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Topic Title: TPN Dist board used as single phase
Topic Summary: any views?
Created On: 02 March 2015 10:03 PM
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 02 March 2015 10:03 PM
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Zs

Posts: 3830
Joined: 20 July 2006

Hello,

Please forgive my typing...it is a laptop in a freezing cold hotel and the proper keyboard is in the van at the job. They honoured me with secure parking so I walked here. I'm not good at laptop but giving this a shot.

I have today encountered something I am not familiar with and in truth I am not comfortable with it. I'd like to run this past you if you have time. This is an I&T of a large, prestigious apartment block. A valued client.

The DBs for the landlord services such as cleaners' sockets and corridor lighting are Memshield 6 way (I think range 3) DBs and are Three Phase and Neutral boards. The main switches are only three phase and the neutral section is blanked, so the neutral is just bolted to the bar. I'm ok with that. On the whole the installation looks to be good and professional. Day one here and only a couple of silly mistakes have shown up.

But, the DBs have been connected so as to use only one phase. So what we've got is an incoming single phase into L1 and then a 6 or 10mm single core between that and L2 and then another between L2 and L3. Effectively , a daisy chain link making the different phase breakers in the board effective. I feel all young because I've never seen it before in such a grand place as this.

Ultra minimal current demand, about 4 amps on each board and only then when I've run up and down stairs to activate sensors. practically no earth leakage, average 1.7mA on each DB so far.

It works.... Way off what I would do, but any reason for it to be condemned?

Except this, which you might help me to understand a bit better..

A low current l-e Zs test from anywhere in that building with an RCBO trips the relevant RCBO.

Unless every other circuit in the entire 17 floor block is in the off position and I am testing L1, in which case it is sometimes fine. That took some time to establish today and I swear I lost about three pounds of weight.

So, this is what I am thinking; The impedance of the line is being intimidated by being forced to flow through a three phase switch and this has an effect on the Megger's ability to run its Zs test.

When everything is in the on position the impedance of the line goes nuts. Pardon the highly technical explanation....?

Ramp tests, 20mA everywhere. I find that low.

If I push it, I can get the test result on R1 but not one I would record because this is not under normal operating conditions. I've been stood there begging my tester to do it's stuff today and now I have Mantras for Zs tests. 'Pleeeeze Mr Megger...you know you want to.'

Just in case, another piece of test equipment is coming to me tomorrow. the Megger was fine at home yesterday on the test socket. I'll let you know if it is in need of attention but I doubt it. It is well maintained.

I'd appreciate your comments on a DB manufactured for Three phases being used for single phase. Type testing, fit for purpose, beyond warranty, and so on. Also anything you can tell me about the impedance of the line if daisy-chained through all of the phases because I can't get a test result on L2 and L3 circuits with RCBO.

All other tests are OK but I have to tell you that some of the boards haves a piece of insulting (sic) tape inside them that says (from about 5 year's ago) Zs 0.09, temp 1.17....I get about 0.23 and this is no longer a temporary builders' installation.

No original certs I'm afraid.

Any views on the three phase boards being DIY adapted to single phase? I have several but would like to hear yours before I comment. One of mine says it should be fine but get some bloody labels on the boards to tell the next electrician what they are dealing with....

Thank you. Off to reception after a big spell check,to see if I can get a hot water bottle.

I think it is now a readable post and will bring the keyboard from the van tomorrow.

Lovely client by the way, I'd rather not be the bad cop.

Zs
 02 March 2015 10:12 PM
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leckie

Posts: 4439
Joined: 21 November 2008

Hi Zs, well you can do this. In fact MEM used to make a triple pole link kit to enable the conversion. I wouldn't fancy doing a cable link though.
 02 March 2015 10:13 PM
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dickllewellyn

Posts: 1410
Joined: 19 March 2010

Evening Zs.

The single phase three phase board is a pretty common beast to be honest. I use them regularly, although I do treat myself to the manufacturers own single phase incomes kit which seems to have been dismissed in your case.

I'm afraid I can't help with the test results, other than to say that memshield 3 RCBOs can be a bit of a pain when testing. I've had many problems with RCD tests where they won't work unless there is a load connected and so on, I wonder if something that causes that is causing you problems with Zs testing?

Is it a 17 series Megger? I find they are more temperamental than the old 15 series when it comes to loop testing, sometimes swapping the Polarity round used to help on the two wire low current test would solve things (advice from Megger), since the firmware upgrade to include the three wire test I've not had so many problems. In general though, I still don't particularly love the low current loop test of the 17 series. A shame, as the rest of the features are brilliant and definitely my meter of choice.

