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Topic Title: when to use 3 pole or 4 pole main switch
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Created On: 21 March 2013 06:49 PM
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 21 March 2013 06:49 PM
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24Hour

Posts: 182
Joined: 06 April 2006

evening gents.

what are the regulations/options regarding using a 3 pole or 4 pole linked main switch on a 3 phase board.

I have a 3 phase board to change,which is supplied with a 3 pole main switch, and am unsure if the neutral needs to be switched as well as the phases, at present on the existing board it doesnt.

I have looked in regs but without your guidance cant find any relevent info, except what i have always reffered to as the neutral being treated as a live conductor.

Does any of the final circuits have a bearing on the switched neutral, from an isolation point of view, supply is tns, and the fuseboard is in a school lab enviroment

I will install a 4 pole main switch , but wanted to know if i could use 3 pole if the need ever came up again.

thanks
andy

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Yes i do do 24/7 everyday of the FLAMIN year.
 21 March 2013 07:07 PM
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GB

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Generally nothing wrong with 3 pole main switch on a DB unless TT system.
 21 March 2013 07:09 PM
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OMS

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try reading 537.2 et seq

that should tell you that a three pole isolator is fine for your application - indeed as it is for almost every TP&N application.

regards

OMS

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Failure is always an option
 21 March 2013 07:46 PM
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jcm256

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Glad you said almost every TP& N application, But you must use, SPSN and TPSN type which indicate switched neutral
*( Neutral makes first and breaks last) for hazard installations.

The type I have inspected at aviation fuel depots and similar (none hazard zone main-switch room) talking about main switch-fuses first : 60-200amp you could see that the neutral switch the two neutral contact prongs were about 1" longer when the switch-fuse was switched off.

Same applies to three-phase distribution boards, again four pole type main isolator usually MCB 4pole.
 21 March 2013 08:26 PM
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24Hour

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Superb thankyou

Nothing like reading the regs book when ur misses is watching eastenders

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Yes i do do 24/7 everyday of the FLAMIN year.
 13 November 2013 12:49 PM
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msandoz

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Came across this thread as I keep asking myself the same question. Alot of confusion existing in the electrical industry over this and very little is written. IEE regs states 3 pole is OK, so alot of Contractors go down this line as its cheaper.

I had a Council boss scream at me once in front of a full office for doing 4 pole isolation. After he left the room a respected engineer came over and said " you are right in what you said you know".

My personal views are slightly different for the following reasons:-

The regs also recognises the neutral conductor as a live conductor, quite rightly, because it is.

All residential properties have 2 pole isolation (L&N) for single phase Consumer units, so why not 4 pole for 3 phase. Is this an oversight, or just an anomoly?.

I have had two near death experiences (one my own, one a friend), my own in a house, and my friends on a large building site, where borrowed or crossed neutrals (a situation where a neutral is wired across two or more circuits) becomes live once you switch the circuit off that you are working on, effectively reversing the live conductor 230V to the neutral part of the circuit you thought you had just isolated. Boom!!!. The reason neither died was that we both fell, effectively forcing us off the conductor, my friend on to a car below, rolled off and went home, myself off the steps and onto the bathroom floor, hurt my leg and my pride.
It was common practice (and probably still is) when wiring lighting circuits across two floors ie:- stairs, when only two cores were taken up to the higher floor, then realising after connecting up, three were needed, therefore a quick fix was to use the neutral from the adjacent room, rather than try to get one up the finished building from the correct circuit, Also when fans were installed, where a switched live was needed from the lighting circuit, but also it was easier to take the L&N from a local power circuit. Also external lights etc etc, usually add ons by electricians taking short cuts. They also frequently get crossed and re crossed on big jobs where many cables run as singles in the same containment system. The end result is the system works. Neutrals definitely get crossed across 3 phase boards too, as in my mates case.

There is also the remote chance that the neutral could become live from a L&N short on the cable feeding the board whilst working on it, which would have the same effect. Poor earthing can also generate what I call floating voltages (intermitently come and go) on the neutral of around 130V which is enough to make you jump off what ever you are standing on.

So 4 pole isolation . Always on 3 phase Boards, and 2 pole on Single phase. Don't compromise. You may already be a killer and not know it.
 13 November 2013 01:23 PM
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MrP

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Sorry buddy
Not with you on this, on a box standard tp&n board four pole are no safer than three pole devices its nonsense to suggest otherwise

The words safe and isolation come to mind
Not killed anyone yet, but there again never worked for the council
Its not a copromise

MrP crimbo only 14days away
 13 November 2013 02:28 PM
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OMS

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So 4 pole isolation . Always on 3 phase Boards, and 2 pole on Single phase. Don't compromise. You may already be a killer and not know it.


