VCB or Fused Vacuum contactor

[jimmy2times]

What are the advantages/disadvantages of using a vacuum circuit breaker as opposed to a fused vacuum contactor for MV motor control. Obviously a fuse is not reuseable. Any other pitfalls in either decision?

 

[RRaghunath]

jimmy2times,
In case of critical motor applications and safety related drives such as fire water pump, you may not like the contactor to drop-off just because there is fault elsewhere in the system causing temporary under voltage.

May be, if you go for latched type contactors that is a different issue.

Otehrwise, the MV contactors save money in cable system design as the cable short circuit withstand could be lower with fuse back up available.

 

[slavag]

Hi.
In additional to Raghun post:
MV vacuum contactor is cheaper then MV vacuum CB.
Regards.
Slava

 

[Gianoli]

In general the operating mechanism of a vacuum contactor is much better suited to frequent switching operation than the operating mechanism of a vacuum circuit breaker. One should expect less maintenance for a given number of operations with a contactor.

The contact material of a vacuum contactor designed for motor switching will be different from the contact material of a vacuum circuit breaker designed for fault current interruption. The contactor will be less likely to generate over voltage transients when switching.

Barring any special circumstances the only valid reason I can think of to use a vacuum circuit breaker rather than vacuum contactor for medium voltage motor control would be in applications were motor currents and or voltages are higher than commonly available motor contactors. Examples could be large 13.2kV motors.

 

[jimmy2times]

Thanks for the info to date.

Gianoli, motors concern are 10kV. Our client in china states that if we allow VCBs then this will give them more options for switchgear supplies. Problem lies with the withstand voltage as I understand. Chinese code requirements for withstand voltage are based on russian standards. We originally were asking for 28kV (75kV) impulse with the contactors with it being a 10kV system. As I understand Chinese code my be more stringent?

 

[Gianoli]

I have no knowledge of Russian or Chinese standards. What I can tell you is if you determine that for whatever reason you need to use vacuum circuit breakers for this application be sure that you’re transient over voltage protection strategy is sound. I would have the motor manufacturer as well as the vacuum circuit breaker manufacturer review it prior to the design being finalized.

 

[dpc]

In the US, I'm not aware of any motor starters above 7200 V, but I haven't looked for a while. Use of VCB for motor control is quite common for large motors, especially at power plants and other facilities where the motors run continuously for long periods of time. If you have frequent starting, then I agree the vacuum contactors may be a better choice - if available. They will certainly be much less expensive.

All MV motors should have surge protection at the motor in any event.