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 02 March 2015 10:21 PM
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Zs

Posts: 3830
Joined: 20 July 2006

Thank you. It is a 1730 and I had the firmware upgrade when I moaned about it.

I assume the genuine conversion kit is a chunk of bus bar and not a few strings of spare cable?

I'll watch on here and see what you all think. Tomorrow I might swap an RCBO for one or two in the van and see what happens. I ran out of time today.....

But thanks so far, really helpful.

Zs
 02 March 2015 10:21 PM
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leckie

Posts: 4439
Joined: 21 November 2008

This is a bigger version of the things I have used before
http://www.edwardes.co.uk/en/p...-single-phase-link-kit
 02 March 2015 10:24 PM
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leckie

Posts: 4439
Joined: 21 November 2008

The thing I have used before was a bit of copper with three prongs, just like a standard busbar from a consumer unit except only three prongs. And loads more money for no possible reason!
 02 March 2015 11:11 PM
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iomtt

Posts: 54
Joined: 30 April 2008

Hi Zs

No problem with the tp&n DB used as single phase as long as labelled and preferably with the manufacturers link kit which unlike your looped out cables is on the load side of the Incomer

On the Zs reading taking out the rcbo , I have a megger 1730 MFT which trips a certain brand of RCBO , as to which I can't remember the make

Havellls I think

Another tester worked fine

Must point out having used Havellls once , not used since!
 02 March 2015 11:54 PM
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AJJewsbury

Posts: 16102
Joined: 13 August 2003

The thing I have used before was a bit of copper with three prongs, just like a standard busbar from a consumer unit except only three prongs

Like these: https://www.tlc-direct.co.uk/Products/HGJK250SP.html ?
 03 March 2015 08:32 AM
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mapj1

Posts: 9692
Joined: 22 July 2004

As everyone has noted 3 phase strapped as one is fine - though usually done with the comb kit - bits of wire are kind of scruffy, but if done neatly and no sign of thermal distress, I'd smile a bit but not worry. - no burn mark, no real impedance to worry about.
So really we have a problem that your RCBOs don't like your meter's cunning way of doing a no-trip test.
Can you take an RCBO out of the box on a circuit no-one cares about and test it somewhere else separately - I suspect they are just a bit frisky - a "30mA" RCD must fire below 30mA and anywhere over 15mA, so 20 is in-spec and is quite believable, just 'frisky'.

You could come over all 1970s and temp bypass the breaker on the circuit during test.
(If you do, please bypass neatly with a live from a fuse or MCB to a spare way please, not just a bit of live core snipped off some flex and stuffed in the company fuse. Actually I'm quite sure you would not dream of it; but some have done that sort of thing for this test, and to join that club you'd need a leather jacket and at least one unsuitable tattoo, perhaps of 'flash' and 'bang' on opposite sets of knuckles. )

Or you could test R2 only by wander lead, and measure L_N loop impedance and halve it to deduce the R1 contribution, and deduce that all is well by adding up the results.


edited for grammar and punctuation.

-------------------------
regards Mike


Edited: 03 March 2015 at 12:06 PM by mapj1
 03 March 2015 09:59 AM
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OMS

Posts: 22428
Joined: 23 March 2004

Single phasing of 3 phase boards is pretty common - although in your case:

1 - You have no means of isolating the neutral (bolted link I think you said) - not neccessarily a breach of BS 7671 (depending on what's upstream of the supply to the Dist.Bd

2 - The home made link kit will have no type approval - MEM make a specific kit for single phasing TP&N Dist.Bd's

So, this is what I am thinking; The impedance of the line is being intimidated by being forced to flow through a three phase switch and this has an effect on the Megger's ability to run its Zs test.


Is it hot? - power is I2R - so if the "I" is constant power lost is proportional to the resistance (impedance) - if the "loops" across the terminals were introducing any real impedance then you'd have a heating problem - looping the switch to make it single phase will add 4/5th of bugger all to the impedance if the terminals are tight

I'd link out (or replace) a few random RCBO's for MCB's and hit the landlord circuits with some high current testing to get a realistic Zs value

All other tests are OK but I have to tell you that some of the boards haves a piece of insulting (sic) tape inside them that says (from about 5 year's ago) Zs 0.09, temp 1.17....I get about 0.23 and this is no longer a temporary builders' installation.


Sounds reasonable - where is the facilty transformer, what rating and can you see the installation through to where you are testing - it might just be worth doing a quick calculation to get the right order of magnitude to compare with measured results (which could only have a resolution of +/- 30% anyway) - also keep in mind the loop test initially was probably in a stone cold installation (both the building and cabling) - now it's under load and has a "reasonable" ambient temperature the resistance (or the resistive component of impedance) will rise with temperature.

