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...

 

[Leesparkykent]

what size is the OCPD for the distribution circuit feeding the kitchen DB?

 

[YoungBart]

Sorry, I've given the wrong info already. 63A breaker supplying new sub board with 80A main switch.

 

[Leesparkykent]

Sorry, I've given the wrong info already. 63A breaker supplying new sub board with 80A main switch.

Put a 63A contactor in then. probably around the same price anyway for a 20A or 63A 4pole contactor.

Edit: I didn't insert the skim link...you can get a 63A 4pole contactor for around £25-£30.

 

[Brightspark2]

As said size contactor for 63A, protective device rating. Remember correct AC rating depending on load being switched.

Sent from my E5823 using Tapatalk

 

[Darkwood]

As above you need to know your load types IE resistive/inductive AC1/AC3 respective, the price different can be 2-3 times but get the wrong contactor and it may be damaged when operating under load. If in doubt or you have a high Inductive nature to the loads then fit a 30kw AC3 contactor with a coil rating of your choice dependent on the control voltage.

 

[YoungBart]

Thanks for the help guys.
The load is pretty straight forward. A couple of vent units. I'll check the data for them and find out more on the load type.

 

[HandySparks]

Clearly, the 'safe' thing to do is to size the contactor to the rating of the OCPD of the distribution circuit. This is probably the pragmatic thing to do if the additional cost for a higher rated contactor is small.

However, if the sole purpose of the board is to supply the kitchen ventilation system, which comprises several known fixed loads, I would not think it wrong to size according to the actual load current. If there is the possibility of overload, one could consider rating to the sum of the OCPDs in the board, especially if there is no possibility of adding more ways.

 

[YoungBart]

Clearly, the 'safe' thing to do is to size the contactor to the rating of the OCPD of the distribution circuit. This is probably the pragmatic thing to do if the additional cost for a higher rated contactor is small.

However, if the sole purpose of the board is to supply the kitchen ventilation system, which comprises several known fixed loads, I would not think it wrong to size according to the actual load current. If there is the possibility of overload, one could consider rating to the sum of the OCPDs in the board, especially if there is no possibility of adding more ways.

That's Where my confusion was coming in, I know the load, etc So was initially planning to size the contactor to that, But with it being a commercial kitchen the chance of someone adding a fairly high load item of equipment is always a threat.
So I've sized it to the OCPD. As you say, To be safe.

 

[Spoon]

Can you not just add a shunt trip to the breaker supplying the DB?

 

[davesparks]

Being a fire alarm interlock do you need to make this a fail safe system?
Something like Spoon's suggestion above but with an undervoltage release rather than a shunt maybe?
Personally I'm not keen on contractors being installed such that they are permanently energised except when an alarm de-energises them, you don't know what may happen to cause it to stick in place and not find out until it is required to operate in an emergency

 

[Darkwood]

Being a fire alarm interlock do you need to make this a fail safe system?
Something like Spoon's suggestion above but with an undervoltage release rather than a shunt maybe?
Personally I'm not keen on contractors being installed such that they are permanently energised except when an alarm de-energises them, you don't know what may happen to cause it to stick in place and not find out until it is required to operate in an emergency

You won't get any real fail safe solutions here, that's why fire alarms and there auxillary operations are checked on a documented regular basis, only way to provide a better safety option would be fitting redundancy into the design so if one item fails then hopefully a second provides the same action to disconnect the supply. If this is for fire safety then a discussion with building control and the fire service may require a more robust solution, a simple fault been pulled through the contactor could weld it closed without anyone noticing hence the need for regular testing.

 

[snowhead]

Are all the loads on the D.B. 3 phase?
If not, either don't switch the Neutral or you'll need an Early make / Late break contact.

 

[Andy78]

From my installation, but not alarm design, experience I would have thought that individual interfaces connected to the fire alarm system controlling the extraction units would have been the usual choice.
Seems a bit clunky to isolate the whole db, including any future additions, via contactor when specific items are required to be operated.

 

[Spoon]

Seems a bit clunky to isolate the whole db, including any future additions, via contactor when specific items are required to be operated.

The OP states "I have a sub DB that need the supply isolating when a fire is detected (Commercial Kitchen)"
I presume that the sub DB is purely for the kitchen so the OP wants the whole DB isolating. Any additions to the DB should also be for the kitchen if that is the case.

 

[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?