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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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 18 March 2013 09:47 PM
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AMN

Posts: 644
Joined: 29 June 2007

Originally posted by: UKPN

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


Well I am happy to say Western Power seem to take their customers needs more seriously. Spending most of the weekend out in appalling weather conditions searching for the fault. They have today replaced a length of overhead cabling and replaced the Tx, so clearly they saw good reason for doing so.
Tests would indicate all is well now and I commend them for that level of service. Thank you to all who contributed to the thread.


AMN
 19 March 2013 09:24 AM
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AJJewsbury

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how many calculations can one forum take?

Just Ohm's Law - hopefully not too complicated for those there.

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

But as I tried to show, that doesn't entirely explain the symptoms - even though the voltage was within absolute limits, it seems to be higher than would be expected in the circumstances (30A load) - which could be indicative of a failing CNE conductor for instance - and I'm sure we all want to avoid that, if only for the sake of the neighbours.


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

What's an rccd?

UKPN. achieving a balance between theory and practice

Ignoring the theory to justify doing nothing (on the supply side) in practice?

- Andy.
 19 March 2013 10:47 AM
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OMS

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UKPN. achieving a balance between theory and practice


Ignoring the theory to justify doing nothing (on the supply side) in practice?



- Andy.


LoL - now that's funny, Andy -

Regards

OMS

-------------------------
Failure is always an option
 19 March 2013 08:34 PM
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UKPN

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no real answer here so far, so for electricians who work in any situation
where the integrity of the equipotential zone is compromised, ie bare
concrete floors with a span to earthed metal, whether a rural end of line
service to a village hall or sports club in the middle of a field, or on the
other end of the spectrum, an urban building with bare concrete floor
for the bathroom, the DNO/Meter operator will want to see one of the
above solutions.
failure to comply will not meet the ESQCR regulations and connection
will be refused on safety grounds.

Regards

would your consultant advise PME without equipotential bonding?
 19 March 2013 08:43 PM
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OMS

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would your consultant advise PME without equipotential bonding?


I doubt it given he couldn't defeat the laws of physics on neutral voltage rise and longer feeders to a point where those micro shocks become tangible.

He would also recognise the ever increasing potential of DNO's to fail to maintain networks with the statistical probability of an open circuit PEN beconing ever greater.

I can't see any consulatant advising any earthing system supplied by a DNO without bonding actually - we understand it's purpose - particularly in protecting from shock risk arising outside the installation - ie network faults on the crumbling UK electrical infrastructure

Regards

OMS

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Failure is always an option
 19 March 2013 09:09 PM
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slittle

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Originally posted by: UKPN

no real answer here so far, so for electricians who work in any situation

where the integrity of the equipotential zone is compromised, ie bare

concrete floors with a span to earthed metal, whether a rural end of line

service to a village hall or sports club in the middle of a field, or on the

other end of the spectrum, an urban building with bare concrete floor

for the bathroom, the DNO/Meter operator will want to see one of the

above solutions.

failure to comply will not meet the ESQCR regulations and connection

will be refused on safety grounds.



Regards



would your consultant advise PME without equipotential bonding?



But the DNO connected it originally (or the supplier) so deemed it safe at the time... So either way, in my book the "other side of the meter from where we play" was wrong..... either it shouldn't have been connected as it breached the ESQCR OR it was connected because it was safe but then a network fault created a danger which it would appear western power have gone to great lengths to sort out.
As I suspect would of UK power networks judging by their performance last time I had a fault with similar symptoms.

Stu
 19 March 2013 09:23 PM
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Parsley

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In my experience all the meter guys ask is, where's the water and gas bonded? Most aren't interested in anything else they wouldn't even check if the bathroom had a concrete floor or if there's an outside tap etc and would they understand why it should have an bonded grid or insulated section of pipe work.


I'm sure there are some that would have a good look around and recommend additional precautions, but unfortunately I haven't had the pleasure yet and isn't that why you have those nice labels.