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 03 March 2015 06:48 PM
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Zs

Posts: 3830
Joined: 20 July 2006

Evening,

I have a keyboard, but waddayouknow, I left the mouse in the tool bag so I'm still a bit out of sorts. I'll do this slowly.

well, I'm grateful for your comments about the daisy-chaining of the switches. I am sure that my client will be wanting to get those changed for type-tested kit.

I had an oppo with me today because I don't test main switch gear unaccompanied. He uses an older Megger of the 15 series. It trips the breakers too but we have to note that it is a three-lead tester.

However...and Megger users are going to love this. My client on this job is one of our in-house genii. He has had this issue in the past and suggested reversing the polarity on the test. So, the red probe into the earth slot of the Megger and the green probe into the L1 slot. Then proceed as normal. It works. Oh it works.

OMS, I'm right next to the O2 Arena. Crikey the wind howls round here. This is an area of massive development. There are more red dots in the sky from cranes than stars. I expect that five year's ago the supply characteristics were vastly different from how they are today. The whole area has the sound of drills which are like a white noise. So, apart from a search I must make for supply incomers I think these readings are understandable compared to the notes on the sticky tape. I have found one incomer, a 100A Lucy jobbie. there have to be more but buggered if I/we can find them.

Digressing a touch, but this is what happens when you don't get certificates from your installer. You end up paying for someone to come in and do the forensics on the original design. We found a half-helpful drawing in the dust on top of a switch cabinet today, but not enough.

By the end of the week it will make a bit more sense. Softly Softly.

I have no Idea how you journeymen and regular hotel-dwellers survive. Already not liking the lack of 'chosen' food and appreciating how I eat and live day -to day. I'm off on foot in search of fresh veg. It will probably cost me about £20 but has to be done.

But the switch linking daisy chain and the Megger tripping RCBOs don't appear to be connected to each other. Thank you for helping me get to the bottom of it.


Zs
 03 March 2015 07:57 PM
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dickllewellyn

Posts: 1410
Joined: 19 March 2010

So reversing the polarity of the Megger still works? That's interesting to note. I must say I've not had any problems since they did the firmware upgrade, but it definitely worked on the old two wire test, so it's interesting it's still a worthy solution.

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 03 March 2015 08:52 PM
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peteTLM

Posts: 3715
Joined: 31 March 2005

Hi Zs, yes its not the most entertaining area.

May i suggest a quick hop over the thames on the jubilee line to canary wharf, shopping centre (jubilee place) and lots of nice food. There is also your favourite waitrose there as well . Royal china on the upper level overlooking the thames is quite good.
South of the water, there is an excellent indian/ himalyan restaurant a quick drive from where you are, i'll try and find the name of it.
Was talking to your 'boss' yesterday you see..

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 03 March 2015 10:09 PM
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mapj1

Posts: 9692
Joined: 22 July 2004

He has had this issue in the past and suggested reversing the polarity on the test. So, the red probe into the earth slot of the Megger and the green probe into the L1 slot. Then proceed as normal. It works. Oh it works.


Hmm. That is a fascinating trick.
Do you hold the body of the tester or place it on a grounded surface during test? I'd be interested to know if it still trips or not in the same way if placed on an insulating thing like a coffee table, or an up turned plastic bucket, and the test started with the minimum of personal contact.

I'm thinking that the leads on a two wire thingy are perfectly interchangeable, except for any current that gets off through the side, via the capacitance of the case to nearby earthed objects (or indirectly capacitivly earthed things like you and me).
In the tester internals there is quite a bit more metalwork connected to the green terminal than the red one, so perhaps it makes a better capacitive electrode to the operator or the floor or whatever.

But I find it hard to believe that any such displacement current amounts to a row of beans in terms of extra current at 50Hz. However it may be significant for high frequency fast switching stuff, as the tester switches on and off a very small test load very fast, and then looks for very small changes in voltages that are in phase with the test load switching, and averages perhaps a thousand such readings in a second.
Quite surprised its so marginal that such a trick makes a reliable difference.
Very interesting, thank you.

-------------------------
regards Mike
 06 March 2015 05:12 PM
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Zs

Posts: 3830
Joined: 20 July 2006

My pleasure Mike,

and it was a pleasure to learn such a trick. I'd have bypassed the RCBOs but that would have added such a lot of time so I was glad of it. The megger hangs around my neck most of the time so it is moving about while the test is in action. That test takes a few seconds to run, just the usual low current test wait though.

A good week but loads of certs and a report to write up now.

Zs
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