If neutral is officially a live conductor, how does pulling the service fuse allow you to comply with the EAWR when replacing a CU?
Working live
- Thread starter ban-all-sheds
- Start date
- Joined
- 11 Jan 2004
- Messages
- 45,921
- Reaction score
- 3,534
- Country
WE have been told it is OK to work on TN systems by isolating the live (obviously we test for dead first, LE, LN, NE).
But, on TT systems, it is imperative to isolate the neutral as well.
How you do that when changing a CU? You need to have a DP iso switch fitted by the DNO.
But, on TT systems, it is imperative to isolate the neutral as well.
How you do that when changing a CU? You need to have a DP iso switch fitted by the DNO.
"live conductor" is a misleading term..
it's a "live conductor" in as much as it carries current back the the substation, but it is not "live" in it's own right.. you can't connect a load N-E an have it work like you can do with L-E..
the neutral is at, or damned close to earth potential.. unless it's a TT system supplied by a local transformer where it can't be guarunteed to be tied to earth.. which is why TT systems require double pole isolation devices..
it's a "live conductor" in as much as it carries current back the the substation, but it is not "live" in it's own right.. you can't connect a load N-E an have it work like you can do with L-E..
the neutral is at, or damned close to earth potential.. unless it's a TT system supplied by a local transformer where it can't be guarunteed to be tied to earth.. which is why TT systems require double pole isolation devices..
I think in order to answer the question you need to consider the fact that you'll always have an element of live working, and the EAWR allows that as long as its unreasonable for it to be dead, its reasonable for it to be live, and suitable precutions are taken.
If it wasn't for the fact that some live work is accepted, we'd never complete the safe isolation procedure as we deem a conductor live until proved otherwise...
The idea that in TN systems that the neutral can be deemed near enough earth potential and need not be isolated works on a similar level of being a commonly accepted idea of meeting the three criteria.
I suppose on a TT system if the supply authority will not provide a DP isolator, then perhaps you could consider it reasonable to after isolating the line conductor, to remove the outgoing neutral conductor from the meter with a VDE complaint insulated tool... the supply authority would generally not approve though
If it wasn't for the fact that some live work is accepted, we'd never complete the safe isolation procedure as we deem a conductor live until proved otherwise...
The idea that in TN systems that the neutral can be deemed near enough earth potential and need not be isolated works on a similar level of being a commonly accepted idea of meeting the three criteria.
I suppose on a TT system if the supply authority will not provide a DP isolator, then perhaps you could consider it reasonable to after isolating the line conductor, to remove the outgoing neutral conductor from the meter with a VDE complaint insulated tool... the supply authority would generally not approve though
I don't know about you guys but, when I replace a CU, the LAST connection will be the tails.
Up to that point the installation is a dead-dodo thingy.
Next conductor will be neutral tail, then line tail.
Main isolator in CU will be off waiting for polarity checks on the CU's incomer.
AFAIK that covers you on the EAWR front.
Up to that point the installation is a dead-dodo thingy.
Next conductor will be neutral tail, then line tail.
Main isolator in CU will be off waiting for polarity checks on the CU's incomer.
AFAIK that covers you on the EAWR front.
Right there with you on that one Taylor. Definately the last connection for me is the tails to the meter. Neutral first then Live, polarity check, energise.
P
Paul_C
If it's not tied to earth then it isn't TT - It's IT.
And are we basing this upon the defined earthing method of the customer's installation? There are still hundreds of TT installations in use which take their power from what are now PME distribution systems. So if the neutral on such a system is considered "safe" at the intake of a TN-C-S installation, it doesn't make much sense to consider it potentially (excuse the pun) dangerous in a TT installation, when the supplier's side is exactly the same in both cases.
- Joined
- 27 Jan 2008
- Messages
- 27,558
- Reaction score
- 3,331
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
I will agree the term “Live” seems to change meaning as you read through the regs. But the main distinction is between a PEN conductor and Neutral and until there is some current flow the Neutral on a TN-C-S is the PEN conductor.
However with TN-S and TT systems there should be a multi-pole isolator and without it then as you say you are working live.
I have been surprised in the past on altitude to “Live” working and having seen the fines handed out by the HSE for live working I can see there is a real problem.
My son was employed to change meters and this did involve live working and to comply with health and safety he had to ware a host of protective items and I would suspect we would need to ware the same if complying to the letter with HSE requirements.
However we all know why we treat the neutral as a live and in real terms we know with care we can change a consumer unit without placing ourselves in danger.
As to drawing the fuse I know there are talks going on at the moment as under HSE health and safety the fuse must be drawn if no isolator is provided but under supply regulations we are not allowed to draw the fuse.
The way I look at it is if the DNO do not want us to draw the fuse then they must provide some other form of isolation and although two wrongs don’t make a right we need to take a positive attitude to safety when faced with a double negative.
Maybe with maths we can have imaginary numbers but in our trade imaginary isolations do not really work if you understand what "i" (j) mean?
However with TN-S and TT systems there should be a multi-pole isolator and without it then as you say you are working live.
I have been surprised in the past on altitude to “Live” working and having seen the fines handed out by the HSE for live working I can see there is a real problem.
My son was employed to change meters and this did involve live working and to comply with health and safety he had to ware a host of protective items and I would suspect we would need to ware the same if complying to the letter with HSE requirements.
However we all know why we treat the neutral as a live and in real terms we know with care we can change a consumer unit without placing ourselves in danger.
As to drawing the fuse I know there are talks going on at the moment as under HSE health and safety the fuse must be drawn if no isolator is provided but under supply regulations we are not allowed to draw the fuse.
The way I look at it is if the DNO do not want us to draw the fuse then they must provide some other form of isolation and although two wrongs don’t make a right we need to take a positive attitude to safety when faced with a double negative.
Maybe with maths we can have imaginary numbers but in our trade imaginary isolations do not really work if you understand what "i" (j) mean?
I agree with taylor.
When changing a CU you remove the old kit, fit the new, the first thing usually is to fit the new tails.....but you leave them loose hanging below the CU until your ready to connect at the end.
And why do we need to use DP isolators for single phase, but it is fine to use TP for three phase and leave the neutral alone? I have always wondered what the reasoning is for this.
When changing a CU you remove the old kit, fit the new, the first thing usually is to fit the new tails.....but you leave them loose hanging below the CU until your ready to connect at the end.
And why do we need to use DP isolators for single phase, but it is fine to use TP for three phase and leave the neutral alone? I have always wondered what the reasoning is for this.
It never really sat well with me, but I assumed the reasoning was based on the fact that on TP you are not going to have an undetected reverse polarity fault at the DB, but could in a SP installation your could, and this could catch out a DIYer who might work on his SP installation without proving isolation (which as I say, doesn't sit right with me, as he is likely to isolate by pulling a fuse or operating an MCB rather than opperating the DP isolator)
Yea, I guess that makes sence.
P
Paul_C
That's never made sense to me either. You could have a single-phase residential service run from the same pole as a 3-phase service to a commercial building next door, yet the former supposedly needs a D.P. isolator while the latter is considered fine without a switched neutral? Totally illogical.
Then consider if that residential service is TN-C-S. You open the D.P. isolator, but the entire earthing system in the building is still connected to the supply neutral anyway.
In the U.S. the main switch/breaker never opens the neutral, either on a 3-phase service or a single-phase 3-wire domestic service.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