Regards

Regards
 21 March 2013 01:34 PM
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weirdbeard

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I must say that UKPN did mention what would be my favoured solution : b) isolate the pipes

This is what I would call a passive solution, ie not relying on electrical connections or devices.

Even in a healthy TT system with an upfront 30mA rcd you could have 10mA or more potential between bonded pipes and the 'earthy' floor .

I realise that this doesn't necessarily help the sparky called in to solve the problem as we tend to make a living by supplying and installing electrical connections and devices, but if you can't stick a bit of push-fit in yourself, the trick is to get a plumber in to do the work, then stick 20 or 30 % on what they charge

Weirdbeard. Achieving on the balance of plumbers charges
 21 March 2013 03:32 PM
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OMS

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Even in a healthy TT system with an upfront 30mA rcd you could have 10mA or more potential between bonded pipes and the 'earthy' floor .


Could you ? - almost certainly you have a neutral earth fault if you do

Regards

OMS

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Failure is always an option
 21 March 2013 03:43 PM
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AJJewsbury

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Could you ?

Maybe so - with a significant electrode resistance and a reasonable "leakage" current, a reasonable p.d. could develop between the MET and true earth without any faults as such or infringing the 50V limit.

In the OP's case, "tingles" (perceived shocks) were reported with <20V difference.

- Andy.
 21 March 2013 03:58 PM
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weirdbeard

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 21 March 2013 05:42 PM
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OMS

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LoL- I see - it would really have to be a significant electrode resistance though wouldn't it - what happened to circa 200 Ohms ? - and that would still need significant leakage current - beyound the tripping threshold of an RCD?

No main RCD and a big leaky system on RCBO's might achieve that with a poor electrode - not really credible though.

regards

OMS

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Failure is always an option
 21 March 2013 05:51 PM
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AJJewsbury

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200 Ohms is to allow for changes in weather/ground conditions, so <200 Ohms when installed could reasonably end up 500 Ohms ish after a dry summer on sandy soil (still well within the 1667 Ohms 50V limit). 40mA leakage would then give you 20V - and the 100mA S-type front-ender shouldn't even blink at that.

For sure a good solid rod with really low resistance should bring everything down to acceptable levels. I guess it's just a cautionary thought that TT isn't necessarily a magic bullet for touch voltages.

Bring back proper TN-S I say!

- Andy.
 21 March 2013 06:15 PM
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OMS

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OK Andy - I agree it could happen, just that I don't think it's credible in a typical set up.

A 300+ Ohm shift in electrode resistance is quite a big variation, a sensible designer would targeting 200 max having taken a view on ground conditions.

40mA of leakage is still pretty high (or very high in a small installation) and we've still not yet reached 50% of the point where a voltage is dangerous as opposed to "uncomfortable"

Touch voltages in a TT system are usually significantly higher anyway -but yes,plan old commmonor garden TN-S has a lot to be said for it

regards

OMS

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Failure is always an option
 21 March 2013 08:33 PM
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AMN

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Originally posted by: jcm256

I am not happy with the set up you now have, the last sports pavilion (council owned) I tested had a three phase TT supply (Due to the load of about 12 showers that could be on at the same time). The consumer unit (yes you can convert a single phase by buying the sliding insulated bus bars) the insulation was melted in parts mind you there is not much of a gap between the little bus bars. The bus bars showed heat stress one outgoing MCB loose and overheated connection to the bus bar. So to your set up you are now putting maybe several showers, on to a single-phase 30mA RCD. Please at least spread the tails conductors under the terminals if you stick them in round they will fall out, what sort of load you have for a single-phase supply bearing in mind the sports people may be showering at the same time.



They are not electric showers.
 21 March 2013 09:51 PM
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jcm256

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Ok got that, carry on.
Conclusion to the problem still outstanding.

Regards
jcm
IET » Wiring and the regulations » TNC-S Earth voltage above true earth

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