Isolation and switching are different aspects. One example where you need 4P switching is a generator transfer switch where the two supplies are on differently earthed neutrals, so you need to break one while running off the other.
I'm not sure of why the UK regs differ in terms of mandating DP isolation for "ordinary user" SP supply isolation while being happy with 3P-only for a TP supply, maybe it is less about the risks of the DNO neutral being at elevated potential (open PEN faults a different story) and more due to the relatively specialised nature of TP installations where less DIY bodgery is anticipated, and where a mistake in swapping N for one of the L is more obvious to the point of equipment-damage-smokin' obvious.
I'm thinking solely about isolation and you raise some of the points which brought this issue to the front of my mind.
I've noticed a trend towards fitting 4 pole main switches in TPN boards where 3 pole was always the norm.
I've asked a few people why they are fitting them and often the answers are along the lines of "Just in case" or "cos it's safer" but they can never explain what it is incase of or how it is safer.
I would have to check but I'm sure that in the past the regulations absolutely forbade the use of a 4 pole switch and only permitted a bolted test link in the neutral of a 3 phase supply.
Little irks me more than the "this is how we've always done it" response. I don't expect people to have all the answers, but "I don't know, let me look into it" would be better than folks blindly following rote.
Although this thread hasn't provided any empirical answers, I'm glad it's not just me that couldn't come up with a definite line of thinking.
Another practical aspect to using a 4P isolator is it simplified any IR testing (part of original thread I think...) as you are not likely to energise the load at all when doing a global IR check, or if you are testing the load on its own you can do so without needing to disconnecting a terminal somewhere (N link in isolator, or maybe at DB N-bar).
Testing is the one reason why I suspected 4 pole isolation is routinely used for equipment, although it could equally be a case of wholesalers keeping 4 pole isolators or people following what they've seen elsewhere. For any equipment which requires a neutral, I would never have considered not providing isolation of all live conductors.
I wouldn't be surprised if it's simply because we like to cling to the past and have just kept the spirit of the old rule which required DP switching and DP fusing in the days when the supply neutral was not guaranteed to be reliably earthed.
I'm inclined to consider 4 pole isolation useful for equipment and the convenience where testing is concerned probably outweighs any potential faults with the isolator. Of course I might now think differently where excceptionally expensive equipment is connected.
3 Phase motors rarely have a neutral so 3P is fine.
Yep, although I often see 5 cores used on motor circuits as the was on hand and neutral ends up parked in a connector.
I had a situation not so long ago with a 4 pole main isolator where it was switched off for some equipment to be changed.
When it was switched back on the neutral did not close resulting in the voltage on the single phase control side going crazy as the voltage raised and cooking some of the single phase equipment.
This is what I'm curious to know. Has anyone else encounted a similarly faulty 4 pole main switch or are such occurrences likely to be extremely rare?
An isolator can of course fail, but neutral links allow for the possibility of human error, should one need to be removed for testing...
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