Sizing a Contactor.

[YoungBart]

Morning all,
Possibly a simple question But a mix of doctor prescribed medication and post Wembley joy have left my brain elsewhere.

I have a sub DB that needs the supply isolating if a fire is detected (commercial Kitchen) So The 3 phase supply for this DB needs a 4P contactor. Do I size the contactor to the current load of the sub board(10A) so Say a 20A contactor.
Or do I need to size the contactor to 63A same as the Sub DB main switch?

Thanks Chaps...

 

[YoungBart]

Thanks for the additional replies.
I hadn't even considered a shunt trip, It would've been a good idea, But a contactor has already been agreed with the client now. I guess its a bit 6 and two 3's, Possibly just a bit more work involved in the contactor.
All the loads are 3 phase, So we'll need the neutral switching.

Andy78 - Completely agree, Our usual way of dealing with this would be to interface the required vent units and isolate them alone. However, Due to the nature of the Building and System already installed that isn't possible. But The sub DB will be clearly labelled showing that in the event of a fire the supply to this DB will automatically isolate.

 

[Darkwood]

I would be looking at swapping out the Sub main main switch with an MCCB if possible and fitting a trip unit to it that runs through the Aux' of the fire alarm, this would be a similar cost but takes the contactor out of the loop.

Going down this road would require an extention box and the busbar kit to change over and usually comes complete (confirm with supplier), the aux trip will need to be specified on order.

 

[Spoon]

As davesparks has already posted, you might want to consider a undervoltage trip. Would be safer than a shunt trip in this case, from a fail-safe point of view. I suppose it depends on the fire alarm system.

 

[Lucien Nunes]

All the loads are 3 phase, So we'll need the neutral switching.

I'm not sure why one follows from the other - a 3-phase motor load might not even have a neutral, although if it does say for control circuits any switching must be early-make late-break. If this were a manual isolator, it wouldn't necessarily switch the neutral. Is there a requirement for this?