Circuit breaker plus contactor on MV motor feeder

[Parkpower]

Hello everyone:

We have a 3MW 5.5kV DOL - started motor installed in a concrete plant. The process designer says that it may be started and stopped several times a day. So contactor shall be used to start and stop it. However, the client requires that vacuum circuit breaker will be used to protect the feeder circuit rated 1.5MW and above. Therefore, both circuit breaker and contactor ( without fuse) will be used on the feeder. Generally we use only one of them, CB or fused contactor, not both. Does somebody use this configuration on previous project? I wonder whether any manufacturer will provide such kind of switchgear? Is there any special protection consideration on this group of CB and contactor ?

Thanks,

Parkpower

 

[rockman7892]

He is wanting to use a circuit breaker to protect the motor branch circuit?

Why not just use a motor relay in conjunction with your contactor and set it to protect both the motor and cables? This is the way I have typically seen it done.

 

[oldfieldguy]

Our typical installation is one of the neat packaged units sold by many vendors, consisting of a disconnect switch, fuses and a vacuum contactor. The fuses handle the short circuit faults and the contactor handles overloads. A single circuit breaker handles a bus full of these things.

You could easily feed this unit with a circuit breaker with overcurrent protection coordinated with the fuses, but that is not an arrangement I've seen in my experiences.

old field guy

 

[TurbineGen]

I agree with oldfieldguy. It's simple to get a prepacgaked vacuum contactor set up. In my experience for a unit that will be started and stopped multiple times per day, a solenoid operated unit lasts longer than a motor/spring operated unit since the solenoid has fewer moving parts and doesn't wear out nearly as fast.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[dpc]

It's pretty common to have a circuit breaker close-coupled to a line-up of MV motor starters. This is basically what you have, except you'll only have one starter. You have to deal with the physical size differences and access requirements of the switchgear versus the MV starters, but this is quite commonly done.

If the customer insists on the breaker and the starter, I would just buy a standard breaker and a standard starter (including fuse) and have the vendor provide it as a package. It's a belt and suspenders approach, but it will work fine.

 

[LionelHutz]

Honestly, I think you'll have trouble co-ordinating the breaker protection and tripping so that it will protect the contactor. If the contactor tries to interrupt a fault, probably over about 5kA or so, it can be bad news for the contactor.

 

[dpc]

Coordination should not be a big problem if you keep the fuses.

 

[LionelHutz]

The OP specifically stated without fuses. That is likely a mistake on a vacuum breaker.

 

[dpc]

Whatever - the fuses will need to be there if he wants a listed motor starter. I think he was just assuming he didn't need the fuses.

 

[LionelHutz]

Yes. The OP asked if he could use a circuit breaker as the protection for a motor circuit using a contactor. We both agree that the contactor can't be used without coordinated fuses.

 

[RRaghunath]

My experience is that the Vacuum contactor with back up fuses should be good enough for MV motors in general. But if the Switchboard fault level (including the contribution of motors connected to the bus)happens to be greater than that the available MV fuse rating in the market, there is no option but to go in for Circuit breaker as back up the contactor. In such case the ON/OFF commands are only routed to the contactors with the protection trips (including motor protection trip) routed to the CB.
The post doesn't indicate the 5.5kV switch board fault level.

 

[Parkpower]

Thanks for all the reply.

We try to persuade the client to adop the option of fused contactor. However, client still stick on the circuit breaker-contactor because of 55 degree ambient temperature. CB is used to replace fuse,and no fuse will be installed.
I hope we can find vendor to supply this kind of product.

 

[dpc]

It can probably be made to work. Contactors are available with a time-delay on drop-out that would allow the breaker to clear a short circuit before the contactor opens. I know the Cutler-Hammer motor starter contactors have this feature, although it is intended to allow the fuse time to clear. The breaker will be slower than the fuse for very high fault currents.