dear motor experts

[mrloy99]

I wonder have any of you motor experts any thoughts on this.I was testing a plantroom today with many 3phase isolators fitted for pumps and panels etc. The isolators used were 4 pole rotary type(20 amp mostly).The installing electrician who wasn't present had connected up only the three phases to the switch and the neutral to the solid link inside the switch.I am wondering why not just connect the neutral thru the switch also.. The only info I can find in the BGB says that it is permitted to connect this way,I had assumed as a cost factor,ie a three pole switch is probably cheaper than a four pole switch...But what advantage is there in having a permanent neutral and having the bother of crimping the blue wire too.I realize that in most of these pumps a neutral wont be required anyway,but it cost me hours today at the panel which I thought was switched off at the isolator,the neutral wasn't and was giving me a very poor reading to earth on ir testing(0.10m ohms neutral to earth) I'm just hoping I havn't damaged something inside the control panel.

 

[D Skelton]

If the three phases and neutral through the isolator feed a panel with control equipment then when the isolator was switched under load, if the neutral broke even a microsecond before the phases then you'd have a floating neutral and damage would be likely to occur to some, potentially all equipment within the panel rated lower than 400V.

It's standard practice fella and is always done this way unless the isolator, switch or fuse has a neutral that is emlb (early make late break).

Remember, the provision for isolation of all live conductors is only required on TT systems, and this can still technically be achieved by a single isolation device with emlb neutral at the origin of the installation.

 

[silva.foxx]

.......but it cost me hours today at the panel which I thought was switched off at the isolator,the neutral wasn't and was giving me a very poor reading to earth on ir testing(0.10m ohms neutral to earth) I'm just hoping I havn't damaged something inside the control panel.

This is why it is important not to make assumptions and know exactly what you are testing. It is so you understand the circuit, work safely and efficiently... and don't break anything.

 

[Pete999]

Normal practice to link the N through a solid connection in these cases

 

[Knobhead]

As above.

You can switch the neutral but as DW pointed out it has to be early make, late break. If the isolator is a standard four pole the installing electrician got it right.

 

[Darkwood]

As above.

You can switch the neutral but as DW pointed out it has to be early make, late break. If the isolator is a standard four pole the installing electrician got it right.

When did i point that out??? ... are you hallucinating again Tony! Deffo my first visit to this thread! :confused5:

I fear I've stolen someones thunder!

 

[D Skelton]

I fear I've stolen someones thunder!

Naa, you'd have only said the same thing lol

 

[Knobhead]

I blame the class "A"s I have to take

 

[mrloy99]

so.....I'd assume then that the main isolator at the dist board where everything is fed from shouldn't be switching the neutral either? which it is, and possibly the mccb feeding the sub main too...all these in theory could have the same effect.....floating neutral? and what about commercial kitchen equipment 3 phase dishwasher,fryers,steamers,boiling plate,should they all have a solid neutral in their isolators,as I'm sure the controls inside these 400volt bits of kit are 230volt.

 

[Knobhead]

Whatever switching is used it should be 3P+N not 4P. The neutral should be late break, early make.

What’s a commercial kitchen got to do with a boiler house?

 

[mrloy99]

What’s a commercial kitchen got to do with a boiler house?[/QUOTE]

Nothing pal,I'm out of the plant room and into kitchen now..So just to reiterate we are now saying that most modern switches,isolators disconnectors,or whatever they are now called with a marked neutral will have an early make late break switching system so its not really an issue?(I was reading up on some previous similar threads)
But your preference is the solid neutral....Hmm
Would seem to not be complying with that reg about switching all lives.
I get your point ok about the floating neutral.
Pity we cant get some sort of testing device that would demonstrate the differences in times that it takes for the various poles to switch.