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Topic Title: Trouble with transformer
Topic Summary: Can there be no incomming circuit protection and is it PME?
Created On: 17 June 2009 07:20 PM
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 17 June 2009 07:20 PM
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alastairfearns

Posts: 6
Joined: 20 August 2008

Hello
I have a two-fold question. I am working on an industrial site where a 1250KVA Transformer is feeding a DB, the max output of the Transformer is 1667A which the cables to the DB are rated for, as are the bus bars within the DB. The Main switch at the DB only has thermal trip that will not operate within 5s (in fact, 60s based on the lowest PFC of 3.33KA between phase and earth and manufacturers data). The cable run from the Transformer to the DB is 8m. All the out going circuits from the DB are suitably protected. Can the main switch be considered simply as a device for isolation, with supply cable and transformer ultimately protected on the 11KV side?

Secondly, the Neutral feed from the transformer (two 400mm2 Cu cables) is linked to the earth bar within the DB via one 70mm2 Cu cable, in addition another 70mm2 Cu cable links the earth bar to the transformer.
I would consider this installation the be PME with the link between the DB and transformer as supplementary, would this be correct? Given the above issue the link between neutral and earth should be at least the same as the neutral itself but would have to be calculated against the energy let through of the transformer and its 11KV protection?

Your help would be much appreciated, if I can provide any further information please don't hesitate to let me know.

Regards
 17 June 2009 11:57 PM
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sfchew

Posts: 589
Joined: 10 December 2002

You are correct to assume the LV switch as an isolating mechanism but it has overload protection. It is not adequately protected though, therefore it depends on 11KV protection if a fault is at the LV busbar. The earth link to neutral need not be the same size as neutral. Adequate protection clearance must be provided if an EF develops whether it is provided by LV or the 11KV end. Regards Chris Chew

Edited: 18 June 2009 at 12:07 AM by sfchew
 18 June 2009 11:22 AM
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alastairfearns

Posts: 6
Joined: 20 August 2008

Thank you Chris,

The protection provided on the 11KV side is outside the scope of ispection and test to BS7671 but if adequate evidence of protection is obtained from the operators of the 11KV side, do you think this would be sufficient for compliance with BS7671 (ie. if levels of protection can be shown to equal the level that would be provided by complying with BS7671).

Alastair
 18 June 2009 01:00 PM
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ArthurHall

Posts: 736
Joined: 25 July 2008

Alastair
The HV system is often considered as being up to and including the first switching device on the LV side of a transformer.
The rational is that HV isolation is required to work on the LV side of the transformer.
Could you argue that the circuit up to your DB is outwith your scope of inspection the same as the 11KV equipment?
As regards the earthing, remember that a transformer needs two earths. The HV earth connects the tank and core to earth. The LV earth connects the LV neutral to earth to form a TN system. I suspect that the neutral earth link in the DB is your LV earth while the other one is the HV earth.
If this is the case you have a TN-S system.
Hope this helps
Arthur
 18 June 2009 01:11 PM
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alastairfearns

Posts: 6
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Hello Arthur,

Thanks for that, if the LV earth comprises of the neutral linked to the main earth terminal within the DB via the 70mm2 Copper would this form a TN-C-S? Why is the HV earth brought onto the DB earth terminal as well? Surly to consider it as TN-S the earth neutral link within the DB should be removed?

Alastair
 18 June 2009 01:23 PM
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ArthurHall

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Alastair
Without seeing your installation I would be guesing what each conductor is. I would expect the HV earth to connect to a main earth bar, but there are various ways to do the job. The transformer neutral must be earthed at some point to make it a TN system. Any fault current flowing to earth has to have some way of getting back to source to complete the circuit.I know it sounds weird, but the LV neutral earth link is where fault current flows out of earth and into the transformer. As I said above this link makes it a TN system as opposed to IT, whether its TN-S, TN-C or TN-C-S depends the wiring systems on the outgoing side of the DB.

Arthur
 18 June 2009 01:52 PM
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alastairfearns

Posts: 6
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Thanks Arthur

Yes, thanks, I am assuming that the neutral link to earth is within the transformer connections at the transformer, I had not considered that the link would be formed at the DB. Given that the DB is 8m from the transformer, this would mean the SWA cable protection between DB and transformer relys on the DB neutral earth link. If this is the case (TN-C-S), it seems odd to have the protective link at the end of the cable. If it is not the case and the transformer is linked neutral to earth within the transformer connections itself, then the neutral earth link within the DB must be redundant (TN-S using SWA and HV earth as main incomming earth)?

