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Topic Title: Isolation for repair of transformer feeder
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Created On: 21 May 2013 08:46 PM
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 21 May 2013 08:46 PM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

In the case of a transformer feeder - in this case a 33kV cable a few km long directly connected (without switchgear) at one end to a step-down transformer to LV - is it necessary under safety rules to earth the HV cable at both ends to make a repair on the cable? Or is it satisfactory to isolate and earth at both ends of the circuit - ie at one end at HV and at the other end on the LV side of the transformer?

I would appreciate your opinions.
 22 May 2013 12:56 PM
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ArthurHall

Posts: 737
Joined: 25 July 2008

Normal practice is to isolate and earth the HV source and on the LV side of the transformer. The LV earth is sometimes omitted on 400 volt circuits as it can be more dangerous to apply than not to apply.
I take it that the isolation will be carried out by a SAP to appropriate rules.
 22 May 2013 03:32 PM
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JohnRRussell

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Joined: 02 December 2003

It's the 'appropriate rules' that's the issue. The circuit is a private wire, the repair will be made by a contractor working to his rules with his SAP, who interpretes the rules as requiring proving, isolating and earthing at HV at both ends. To isolate and earth at HV at the transformer end he's proposing to open the transformer tank to prove dead then open the HV cable box to isolate and earth. If those are the contractor's rules then they don't seem to me to be appropriate rules.

The isolation and earth on the LV side of the transformer is required, because the LV side has a generator.
 22 May 2013 06:09 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

To open the transformer you will need a HV permit to work, which means earthing the HV and isolating the LV; you are then going to need some form of method to prove the transformer dead before issuing the permit and opening it up - how is he going to do that? I would suggest that proving the transformer dead by testing the LV dead is not good enough.




The method to fix the cable would be:

1) Isolate and earth the circuit via the HV CB feeding the HV cable
2) Isolate the LV on the LV side of the transformer (earthing the LV is not a good idea)
3) Issue a sanction for test and identify the cable at the point of work, then spike the cable under the SFT
4) Cancel the SFT and issue a PTW at that point
5) Do the repair under the PTW at the point identified.
6) Cancel the PTW
7) Issue another SFT to test that the cable repair is good
8) Restore the circuit.


If the cable requires multiple repairs it will need to be suitably identified at each point

Edited: 22 May 2013 at 06:18 PM by Zuiko
 22 May 2013 09:57 PM
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JonathanHill

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It's understandable that the contractor wishes to apply an earth at the HV terminals of the transformer. Most Rules I've had acquaintance with require that any HV apparatus that has been "on the system" is maintained discharged to earth before a Permit is issued. As the HV cable is going to be cut to effect the repair, the transformer side would otherwise lose the earthing provided by the switchgear.

Even if the Rules don't make it explicitly clear that earthing of both sides of the cable is necessary, a risk assessment would identify this as a necessary mitigation due to the risk of induced pick-up from any adjacent circuit over the long length of this cable.

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Jonno
 23 May 2013 09:27 AM
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ArthurHall

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At voltages up to 33kv not all safety rules require plant to be connected to earth. The DNO I used to work for required that an earth was applied between the point of isolation and the point of work, but the point of work did not have to be earthed. This is a practical way of working as it is not always possable or desirable to earth every part of the apparatus. As well as transformer feeders, think of the busbars of ring main units, lengths of cable, or overhead lines.
That said if you still want to earth the cable at the transformer many 33kv transformers have link boxes so that the windings can be disconnected under a permit, if this transformer has a link box this would be the best place to earth the cable. The normal procedure for working on links is to isolate and earth then issue a permit, the permit has the specific instruction that the links must be proven not live as soon as the link box lids are removed.
One big danger when working on transformers that is often overlooked is correct identification, to many times people have started working on the transformer next to the one they should be on.
 23 May 2013 10:31 AM
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JohnRRussell

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Originally posted by: Zuiko

2) Isolate the LV on the LV side of the transformer (earthing the LV is not a good idea)


Thanks Zuiko. When you say that 'earthing the LV is not a good idea' you imply that there's a problem with it. Although earthing the LV may not be sufficient under some circumstances, such as Jonathan's induced voltage, are you suggesting a problem?
 23 May 2013 10:45 AM
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JohnRRussell

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Originally posted by: JonathanHill

It's understandable that the contractor wishes to apply an earth at the HV terminals of the transformer. Most Rules I've had acquaintance with require that any HV apparatus that has been "on the system" is maintained discharged to earth before a Permit is issued. As the HV cable is going to be cut to effect the repair, the transformer side would otherwise lose the earthing provided by the switchgear.


