Min. Short Circuit Breaking Capacity of Vacuum Contactor

[sykimk]

I need your advice how to select the minimum short circuit breaking capacity of vacuum contactor (VC) for motor controller.

If SC rating of MV system (6kV) is 31.5 kA (Breaking), I think that the required SC breaking capacity of VC will be quite lower than SC rating of swithchgear due to fast acting behavior (tripping) of power fuse. But it is hard to determine the minimum short circuit breaking capacity of VC for specification preparation at the early stage of engineering.

Would you anyone else advise me how to select and determine the minimum short circuit breaking capacity of VC for 6kV motor controller?

Thanks in advance for you advice.

 

[gsimson]

Vacuum contactors are always chosen with fuses for short circuit breaking. Contactors takes care of only overload. And they should be able to withstand the shortcircuit till the fuse is blown. so any tating of app. 5 - 6.5 ka should be sufficient. Details are to be worked out for individual systems.

 

[Stefa]

How about in case of let-through RMS current of fuse is still higher than breaking capacity of contactor,does the contactor short time rating lower than the let-through RMS current make any harm to contactor before fuse break the current.For example if short circuit RMS current of system is 25 kA and from HRC fuse let-through current curve give the let-through RMS current of fuse of 10 kA but contactor have 4 kA short-time rating at 1 sec,does the short-time rating of contactor is still enough or its short-time rating shall be selected to be more than 10 kA.
Thanks

 

[jbartos]

Suggestion: The contactor may be protected by Ground Fault Monitoring relay that complies with UL 1053 Class II. Essentially, it will block the contactor opening until the upstream protector clears. This will safeguard the contactor contacts; however, the contactor must withstand forces due to short circuit current (including peak) since the electromechanical forces will be significant.

 

[Stefa]

jbartos
Could you please elaborate the electromagnetic force awareness,does it mean that we should compare the I^2t of contactor (25^2)x0.01 s with contactor strength(4^2)x 1 s ? which from this,it is enough.

 

[jbartos]

Suggestion: The contactor, just like upstream circuit breaker, should withstand Close & Latch value of the short circuit current which may last a number of cycles given by the upstream breaker rating, e.g. 2cyles, 5cyles, 8cycles, etc.

 

[swgrmfg]

Actually, when combined with a current limiting fuse that interrupts in a 1/4 cycle, extensive testing has been done on vacuum contactors and has proven that the limited let through does not harm the contactor. Also, even if the contactor opens during a fault (say when the blowing power fuse drops the phase that feeds the control power transformer) an arc of about 200V in magnitude (due to metal gas plasma) is generated across the interrupter inside the vacuum chamber, and current keeps flowing through until complete interruption of the fuse occurs. Luckily for the interrupter (actually by design with 'low surge' type contact materials) current does not get chopped because of this internal arc. That is why, most combination-type (contactor-fuse) starters are rated at 50KA at 5KV.

GH

 

[cherryg]

We have recently executed a project where we used ABB Unigear ZVC vacuum contactors upto 400A rating @ 6.0kV. The contactor has a maximum short time rating of 4kA. The fuse takes care of the short circuit interruption and is rated upto 50kA. For more details refer to ABB site library.abb.com/GLOBAL/SCOT/scot235.nsf/VerityDisplay/ 6E839314555D9D7B48256D670008B174/$File/1VGA671003.pdf -