You should be very careful where you deploy 4 pole isolation in three phase systems - I've seen a whole sequence of catastrophic failures and some pretty dangerous situations when 4 pole devices have been deployed with UPS and generator systems - you really do need to understand the role of the neutral and where N-E bonds are before randomly specifying them.

There is a reason why the wiring regs consider the system neutral to be at or around earth potential and generally only require 3 pole devices.

For many of the scenarios you describe above, switching the neutral would make the system more unsafe if 4 pole devices are used.

The wiring regs are quite clear on the issue of borrowed neutrals - 4 pole devices aren't the solution - correct wiring is, coupled with correct isolation and proving techniques

Regards

OMS

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 13 November 2013 02:51 PM
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calumbtw

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Originally posted by: OMS

So 4 pole isolation . Always on 3 phase Boards, and 2 pole on Single phase. Don't compromise. You may already be a killer and not know it.




You should be very careful where you deploy 4 pole isolation in three phase systems - I've seen a whole sequence of catastrophic failures and some pretty dangerous situations when 4 pole devices have been deployed with UPS and generator systems - you really do need to understand the role of the neutral and where N-E bonds are before randomly specifying them.

Regards

OMS


For the less clever people (myself), could you please explain a bit further. Curious to what failures have occurred with UPS + generators..?

Thank You
 13 November 2013 03:05 PM
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OMS

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OK - if you interpose a 4 pole device between an intake position where the N-E bond sits and the UPS, then you break the neutral to the UPS

If you look "through" the UPS, you'll find it's common reference is the neutral, but now it's broken - so that gives you two problems.

You have 3 phases seperated by 120 degrees (so 400V between) from the invertor and battery but no idea where those phases are with reference to earth or neutral - so any form of ADS is highly unlikley to operate.

You also now have a high impedance neutral, that, due to system harmonics, capacitive coupling etc is now rising uncontrolably in voltage - as it rises, the UPS really doesn't like it, get very unstable and tries to go to internall bypass - which is probably dead as a result of the 4 pole isolation - and you lose the critical load.

So if this UPS is backing up say a hospital HDU unit, you could easily have a few very dead individuals because of that 4 pole rather than 3 pole device.

You really do have to understand the role of the neutral for earthing before coming up with some random scheme that assumes 4 pole is safer than 3

Regards

OMS

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Failure is always an option
 13 November 2013 04:22 PM
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calumbtw

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Originally posted by: OMS
You also now have a high impedance neutral, that, due to system harmonics, capacitive coupling etc is now rising uncontrolably in voltage - as it rises, the UPS really doesn't like it, get very unstable and tries to go to internall bypass - which is probably dead as a result of the 4 pole isolation - and you lose the critical load.

Regards


OMS

I'm not sure I'm 100% correct.

The high impedance neutral is only due to the 4p switch?
I'm not entirely sure about the rest, but when it is disconnected the transistors in the UPS(?) are trying to find the common reference (Neutral, or Earth) to cause the increase in voltage?

Is Earth considered a common reference as it is the same voltage as Neutral? Does anything differ if it took its common reference from Earth instead of Neutral? Is it even able to do this?

Why does it try to go through the internal bypass? Low resistance route?

OMS, Thanks for the above.
 13 November 2013 05:43 PM
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OMS

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Originally posted by: calumbtw

Originally posted by: OMS

You also now have a high impedance neutral, that, due to system harmonics, capacitive coupling etc is now rising uncontrolably in voltage - as it rises, the UPS really doesn't like it, get very unstable and tries to go to internall bypass - which is probably dead as a result of the 4 pole isolation - and you lose the critical load.



Regards





OMS


I'm not sure I'm 100% correct.



The high impedance neutral is only due to the 4p switch?

Yes - as it's now disconnected from anything by the 4 pole switch

I'm not entirely sure about the rest, but when it is disconnected the transistors in the UPS(?) are trying to find the common reference (Neutral, or Earth) to cause the increase in voltage?

The system does not have a common reference anymore due to the 4 pole device being open - it's effectively floating

Is Earth considered a common reference as it is the same voltage as Neutral? Does anything differ if it took its common reference from Earth instead of Neutral? Is it even able to do this?

You shouldn't combine N and E within an installation, to do so creates a TN-C system prohibited by ESQCR (in the UK)

Why does it try to go through the internal bypass? Low resistance route?

No, the rising voltage on neutral makes the monitoring and sequencing electronics very unstable - so the UPS tries to protect itself by switching to internal bypass and taking the invertors out of circuit until the problem stabilises.

With an open circuit neutral, and an effective three phase system - the UPS is trying to find a stable zero volts reference in the neutral whichis now some sort of quasi TN and IT arrangement



OMS, Thanks for the above.


Regards

OMS

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 13 November 2013 06:22 PM
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MickeyB

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The simple view I took on the use of 4 Pole devices was to use them when I had a separately derived N-E reference that I need to isolate to avoid circulating currents.... parallel paths etc.... Typically TP&N for a TN based installation is all that is required.....