Alastair
 18 June 2009 01:56 PM
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jafarpour

Posts: 198
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1-In some case all secondary electrical faults cannot be covered by primary protection systems. For more information you can refer to Protection Riddle No.35 from http://electrical-riddles.com

2- TN-C-S :

a-Considering of common conductor for earth and neutral electrical path between transformer and DP
b-Separation of earth and neutral path between DP and loads

TN-S : Separation of earth and neutral path in whole of circuit except star point of transformer
 18 June 2009 03:06 PM
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ArthurHall

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Alastair
The neutral earth link is often in the DB and can be disconnected for testing. For example with the circuit isolated and the DB incomer open. If you disconnect the neutral earth link and megger between neutral and earth, you will be testing the transformer LV winding and cables to earth.
You are correct in saying if the neutral is earthed at the transformer then it should not be earthed at the DB. However I would guess that it is only earthed at the DB.
The neutral must be earthed in every system apart from IT. Just because the neutral is earthed does not make it PME.
TN-C and TN-C-S is where one conductor is used as neutral and earth. TN-S has seperate neutral and earth conductors even though they are connected together.
I think it is a short coming of the regs that they assume every installation starts at a cutout. There is a fair bit of confusion about transformer earthing.
 18 June 2009 04:25 PM
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alastairfearns

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Many thanks Arthur,

We shall be testing the location of the neutral/earth connection at the next available opportunity. I will (until then) consider the installation as TN-C-S with the HV earth from transformer to DB as supplementary (for BS7671 purposes). The 'primary supply overcurrent protective device' will be considered inadequate given it's tripping time of 60s with a recommendation to replace it. But if after consultation with the HV engineers, evidence of suitable protection on the 11KV side is provided (i.e. achieving disconnection within 5s of a fault on the LV side occurring) it will then be considered adequate.

Many thanks for all your input.

Alastair
 18 June 2009 05:12 PM
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alancapon

Posts: 5772
Joined: 27 December 2005

Alastair,

I agree with Arthur here regarding the supply type. The installation is almost certainly TNS. It is usual to place the neutral - earth link at the first distribution board, but before any fuse, switch or link in the suppply. The reason is that any volt-drop on the neutral cable from the transformer is seen as a loss in the transformer, reducing the neutral to earth voltage within the customer's installation. This is a standard method that is employed by UK DNOs where a transformer feeds LV to a single customer only, without any other LV network connections.

It is usual for the HV protection to cover the transformer and LV cables up to the first LV protective device.

I do not however agree with Arthur's suggestion of removing the neutral - earth link. This must only be done under controlled circumstances with the transformer HV isolated from the source of supply. Depending on the condition of the transformer winding, you could find up to the HV phase - earth voltage appearing on the LV terminals. This would probably damage insulation on the LV side that would not be adequately rated, as well as possibly killing yourself too. The same risk also applies to disconnecting the "tank earth" with the transformer energised.


Regards,

Alan.
 18 June 2009 05:24 PM
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ArthurHall

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Joined: 25 July 2008

Alan
I did state that the circuit would have to be isolated and the DB incomer open.
I totaly agree with you that this test should not be done live.
In reality the HV side would be earthed and a permit issued.

Arthur
 18 June 2009 06:16 PM
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alastairfearns

Posts: 6
Joined: 20 August 2008

Hello Alan

can I reasure you thet no links will be removed whilst the intstallation is energized on the 11KV side, and all works pertaining to the 11KV side will be undertaken with suitably qualified and competant persons.
 18 June 2009 10:09 PM
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deansumner

Posts: 25
Joined: 05 March 2009

Firstly an overload protective device only does not sound adequate for a main LV Incomer, yes the HV will protect the Tx tails and the Tx and is very likely to trip within 5 secs.

Based on this, an O/C fault downstream of your main LV Incomer, will also trip the HV (not good practice).

A REF (Restricted Earth Fault Relay) can always be installed in your LV DB to protect your transformer by any occurrence of a slight earth fault, it would trip instantaneously.

Your earthing also requires some investigation, it sounds like you have a split HV & LV earth nest with 70mm earth conductors for each nest. 70mm is likely to be fine for the HV however it probably is not large enough for the LV.

I would get somebody whom knows what they are doing to have a good look at your system, I would suggest first to try and contact who initially installed it, it may be that everything has been designed and installed to standard.

Regards
Dean
 22 June 2009 08:52 PM
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alancapon

Posts: 5772
Joined: 27 December 2005

Originally posted by:
I did state that the circuit would have to be isolated and the DB incomer open.

I totaly agree with you that this test should not be done live.

In reality the HV side would be earthed and a permit issued.


Arthur, yes, sorry about that. It was actually Alastair who implied that he could remove the link. As I am sure you realised, I was trying to stop him possibly killing himself by removing the only earth reference on the LV winding with the transformer energised.


Regards,

Alan.
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