Contactor ok for 2 x 32A rings?

[cornburn]

I've never designed or installed using contactors before but a client has just asked if they can have a keyswitch to knock off 2 ring circuits in an office at the end of the working day. There is currently a small 5 way board with 3 spare ways and 2x 32A RCBOs for the 2 ring circuits.

My question is really whether a single contactor could switch both of these circuits and from your experience would it be best to fit one to the existing 5 way board or use a separate unit of some sort? I assume I'd need to find a 63A contactor that could switch 2 circuits and power it from a separate MCB, so although tight that should fit in the 3 empty spaces I have on the board?

Cheers

 

[mikejashley]

Personally I would go for 2 x single phase 32A contactors fed from 2 32A MCB's....Loads of them about that are the same size as a single MCB so you should be able to fit two in there, both can be switched from the same keyswitch. Going for a single 63A contactor you would only be able to feed it from one MCB ie 32A or 63A and you cant feed it from a 32A as with both ccts pulling would trip it out.

I would also build it into an adaptable box and mount it seperate to the fuseboard. Will make the job a lot easier and give room for future expansion.

 

[cornburn]

Thanks, very useful. Looks like the list for parts is going to add up a bit then, I think they were hoping for a £50 "just stick a keyswitch in" job. Wouldn't a contactor that did 2 circuits just need one small MCB to feed the electromagnet (eg 6A), which completes the circuit to the 2x 32A RCBOs already installed? Current for these 2 circuits doesn't actually flow through the feed for the contactor does it? Or maybe there are different setups.

 

[malcolmsanford]

I did something like you want to do for a company selling display untis. They had several different types in a show room and when they had prospective customers coming they had to individually switch on each item from the socket.

This was 2 radials though and so they wanted the same to come into the showrooom and key switch so all the units came on, then just switch them off when leaving.

I fitted a 32 amp 3 phase contactor into a box, slotted the box and fitted some mesh inside to ventilate it but keep a good IP rating. You will though have to label the box for 2 different supplies to the contactor, and I also marked on the box what DB the mcb's where in and what the circuit number was.

They had a grid switch so got an MK emergency grid and picked up the contactor supply from the lighting with the emergency switch controling the coil. Again marking at the box the DB and the circuit for this as well. May worlk for you

 

[cornburn]

So you fitted a 3 phase contactor in there and put each of your 2 circuits on a separate phase connection? I suppose if the total rating of the contactor is greater than your 2 circuits this makes perfect sense. I've drawn an awsome picture of what I'm thinking if you'd be ever so kind to give the thumbs up/down

 

[malcolmsanford]

If your ring final circuits are on 2 different phases you will also have to put a 400v sticker on it as well. Seems like that may float

 

[Spudnik]

That is indeed an 'awesome' picture

And yep, looks fine to me.

 

[cornburn]

Thanks, copies available for a fiver (non framed). Luckily they're same phase so no worries about 400V craziness!

 

[Darkwood]

Not sure if the customer has specified the type of keyswitch, but may a suggestion for future reference be to put a keylock rotary isolator in 4pole, cheap way of achieving the same task and no issue with contactor failure, control circuit tripping etc and quick and simple to install.
This method of course wouldnt allow multiple key switch points or remote keyswitch but i dont know the job spec.

 

[cornburn]

No spec has been given, just that they want to be able to isolate both ring circuits at once with a key, simple as that. A rotary isolator could just be fitted in line after the RCBOs then next to the board, just break into the conduit? That would be much simpler but as it's not something I've done before I didn't know such a thing existed! You might have just made my day

 

[cornburn]

Just thought, using a 4 pole isolator to power off both circuits would mean creating a figure of 8 in each ring, as the line conductors of each half of the ring would have to go under the same terminal, as would both legs of the neutral. So unless they do an 8 pole isolator I'm either looking at putting a 4 pole in for each ring or not switching the neutrals (which isn't the best idea as it's supposed to be an isolator)

As it's all wired in singles it would be much easier to just extend the line conductors out of the conduit and switch them, leaving all the neutrals and CPCs intact as they are. Not sure if this would go against any regs though. Will have to have a read in the morning. Thanks guys

 

[Pinto Baz]

If you check out the definiton and parameters of a isolator, I don't think a contactor could ever be considered an isolator, and you are not required to have one in this ring circuit. You can switch the four lives only using a four pole isolator or contactor. If you go for a four pole 230v contactor I would mount on DIN rail in a box and get my feed for the control from one of the four lives feeding into the contactor and pass that through a 1A to 3A cartridge fuse mounted in a carrier on the same DIN rail.

One thing that you need to watch: The customer will no doubt be expecting to just flick it all off at night with the key/isolator and then staight back on in the morning. Computers are highly inductive and give a current surge when switched on, if there are several on the circuit it might well be too much for the 32A breaker. I would check this first. Switch off the MCBs and try switching them back on. If they can't be reset under full load the customer will still have to switch at least some of the load on manually. No doubt they won't be happy, so check this and warn them first before doing the job

 

[Darkwood]

You can get multi-pole cam style rotory switches although the expense does start creeping up with this type and as Pinto has mentioned it likely the computers will be just sitting in stand by mode when isolated thus come the morning turning the key will create a large inrush that even type (d) mcb would struggle to hold, whichever method you use to isolate it will need to cope with the inductive nature of the circuit so fitting a 40amp isolator will probably only be good for 40amps resistive load switching, its inductive switching could be as low as 10amps, which ever method you employ you need a isolator or contactor with an AC-3 rating of 32amps (AC-3 is a rating designed for switching squirrel cage motors so should be better at this type of load).
Most isolators, contactors, switches etc state their AC1 rating which is non-inductive and slight inductive loads, this is where most sparkies get it wrong especially with inductive lighting hence the contactors dont last, burn out or stick in closed position.

 

[cornburn]

Thanks I'll bear that in mind. There are only 12 computers over the 2 rings and they're all new low power eco ones that just go back into standby when the power's put back on. I'll test this properly though before starting the job.

Thanks everyone, some very useful info here

 

[wade88]

Hi guys,

We have just fitted 2 three phase contactors at my work both with individual timmers. However, one of the contactors when activated makes an incredibly loud high pitched buzzing sound. What may be causing this?

Thanks

 

[benji]

supprising how many people think a 3pole contactor is 3phase

 

[wade88]

No these are 4 pole, 3 phase contactors, (ABB A26-40-00) is the model