I have seen the use of 4 Pole devices (incorrectly in my opinion) on a static or rotary UPS supplies many times....... to which I point out about the 'neutral'........ as OMS has noted the 'neutral' on a static or rotary UPS with batteries is typically derived from the incoming neutral.... therefore if you switch OFF the UPS Mains 1 supply the UPS will try and switch to Mains 2 (if installed) and then battery.....

There should only be a single neutral supply into the UPS to which the UPS needs to reference to..... which is why you find a 'taped up' neutral wire typically hanging in the bottom of the UPS panel from Mains 2 supplies..... If you make Mains 1 a 4 Pole device you will isolate the neutral which causes the UPS to 'float'.....either on battery or Mains 2 which is has no neutral connected.......which is taped up...... the UPS may be able to handle this.... it may not...... Keep it simple..... keep it TP&N.... unless you need to isolate a separately derived neutral.... which does not typically come with a UPS....
 13 November 2013 09:51 PM
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AJJewsbury

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All residential properties have 2 pole isolation (L&N) for single phase Consumer units, so why not 4 pole for 3 phase. Is this an oversight, or just an anomoly?.

There's a danger with open N on unbalanced 3-phase systems - in that equipment on lightly the lightly loaded phase can be exposed to 400V - when it was only designed for 230V - usually causing permanent damage. As moving parts are inevitably less reliable than solid parts, there is a small increase in risk by switching the N rather than having a solid link.
- Andy.
 14 November 2013 04:55 AM
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leckie

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Have a read of page 8 of this"Isolation & Switching".
Pretty good explanation of the basic requirements.
 14 November 2013 01:32 PM
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lyledunn

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There are always advantages and disadvantages in such things. OMS has described a disadvantaged that he has witnessed. I have witnessed the damage caused where failure to reconnect an incoming neutral following its disconnection at a TP+N board to allow IR testing on the final circuits without the influence of the neutral connection with earth on the supply side. On one occasion the devastation resulted in the complete destruction of 80Ks worth of electronic equipment.

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Regards,

Lyle Dunn
 14 November 2013 03:11 PM
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OMS

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Indeed Lyle - although it could have happened by maloperation of a 4 pole switch anyway - if that neutral opens slightly early or makes slightly late then there's a problem.

I guess the difference here though is the wrong application of a 4 pole device by design is inherently dangerous

On the other hand, Forgetting to remake a neutral link is just plain bad practice/workmanship etc.

Regards

OMS

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 14 November 2013 07:59 PM
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cookers

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OMS is correct in my opinion 4 pole devices are used without sufficient care and thought.

The "Memorandum of guidance on the Electricity at Work Regulations 1989" gives advice in regulation 9 which is often ignored or misunderstood and danger can result.
 16 November 2013 09:55 AM
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weirdbeard

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Originally posted by: OMS


You should be very careful where you deploy 4 pole isolation in three phase systems


Must admit I'm a bit confused by this - if the inferred problems may occur by switching the neutral for isolation in TN systems, wouldn't it be the same for a TT system where the neutral must be disconnected for isolation?


- I've seen a whole sequence of catastrophic failures and some pretty dangerous situations when 4 pole devices have been deployed with UPS and generator systems - you really do need to understand the role of the neutral and where N-E bonds are before randomly specifying them.


I have to say that this doesn't sound like the issue is with the device for isolation but more to do with the lack of an interlock or not following the correct isolation procedure as per the durable warning notices, 537.1.6

There is a reason why the wiring regs consider the system neutral to be at or around earth potential and generally only require 3 pole devices.


For many of the scenarios you describe above, switching the neutral would make the system more unsafe if 4 pole devices are used.



The regs say "In a TN-S or TN-C-S system it is not necessary to to isolate or switch the neutral....."

This does not seem to suggest that a 4 pole could be more unsafe as it's under the general requirements for isolation.
 16 November 2013 09:10 PM
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cookers

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I can only reccomend you read the HSE Memorandum of guidance to Regulation 9 of the Electricity at Work Regulations 1989 (free to download!). Isolating a reference conductor can cause danger and very little of the circuit protection arrangements (overcurrent, earth fault etc) will protect you and others from the dangers created.
OMS is correct to warn of the dangers of fitting 4 pole devices to 3 phase systems. Double pole switching on single phase systems is a different issue and is a perfectly safe and reasonable thing to do.
4 pole isolation on 3 phase systems needs careful thought and if you don't know why you are fitting a 4 pole isolator then my advice would be "don't do it!" fit a 3 pole. I always specify 3 pole isolation and wait for convincing evidence why a 3 pole is unsafe.

My thoughts are 4 pole isolation will not protect you from crossed neutral problems as some have suggested.
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