available fault current

[jimbo123]

If two motors of the same Rating , one started across the line and the other using a freq.drive. will they both have the same rating in a arc flash analysis?

 

[ron]

Typically the motor with a VFD will have a different value. Especially if there is no bypass around the VFD, the available fault current at the motor will be less than the across the line motor, and the arc flash calculation will be different.

 

[charlie b]

I agree with Ron. But I will add that with a lower contribution to the total fault current, the breaker that will eventually trip to terminate the event will take longer to trip. That may actually make the arc flash hazard worse, and may require a higher level of PPE.

 

[jimbo123]

Most of the bfd have a bypass incase the drive is bad. But 2 are started across the line because no drive. These are 125/150 hp motors

 

[kwired]

At the motor there will be different fault current levels. At the line side of controller assuming everything else is same they will be same.

 

[ron]

You would likely need to run the calculation for the motors with VFD's (and Bypass) in both bypass and VFD mode. Post the worst case sticker. Or if the bypass is a manual operation to transfer to, then put both stickers with instructions for use.

 

[danw@sor]

I am confused at some of the replies. A VFD would not have a bypass contactor, I think you are talking about a soft start.

I am also confused about the original question. Are you asking about the available fault current at the VFD or at the Branch Circuit Overcurrent Protective Device? The fault current at the controller would be no different, the fault current at the motor may be slightly different.

Another statement was that a VFD would take longer to trip, but VFD's have extremely fast acting semiconducter fuses so they should be faster in a short circuit condition.

 

[jimbo123]

No they are vfd's and the motor can be used with the vfd or across the line . They are connected to another panel and have a option of using the vfd or not. All that is done is setting a selector switch to bypass or vfd. Each of these vfd's has a instantainous breaker within its panel which are fed from the main switchgear.

 

[kwired]

I am confused at some of the replies. A VFD would not have a bypass contactor, I think you are talking about a soft start.

I am also confused about the original question. Are you asking about the available fault current at the VFD or at the Branch Circuit Overcurrent Protective Device? The fault current at the controller would be no different, the fault current at the motor may be slightly different.

Another statement was that a VFD would take longer to trip, but VFD's have extremely fast acting semiconducter fuses so they should be faster in a short circuit condition.

The OP only said motors. The reality is that the motor with the VFD likely has less available fault current, because the VFD is going to limit current to some degree. Most better drives will trip without destructive levels of current as they tend to protect themselves with their own circuitry. At least I have witnessed this with a short circuit or ground fault that is there at startup. Maybe you will see higher fault currents if the short circuit or ground fault happens when the drive is already up to speed when the fault occurs.

Do people normally do an arc flash analysis at motor location? Motor disconnect I can see but the motor itself is not likely to need to do any work that would expose you to the hazard. Not much you can do at a motor while it is running except use a clamp on meter or maybe temperature tests.

 

[don_resqcapt19]

I am confused at some of the replies. A VFD would not have a bypass contactor, I think you are talking about a soft start. ...

We have a number of critical pumps and blowers at the plant I am working at that have VFDs with bypass contactors. That lets us run the pump or blower if the VFD fails.

 

[ron]

Do people normally do an arc flash analysis at motor location? Motor disconnect I can see but the motor itself is not likely to need to do any work that would expose you to the hazard. Not much you can do at a motor while it is running except use a clamp on meter or maybe temperature tests.

We do it at the motor because there are times that IR scans are done for the terminations in the ....... we'll I'm not sure the correct term that doesn't sound rude.

 

[tish53]

one other consideration is the motor contribution to the fault current will be different. A VFD will normally not pass motor contribution back to the system during a fault.