4 pole Isolator on a 3 phase system

[wade88]

Hi guys,

Again, its been a long time in between posts for me. Taken a new direction in my job role and been focusing on other things. Frankly trying to do less and less electrical bits and bobs and i frankly feel less and less comfortable doing it as its all getting pretty heavy here and I just don't want my head on the chopping block anymore as my skills and knowledge just arent up to the job, so will be looking for a local industrial spark to add to my preferred contractor list in Hampshire (if anyone is interested or can recommend a decent spark). Anyway....

Having the EICR done across all the buildings today, chaps have turned up and instantly laid into me about the 4 pole isolator i installed on a 3 phase supply. The system is PME and had no means of isolation, so i installed an isolator as i needed to do some work on the system. I put a 4 pole on after taking some advice, but this bloke is giving me hell saying i should never isolate the N when part of a PME system as any potential fault currents wont be able to earth etc.... but how is that any different to a domestic supply when isolating the N on a double pole switch?

The regs say all live conductors must be isolated, but i also understand exceptions can be made for TN-S and TN-C-S systems but no where does it say you should NEVER isolate the N? Im getting the whole "ive been in this 22 years mate blah blah" and if i even try and start a discussion to further my understanding, when asking him to just explain his thinking to me, he just tells me hes not prepared to have a discussion he knows best and it will going down as "dangerous" on the report.

Can someone set me/him straight on this. His experience and knowledge is undoubtedly in excess of mine, i just dont feel i am totally wrong on this one.

cheers,

 

[Tiny Spark]

Think it has all been said but I agree with isolating the Neutral.
Regs say all live conductors need to be isolated and the N is classed as a live conductor therefore needs to be isolated along with the 3 phases.
We do have 3 pole isolators on site but all new ones are 4 pole.

At the end of the day as long as you dont break the earth then there is no problem!
And obviously not to break the PEN but thats a whole different matter.

 

[Darkwood]

Think it has all been said but I agree with isolating the Neutral.
Regs say all live conductors need to be isolated and the N is classed as a live conductor therefore needs to be isolated along with the 3 phases.
We do have 3 pole isolators on site but all new ones are 4 pole.

At the end of the day as long as you dont break the earth then there is no problem!
And obviously not to break the PEN but thats a whole different matter.

Re-read your Reg' 537.2.2.1 it does say all Live supply conductors but also says subject to 537.1.2 which states the N conductor need not be isolated or switched where it can be regarded as reliably connected to earth like TN-S and TN-C-S so no you dont have to switch it. The exception is Reg 537.1.4 does state household or similar installs where ordinary person can be expect to operate it then both Live conductors of the single phase supply require the ability to be isolated/switched.

 

[Marvo]

......and he said he would bring in the literature that backs up his argument.....

I'd be interested to know what proof he comes up with. Please let us know.

 

[Darkwood]

Probably printing out of his 15th edition yellow regs as we speak

 

[netblindpaul]

Wade,
I know we have crossed swords on here before, however in this case you are totally correct.

Not sure about the rest of the guys, however, if you list 10 "electricians" then you can list at least 2 professionally qualified engineers who back you up also, E54 & another...

Just remember that the BGB has just been amended with Amd2 (Aug 13), also don't forget Corrigendum 1 (June 13), both are irrelevant to your situation, but have been released, and unless your muppet is well up on things he won't know this!!!
However, he should now be working to the latest version!

 

[Dcf]

think he is talking pish trying to big himself up in front of you.
what about a petrol station? the neutral has to be switched on that.including the outgoing devices that have to be DP
i would say that 4 pole switches are more common now than they used to be thats for sure.
it really is a pet hate of mine reading the "inspection polices" reports that are solely designed to score unnecessary remedial work

 

[Tiny Spark]

Re-read your Reg' 537.2.2.1 it does say all Live supply conductors but also says subject to 537.1.2 which states the N conductor need not be isolated or switched where it can be regarded as reliably connected to earth like TN-S and TN-C-S so no you dont have to switch it. The exception is Reg 537.1.4 does state household or similar installs where ordinary person can be expect to operate it then both Live conductors of the single phase supply require the ability to be isolated/switched.

Thanks, and fair enough.

