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Topic Title: Village Hall High Ze and high voltage
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Created On: 11 March 2013 04:04 PM
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 11 March 2013 04:04 PM
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Zs

Posts: 2630
Joined: 20 July 2006

Hello, I'd appreciate your views on this.

A village hall in the grounds of a church. 18 months old. Never tested until a few week's ago.

It is a new supply, TNCS. The church itself is TT.

I carried out an EICR on both a few weeks' back and the village hall needed a great deal of work to put it right.

Amongst other things a high Ze and a voltage of 255. It is about a quarter of a mile from the sub-station in the village.

The supply authority came out, agreed with me, and said they would get a linesman out. Albeit his test result was only marginally high ( he only ran a high current l-n.)

The Ze as three lead low current is still high today at 0.85 Ohms. High current l-n it is 0.17 . The voltage is 251.

With both kettles on, the heating on and the water heater running on full, the voltage drops to 231. So that is a high percentage.

The entire installation is protected by two RCDs which perform well.

I don't know what, if any work has been carried out on the line since then. It is overhead from the local TT pole about 30 metres away and then down a new pole, new when the building went up, into the ground beside the village hall, up into the supply cut-out as a TN-C-S.

Given that the l-n impedance test result is much better than it was, do you have any views on the safety of this installation as it is now?

The supply authority seem to think it is ok. I'm concerned about the low current test result and the difference between the two. But were I to get the supply authority to confirm in writing that they are happy, do you think this one can be left as it is?

Zs
 11 March 2013 04:17 PM
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AJJewsbury

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The Ze as three lead low current is still high today at 0.85 Ohms. High current l-n it is 0.17

On TN-C-S??
What's a high-current test show L-PE? (before the RCDs of course).

I'm thinking either a loose earth connection or Zs has a dodgy meter...

High current l-n it is 0.17 . The voltage is 251.

With both kettles on, the heating on and the water heater running on full, the voltage drops to 231. So that is a high percentage.

What does that add up to in amps? Loosing 20V over 0.17 Ohms would imply a draw of well over 100A (117.6A by my calculator) - is that feasible? (Either that or some local large load came on at the same time.)
- Andy.
 11 March 2013 04:35 PM
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Zs

Posts: 2630
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The meter is showing normal results here at home right now but believe me, I jumped at that suggestion.....megger 1730.

In terms of Amps, I didn't measure, but I reckon at a real upward push 87 maximum. That's a gross over calcualtion though, using 3kW for the heaters (four of them) which clearly state 2kW on the labels, and 6kW for the water heater which isn't clear but it is on a 45A switch to a 32A breaker and is very small (and not very effective). Obviously, I used 251 as the voltage.

There is no high current l-e test on the 1730, unless you start conning it by juggling how you plug it in, which I didn't do. Or unless you can enlighten me otherwise.

Zs
 11 March 2013 04:59 PM
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dg66

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Con the instrument , do the two lead test, but connect to earth instead of neutral

-------------------------
Regards

Dave(not Cockburn)
 11 March 2013 05:03 PM
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AJJewsbury

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There is no high current l-e test on the 1730, unless you start conning it by juggling how you plug it in, which I didn't do. Or unless you can enlighten me otherwise.

Ah, this new fangled technology. My old megger 1502 just gives you 2 wires on a hi current test ... so you can put them on whatever terminals you like! Can you put red to L and both green and black (blue?) to N?

In terms of Amps, I didn't measure, but I reckon at a real upward push 87 maximum. That's a gross over calcualtion though, using 3kW for the heaters (four of them) which clearly state 2kW on the labels, and 6kW for the water heater which isn't clear but it is on a 45A switch to a 32A breaker and is very small (and not very effective).


So maybe losing 20V with say 80A - that would imply a loop impedance of about 0.25 Ohms - which might suggest your 0.17 reading is a bit low, but at 0.08 Ohms out, maybe not implausibly so. If a neighbour happened to turn a load on at about the same time as your test, that might affect your results too.

On paper things are OK - voltage stays between 216.2V and 253V, Ze below 0.35, so the DNO can certainly claim it's within limits. There's something that doesn't quite add up though - maybe it's a loose or tarnished connection somewhere (maybe the meter's leads if not in the installation).

I'd be happier if you could repeat your loading experiment (I think that's a good practical approach) - perhaps a couple of times to reduce the chances of other loads in the neighbourhood interfering unnoticed, ideally with a clamp meter to measure the actual load, plus a couple more high current loop tests at the origin.

