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 Post subject: Breaker & Relay Trip & Opening Times
PostPosted: Tue Apr 26, 2016 8:43 am 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
I'm attempting to adjust relay and breaker trip times based on realistic expectations of performance in my calculation software. If I use the straight data on relays and breakers provided within the software no safety factor is applied for non-intentional delays of the devices in tripping and opening the circuit. I want to know if I should be padding reaction times to provide a more conservative assessment of my system. I would hate to derive a very low arc flash energy for a bus only to be told by someone wiser that I should have padded the reaction times by 10, 15, 20 percent due to some set of factors I was unaware of.

Does anyone have a methodology in figuring practical (not too liberal, not too conservative) 'fudge factors' that would be used to pad the reaction times of relays and breakers in software programs? If a system is 6 months old it can be expected to response at or near the typical performance data provided by the vendor but how to approach a system that is 16 years old? Or a system that is old but has a high quality record of service & maintenance record vs. a system that is never serviced regularly maintained?

I understand that electro-mechanical relays have larger time margins vs. solid state/static trip relays but what are those typical time margin frames that I can reference? Are they manufacturer specific? Do people on this forum bother going in and padding the software response times?

The software I use has the following discussion on this topic from one class I attended: "The time entered should be Relay Time + Breaker Opening Time. A 5 cycle HV breaker opens in 0.083 seconds. The relay time would be added to this. To determine relay operation time, consult the manufacturer's specifications for the maximum operate time. It is also advisable to add an additional 15-20% as a safety factor. Times in the 0.1 to 0.15 seconds will be typical."

Any insights or suggestions are appreciated.


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 Post subject: Re: Breaker & Relay Trip & Opening Times
PostPosted: Tue Apr 26, 2016 9:48 am 
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Location: North Carolina
Electromechanical relays are +/-15% but that's rule of thumb. The correct value to use is the specification limits that you put in your relay testing specification.

It's not very common to find a 5 cycle circuit breaker. May want to recheck that.

Also if you have a bunch of extra layers for relaying such as electroswitches will add delays.

As to adding a "safety margin" beyond this, there is no need. IEEE 1584 is based on roughly a 90-95% confidence interval on the values. It uses a two step process (use the original available fault current, and 85% of that) which adds a considerable safety margin to everything. There are multiple safety margins built into the IEEE 1584 calculation as well as the ATPV rating. On top of that there's the fact that it is extremely rare to be wearing PPE which exactly matches the incident energy rating. Typically it is significantly higher just because it's not practical to have PPE around that covers every conceivable incident energy value. Add to that for instance that most workers will be wearing say an undershirt but the standards only use the outer layer of clothing as PPE. So numerically the chance of using everything "to the limit" is highly unlikely.


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