Contactor Performance during fault

[victa1288]

Hi,Guys,

I am dealing with One medium voltage starter company.I don't want to mention its name.

This company chose a fuse-contactor combination and our senior enginner approved it. But later on, after I did short circuit calculation, i realized that fuses can't well protect the contactors. What would happen is, during the fault, the contactors will open itself because of voltage dip on its coils before the contactors can clear the fault(The fuses are too big), meaning the contactors would have to interrupt the fault above its interrupting capacity. It is very dengerous.

But that senior guy and the manufactuer insisted on their doings. His argument is, the manufactuer deigned the fuse-contactor- starter for the motor protection. Therefore, their contactors will be well protected, because this is a international company, they cann't make this kind of mistakes.. But how could they choose the fuses without knowing the fault levels?

I got so frustrated about what they are doing.

I would appreciate it if somebody can direct me to a right way to deal with them. Should I quit or live with it?

 

[RajT]

Are you sure you are correct in your findings as it is most unlikely that the switchgear supplier can get the fuse size incorrect.
You could also present/ send your findings to the supplier for his comments and if you are correct then he should be grateful.

 

[LionelHutz]

Sizing fuses to protect the motor and using the right fuse to protect the contactor are two completely seperate requirements. Picking a fuse that protects the motor does not mean that the contactor is also protected.

You could just go directly to the manufacturer with your findings and confirm your findings with them. After all, the fuse/contactor co-ordination is part of the fault study so contacting the manufacturer to confirm your findings is part of that study.

Oh, just being a big company doesn't make them immune from making mistakes.

 

[sitl]

You should check once again why contactor would open. From where is that voltage dip coming from? The problem here might be the dynamic and thermal withstand of contactor while conducting short circuit current, not clearing it.

 

[rcw retired EE]

Does the contactor have a DC coil? Many medium voltage and large low voltage contactors use a DC coil, even with AC control power. A full wave rectifier bridge with a smoothing capacitor provides the DC. The capacitor may also hold the contactor in during a voltage dip.

In another case, we thought the fuse was too large because at the contactor's maximum rating of 6000 Amps, the fuse took some time to clear the fault. The manufacturer showed that the contactor drop out time was longer than the fuse clearing time. The contactor would never open during the high fault before the fuse blew.

 

[nightfox1925]

Check your contactor drop out time against the associated fuse clearing time at the specified fault current value. Due to the fuse current limiting effects, a let-through current will persist and this current value should be checked against the contactor withstand rating. If the fuse blows fast enough before the contactor opens and the contactor withstand rating (make rating)is adequate for the current let-through then, we can say that the selected fuse will protect the contactor. You may want to double check the manufacturer's fuse TCC for this. Normally, you specify your MCC with a symmetrical short circuit rating which becomes a base line for the manufacturer to select the minimum short circuit rating of the fuse. This starters are also tested as a complete combination as per UL.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[sitl]

Well, I have never encountered problem that voltage dip in medium voltage cables during short circuit from the point of stiff voltage would be ~30% of the nominal voltage, in order to cause contactor opening. On the other hand, in all motor control circuits for medium voltage motors we use either DC voltage from batteries or AC voltage from UPS, in that way preventing unintentional contactor opening/closing.