- Andy.
 11 March 2013 05:13 PM
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Jobbo

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Zs

Perhaps im reading this wrong, but on the 1700 series, you press the button with the arrows to switch it to high current. It automatically starts with low current to prevent tripping out of rcds as a safety feature.

Jobbo
 11 March 2013 05:23 PM
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Zs

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That changes the perspective of my question somewhat but I'll go there. You may be right, I just don't know.

Why would a con-job on a test meter make any difference? Surely I buy, and calibrate a meter to do what it should, as purchased? Why would any test which involves doing what we do out of enthusiasm and geekiness really be required? Is it worth going back and would it prove anything? Better to go back and measure the Amps maybe?

But, if there are Complaints About the Megger 1730 she says, making sure it goes onto google because it cost a packet then we all need to know about them.

However, this is a TNCS in a 90% or so, probably more, TT village. I have yet to encouter a home there which is not TT and I have 14 on my books. The voltage is pretty much indisputable because my test socket here at home is showing the same as it does once a fortnight, as it is with a three lead low current Ze and a hign current l-n. I should add that the same high current button also states l-l for your info, so the same button with different names to choose from.

I'm ok with allowing this to go because the l-n is perfectly acceptable and it is a TNCS.

What I'm not ok about is the relationship between TNCS and TT in this location.

It smells funny that a test down the green and yellow doesn't come up to scratch if you see what I'm saying?

Andy, when the supply guy came out last time we checked all the connections and messed around putting the kettles and heating on etc. His meter agreed with mine give or take a volt or two, hence the linesman thing.

If it were a TNCS village, I'd not be asking. My inner alarm is calling to me about a connection or change from TT to TNCS. Does that make sense?

What is the maximum parameter for UK Voltage?

Zs
 11 March 2013 05:31 PM
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AJJewsbury

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What is the maximum parameter for UK Voltage?

230V + 10% = 253V.

I don't think the neighbours being TT makes much of a difference - the supply side is should be configured the same whatever (other than the earthiness of the N being not tested in the field as it were).

It smells funny that a test down the green and yellow doesn't come up to scratch if you see what I'm saying?

I concur - you'll know your meter better than me (mine typically adds 0.1 Ohms on a low current test), so if it's showing different numbers than normal for the same nominal situation, there's an "unanswered question" there still. That fact that it's on the earthing conductor does make it feel even more uncomfortable. Hence I'd do two high current tests, same meter, same leads, same time of day, just one L-N and the other L-PE.

- Andy.
 11 March 2013 05:52 PM
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Zs

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I will gladly do that and get back to you.

It will be a good few days though, I'm driving the desk job as well as the van for a few weeks and the church will be locked by now, where they hide the key to the village hall you see.

Thanks, I will bump this thread when I have been, possibly early next week.

I'll be back... ... ...
 11 March 2013 06:31 PM
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sparkxelectrical

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Zs,

Don't forget UKPN are not bound by the same regulations as us, and if they believe a Zs to be adequate (even if it's outside our parameters) then its ok.....

(I've just been on the phone to a lovely UKPN engineer..)

I have also seen installs where the Zs could not be brought down to a half decent value - the UKPN solution was to install a 4 pole 30ma RCD. It was endless fun on a large multiple occupancy farm......
 11 March 2013 06:53 PM
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dbullard

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Hi Zs,

I have a 1720 .................. when you turn on to Ze function you can choose either 3lead low 2lead hi or 2 lead low it is this darn arrow button <> has yours had the software upgrade ??? which basically turns it in to a 1552/3 series.

I didn't realise until my full scope that the 1700 series can do a functional test on S rcd's only my inspector had the knowledge as the pamphlet that comes with the meter is sheite.

If yours has not had the firmware upgrade get it back to MEGGER also can you check your red leads, as I have had all the red leads break at the point where they enter the meter, I think the red colour is defective or brittle, and I am waiting for the Megger stand at ELEX Exeter to fire one across their bows ...............As I have replaced the 3 lead plug top set and one set of red leads in 12 months, no other colour

edit ::: Zs is it a what I call and home converted TNC-S connection, as I have a farm with some rental cottages on that have a very dodgy PME service where the CPC enters the neutral part of the service head and is sealed, so done by the DNO these have a voltage of 253 also....... but the Ze is fine, Transformer pole mounted around 750 yards away
Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 11 March 2013 07:08 PM
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slittle

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Zs,

I'm assuming this is a single phase TNCS supply and you're not measuring one of several to Neutral ??

Have you stuck a suitably long screwdriver in the soil and tested between the tncs earth and the screwdriver ?? that can be quite revealing.

I have to say I'd be smelling a rat too, it smells to me like there could be a fault on a non rcd TT property pulling the supply away from where it should be.

