LV CB Open time with electronic trip for arc flash 1

[EP007]

For a LV molded case circuit breaker, equipped with electronic trip unit, the tripping curves may only include the trip unit time, but may not include the CB opening time thereafter. Has anyone seen manufacturer's data on this? The application is arc flash analysis.

 

[dpc]

For any molded case circuit breaker (at least in the US), the manufacturer's TCC for that breaker will include the breaker operating and total clearing time, regardless of the type of trip unit used. You don't need to add any additional time. Modern molded case breakers are very fast at the high end of their fault current rating, due to the physical design of the main contacts.

With electronic trip units, you have to make sure you are using the curve for the appropriate type of breaker (molded case vs insulated case vs power circuit breaker). There will be separate TCCs for each type of breaker even though the trip unit model sounds identical.

David Castor

http://www.cvoes.com

 

[EP007]

dpc, I have been in agreement with what you have said. However, the Consultant maintains that he has looked in detail at C-H NDC with RMS310, discussed with the manufacturer, and concludes the curve is only for the trip unit, and does not include CB operating time, which I am asked to include. I see the trip unit curve in the INST region is about 20 ms. As a comparison, some shunt trips on a TM CB is about 50 ms. I believe the consultant is knowledgeable and thorough and therefore I am examining this.

 

[dpc]

I would contact Eaton directly. I have never heard of this before. If you can post the TCC in question, I'd be interested in looking at it.

If the TCC references "Total Clearing Time", that has to include the breaker clearing time as well as the trip unit time.




David Castor

http://www.cvoes.com

 

[EP007]

The TCC is on page 2 of the following:

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=168977

 

[dpc]

Note the top curve referenced as "Total Clearing Time". This has to include the breaker time by definition.

Also, the TCC is for a particular circuit breaker with a particular trip unit - not for the trip unit alone.

What is your consultant seeing on this TCC that indicates the times do not include the breaker time?

I don't see anything unusual about this TCC that would change my opinion. Let us know if C-H says something different.

David Castor

http://www.cvoes.com

 

[davidbeach]

What dpc said.

 

[EP007]

I believe you are correct dpc. On the curve it does state Total Clearing time. I will now ask the source of the consultant's info. Thanks.

 

[rhatcher]

DPC is right with his answer. I have direct experience testing circuit breakers including the various versions of the Cutler Hammer RMS trip system. The total trip time for the RMS trip system is within the curve that is shown. Note that the RMS trip system uses a dedicated trip actuator. This is a separate actuator from a shunt trip.

The trip time for an (external to the breaker) 'arc flash protective device' that uses a shunt trip to trip the breaker will be different from trip time for the RMS trip system that is shown on the curve you are referencing.

So..it may be that DPC and your consultant are both right.