doors board off and hey presto it went. Turns out it was next doors water pump at fault

Don't know the outcome, as i was only asked to find the fault.

So theres a small chance it might not be the DNO
 17 March 2013 05:19 PM
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jcm256

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It is a good that not many take a shower when the sunshines, otherwise people would be leaping about from pillar to post. Tabloids hunt around forums like this for gossip, just watch what you say what you come up with in your hunt for an answer to this tingling question. The main requirements are that the answer should be logical, the evidence marshalled should be relevant, and the reasoning made clear to the readers of this forum.
Metaphorically speaking Electricians and Engineers, cannot flout conventional rules, one section of blame not developed to the exclusion of another merely because it is more interesting and may not lead to a proper conclusion?
 18 March 2013 10:47 AM
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AJJewsbury

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With only a small load on there is only a few volts 3-4 V. When the load was increased to 20-30A this rose to over 15 volts

The supplier is permitted to loose 16% or 230V in their supply lines = 36.8V and so for a single phase supply it might be reasonable to expect half of that to be along the supply N, so in the region of 18.4V. So seeing 15V above true earth shouldn't of itself be thought of as exceptional.

But for it to increase by 11V for say a 25A increase in load implies a impedance of around 0.44 Ohms TO EARTH - if that was simply due to the impedance of the single-phase supply lines with all joints sound, then the overall loop impedance would be expected to be around twice that - 0.88 Ohms - which would mean the supply would have to be limited to around 40A to stay within statutory supply v.d. limits. Presuming the supply is actually rated between 60A and 100A, then there would seem to be a real problem.

Even if the installation is TT'd (which seems like a good idea anyway - ideally eventually with something that offers a bit more discrimination than a front-end 30mA RCD) the high loop impedance will likely cause low voltage problems at high loads, so should be looked into all the same.

- Andy.
 18 March 2013 09:01 PM
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UKPN

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how many calculations can one forum take?


a normal, rural, end of line supply, long lead-ins and an
incorrectly utilised PME terminal.

solution=
a) equipotential grid. (not practical here).
b) isolate the pipes |(plastic inserts)
c) rccd that part of the installation (not practical here)
d) rccd the whole installation

Regards

UKPN. achieving a balance between theory and practice
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