Is it UKPN's ground or are you up far enough for central networks ?

Stu
 11 March 2013 07:21 PM
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dg66

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When i had the misfortune to be using a 1730, i was doing the high current L-N test for earth loops and connecting to L+E. The instrument doesnt know the difference between N or E .

-------------------------
Regards

Dave(not Cockburn)
 11 March 2013 07:25 PM
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John Peckham

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The 1700 series tester has the same functionality as the LTW range in that you can do a high or low current 2 wire test. For testing at the origin I would always use a 2 wire high current test. Low current or no trip testing is for RCD protected circuits only and unless you are using a Robin 4120DL all the other meters will give fruit machine readings on low current especially if there are harmonics on the circuit e.g there is discharge lighting on the circuit or electronic devices. Why fart about doing 3 wire tests when you can 2 wire test unless you can toggle between LN and L-PE loops? It could be a case of "if all else fails read the instructions" for the meter of course I only do this as a man if I have tried everything else first.


Your 2 wire high current test at the origin should give the same readings L-N and L-PE on a TN-C-S supply as the loop path is the same (a common 2391 question). If you are not getting the same you either do not have a TN-C-S supply or the earthing conductor has a high R where it is connected in to the head (unlikely but not impossible)..

If you do have a loop of around 0.17 ohms then an 87A load would give a volt drop at the origin of 14.79V. That assumes as Andy says there are no other loads on the cable from the Tx which there probably are.

If you have a high voltage at the origin with no load it could be the DNO have jacked up the voltage for the long run of supply cable to compensate for the volt drop under heavily loaded conditions.

I assume we are talking about a single phase supply here? What is the outside diameter of your concentric cable? and how long is the run? Do you know what size the Tx is?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 11 March 2013 07:31 PM
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John Peckham

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PS Meggers come with sh***************************t leads, probes and croc clips both new and old pattern. You have the old set leads I think? These are double Sh*****************t! Get yourself a set of Kewtech leads they seem to be the best around.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 11 March 2013 07:53 PM
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dbullard

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Thanks JP,

Yes the leads are poooooooor but I prefer to get mine free from megger as the ones they supply should last more than a few months, The Kew Teknic range I have a friend who has just replaced a third set, seems we have no quality in anything at this moment in the good old electrical industry ...........

I could rant for ever, but my first port of call for any manufacturer is the rep, I don't get many reps asking me for my views on products anymore as the answer often offends.


Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 11 March 2013 09:58 PM
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UKPN

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its up to the designer to work to the supply parameters. UKPN
dont guarantee loop figures.
this is a rural network, high loops are expected.

Regards
 12 March 2013 01:50 PM
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OMS

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Originally posted by: UKPN

its up to the designer to work to the supply parameters. UKPN

dont guarantee loop figures.

this is a rural network, high loops are expected.

Regards


High loop impedance might be expected - the difference between L-PE and L-N loops is what is causing concern in this case.

With an approximation of load and knowledge of the voltage depression and given the numbers that arise, then certainly something is adrift - more resistance at the point of testing P-E than you'd expect.

As Stu mentioned, I'd test to a screw driver in the flower bed, and I would also suspect an uncleared L-PE in the TT network side of the syetm from which this PME is derived.

I'm pretty sure the designers would work to the supply parameters - but I'm equally sure the Ruthies on the front desk would have quoted 16kA and 0.35 ohms as part of the script

Regards

OMS

-------------------------
Failure is always an option
 12 March 2013 07:16 PM
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OMS

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Thinking about this a bit more, church is on TT, adjacent village hall on TN-C-S, abnormally high supply voltage - would there be a common bonded conductive water pipe between the buildings perchance ?

I'm still thinking a L-E fault (on a different phase to the church hall) pulling the neutral away from earth and this now being expressed on the PEN - depending on how far the neutral displacement is, it could be a scenario of a (negative) voltage on the MET added to the normal line voltage giving you that 253V value.

It's an abnormally high value for a rural supply (even lightly loaded) - add to that a sole TN-C-S system in an area of TT systems and TT systems often suffer unrevealed P-E faults of some impedance.

Test from live and from neutral and from earth to a "clean" earth reference (screwdriver in the lawn for example) - I think you'll measure about 240V ish, about 10 - 12V ish and zero on the end of the earthing conductor when seperated from the neutral at the cut out

Just a guess though

Regards

OMS

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Failure is always an option
 12 March 2013 08:02 PM
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UKPN

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"just a guess though"

"the supply say this service is ok"

Regards
IET » Wiring and the regulations » Village Hall High Ze and high voltage

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