Thanks Jonathan. The cable is also going to be spiked. Would you say that, after HV isolation and earthing at one end and LV isolation at the transformer end, then spiking, the application of earths on both sides of the point of work before cutting would satisfy the need for continuing earthing?

Even if the Rules don't make it explicitly clear that earthing of both sides of the cable is necessary, a risk assessment would identify this as a necessary mitigation due to the risk of induced pick-up from any adjacent circuit over the long length of this cable.


In general I can't argue with that! In this case there is no other nearby circuit, it's a transformer feeder into the middle of nowhere. But I'm not the man who's going to touch the thing, and if he's concerned about induced voltage I'm not going to ignore it.
 23 May 2013 10:47 AM
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JohnRRussell

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Originally posted by: ArthurHall
That said if you still want to earth the cable at the transformer many 33kv transformers have link boxes so that the windings can be disconnected under a permit, if this transformer has a link box this would be the best place to earth the cable.


Thanks Arthur. I've seen 11kV transformers with link boxes but not 33kV. Is that a common solution to the transformer-feeder isolation and earthing problem?
 23 May 2013 01:35 PM
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JonathanHill

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John, What you suggest sounds fine, however I'm not clear how you're going to access conductors close to the point of work if it's an underground cable. If it's an overhead line, then it'll be straightforward to apply portable earths (subsequent to applying the Main Circuit Earth at the HV switchgear)

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Jonno
 23 May 2013 06:38 PM
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Zuiko

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Conductors, plant or equipment under ptw does not need to remain connected to earth once the ptw is issued: this is obvious, because working will often require disconnecting them from the system. You cannot however, remove a circuit main earth from the system under a ptw (you can under a sft).


If the contractor insists that the HV side of the transformer earthed towards their point of work you are going to have to:

1) Isolate the LV side of the transformer (solid links or ACB?)
2) Open, isolate and earth HV CB feeding the cable towards the transformer
3) Use some method to prove the transformer dead (you can test across the LV terminals in the LV board or if there is an LV cable box) note: this only proves that the LV is not live (dead is a different thing!)
4) Issue a PTW to open the transformer (lid/end-box?)
5) Use a HV voltage detector to test the HV is not live
6) Connect the HV conductors at the transformer to earth, and short them out (using insulated portable earthing device with of sufficient length - at least 1.5m). These are additional earths.
7) Once the HV conductors are earthed disconnect them from the transformer
8) Cancel/suspend PTW

9) issue SFT to ID and spike the cable at point of work
10) issue PTW to fix cable
11) cancel PTW
12) issue SFT to test cable then cancel SFT

13) Reissue/Issue PTW to re-connect conductors to transformer and remove additional earths
14) Cancel this PTW

15) issue ANOTHER SFT to re-test the entire cable from HV breaker, through repair, to new terminations at transformer
16) cancel SFT

17) remove earth at HV CB
18) remove isolation and close the LV links or breaker
19) remove isolation at HV CB then close, trip, close the HV breaker



I said it was not normally a good idea to earth the LV because very few LV boards/pillars/end-boxes are designed for the fault.

Edited: 24 May 2013 at 07:59 PM by Zuiko
 23 August 2013 09:39 PM
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neilmcd84

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Joined: 02 June 2003

There is a common misunderstanding about the function of an earth applied on high voltage apparatus. The purpose of a primary earth (or sometimes called CME) is to protect against the failure of a point of isolation. It is not to maintain the point of work at earth potential. It is very common to have primary earths applied at the remote ends and conductors cut so the earths are no longer applied to the conductor being worked on. Having said that, if there are concerns about the presence of induced voltages drain earths should be applied. This isn't a huge concern on cable networks.
Isolation at the remote ends of the circuit is fine, with no earth on the LV side, assuming you mean <1000V LV.
Remember you can take isolation out past the initial piece of switchgear if required - eg to be able to apply a padlock.
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