Switch the neutral or not in isolator

[G1BBO]

Hi,

i just have a quick question as regards to a situation at work.

One of the apprentices is replacing a single phase pump that's been out for awhile and has ordered a new 3 pole isolator (other apprentice broke the bottom). He had two questions for me. The first was it was a 3 core 3 phase cable supplying it and asked what he would use for the live neutral and earth. I said he would have to trace it back to the supply and see what they have used and make sure you identify the conductors with the appropriate colour sleeve. The second question was should he switch the neutral. Now this is the point I wasn't 100% on. I know it's good practice to switch live and neutral conductors and in a fault condition neutral could become live. However it is supplying a three phase motor supplied by single phase so there is a capacitor in there. So my thoughts where if you where to isolate live and neutral there would still be a charged capacitor or would this discharge through the resistance of the windings?

Hope this made sense as it's my first post, apologise if not.

Thanks in advance for any replies.

 

[Pete999]

What was there before?

 

[G1BBO]

What was there before?

I honestly couldn't tell you, the motor/pump has been out since I've worked there.

 

[Megavolt]

How bigs this pump

 

[Guest55]

the capacitor will still remain charged whether neutral switched or not.
this should dissapate over a relatively short space of time.
neutral should still be switched at the isolator though.

 

[G1BBO]

the capacitor will still remain charged whether neutral switched or not.
this should dissapate over a relatively short space of time.
neutral should still be switched at the isolator though.

Cool am happy with that, thanks a bunch.... Ermm it's only small so couldn't imagine anything more than 3-4kw. Al get back to you on that one when I am back in on Wednesday if you would like.

 

[camerabloke]

yes please, let us know

 

[G1BBO]

Hi, It was 1kW. I apologise for my over estimation.

 

[Marvo]

......However it is supplying a three phase motor supplied by single phase so there is a capacitor in there.

This confused me, are they trying to run a 3-phase motor from a single phase supply?

1kW is a bit of an oddity, standard motors are usually 0.75 or 1.1 kW (one horse or one and a half horse in old money)

 

[Croney]

Marvo:

It could be controlled through one of These I have used them a couple of times before, good units but noisy in sound and generation.

 

[Lucien Nunes]

The question is a bit muddled. If a motor with an integral capacitor is fed from a single-phase supply, it is serving as a single-phase motor (even though it might have a 3-phase winding), all the normal single-phase methods apply and you can use a DP switch.

If it is a permanent-split-capacitor motor, the capacitor does not remain charged after use at all; it will discharge into the winding within a fraction of a second of the supply being switched off. If it has a start capacitor that is switched out of circuit by a centrifugal switch, that can remain charged until its bleeder resistor discharges it over a minute or so, but this has nothing to do with the method of isolation.

A three-phase motor on a three-phase supply would not normally have a neutral connection, therefore the question of switching the neutral does not arise. If it is a three-phase motor with a separate capacitor box to enable single-phase operation, you should not be switching the circuit between the capacitor box and the motor as one leg runs at higher than mains voltage. You should switch only the single-phase supply, in which case it is still serving as a single-phase motor.

 

[Marvo]

I'm kinda hoping if his motor is 3-phase and it's running via a converter on single phase he might have mentioned this in the OP . Either way it would be nice to get some clarity on exactly what the setup is.

 

[G1BBO]

Sorry, I asked the question again and he said it is 0.75KW. My apologise, Friday and Saturday 12 hour night shifts do funny things to you!. I've spoken to the apprentice since and he said it tells you on the back of the terminal cover how to wire it single phase. The picture he took of the terminals was a standard 3 phase set up with an extra two separate to the left if I remember rightly. There was also some jumper cables connected across different terminals from the manufacturer. To solve this question on Thursday I will get a picture of the terminal box and the data plate to give you more information. Once again I apologise for the muddled and lack of information.