I shall continue with my N isolation all the same though

 

[shagbite]

wade, you might want to consider sacking this bloke. if the report is costing 2 grand, the last thing you should be worrying about is his competency.

the fella sounds like a blowhard. i'd contact the company he works for and kindly ask them to send someone who's fully versed with the reg's.




or you could just give him a spinning back-fist.

 

[Engineer54]

When it comes to LV power distribution I’ve always used a 3P switch with a removable neutral link.

For a small installation incomer then yes, the neutral should be switched. You don’t have control of the incoming supply.

Like everything it’s horses for courses. I don’t like a neutral to be broken anywhere unless for testing.
There again, I’ve never come across stupid things like shared neutrals in 40 years.

Think we've had this conversation before... lol!!

Main incoming LV breakers off the Supply Transformers onto a main Switchboard will always be 4 pole, especially when that main switchboard incorporates a number of TX incomers and where sections of the switchboard can be supplied by more than one TX source via bus couplers, which will also need to be 4 pole breakers.

Doesn't matter much if the outgoing feeder circuits have 3 or 4 pole breakers, as most of our MDB's and Sub DB's are 4 pole incomer breakers...

 

[Knobhead]

And our incomers were 3 pole with links. The reasoning being, switch the neutral and you remove one of the neutral earth paths. To break one of those links had to be a deliberate act.
There is the danger of closing the switch without the link fitted. But for the link to have been removed in the first place a responsible person would have authorised it and be accountable if not replaced.

The only time I’ve fitted 4 pole ACB’s was for stand by generators.

Sorry kid, we’re poles apart :yes:

 

[Engineer54]

And our incomers were 3 pole with links. The reasoning being, switch the neutral and you remove one of the neutral earth paths. To break one of those links had to be a deliberate act.
There is the danger of closing the switch without the link fitted. But for the link to have been removed in the first place a responsible person would have authorised it and be accountable if not replaced.

The only time I’ve fitted 4 pole ACB’s was for stand by generators.

Sorry kid, we’re poles apart :yes:

Thanks for the kid quote, ....if only....lol!! :yes:


Now think about having Multiple 3 pole incomers from TX's and 3 pole bus couplers, all the TX neutrals commoned!!! How about having a faulty TX tripping out, but still having it's neutral being backfed even when it's out of service, well until someone actually goes and physically removes the neutral connection(s) anyway. ....So Yep, we sure are poles apart, all our switchboards are fully automatic both operationally and protection wise. When a supply needs to be fully isolated from the switchboard, we don't want to be messing around with a socket wrench or spanner to disconnect a neutral conductor, which in many cases could be several neutral conductors where parallel feeder supply's are involved!! No, i'll be definitely be sticking to the industry standard of 4 pole ACB's on main multi section switchboards. It's an absolute no brainer as far as i'm concerned...

 

[Knobhead]

Using REF both the supply and load VCB and ACB open under fault conditions. OK there could be a path through a faulted secondary winding but with the neutral being solidly bonded the current would be minimal.

From my days working under M&Q the neutral was never switched (if it was used at all). It was the same in the steel industry.
Most of the LV switchgear I installed were 3 pole + neutral links, it’s what I’m used to.

At the end of the day, we’ve both worked in different fields. I was brought up with 550V and 660V systems.
Neutral? What’s that?

DNO distribution LV boards don’t switch the neutral as this picture shows

BTW I do like these ABB SAIF pillars. They are designed for load break / fault make. Better than pulling fuses by hand.

 

[Engineer54]

Yes, but we're talking here about multiple TX neutrals that are on 3 pole ACB's for incomers and couplers. isolation of any of those commoned TX neutral becomes a real pain in the neck.... On our 4 pole system, the TX and switchboard bus bar neutrals never get to be commoned up, it is only present at the section(s) of the switchboard it's corresponding phase conductors are also present!! It's an all win, win arrangement over an identical but 3 pole arrangement!!

 

[Knobhead]

I’m used to multiple transformers having commoned neutrals. The biggest board having seven transformers.

It would be once in a blue moon the bus sections would be closed but the neutrals were never broken.

We’ll have to agree to disagree on this one.

 

[Tiny Spark]

I may be talking crap here and mixing up regs but a thought popped into my head this morning.

Is there not a reg somewhere that says something along the lines of if you have a 3 phase machine being worked on by unskilled staff the local isolation must switch all live conductors?

Sure i remember seeing something about it in the past but cant remember and my regs book is at work lol