Sometimes protective devices fail to open under overload or short circuit conditions. In most cases, the fault clearing time of an upstream protective device is used to define the duration of an arc flash.

Here is this week's question:

Have you/your client ever had a protective device misoperate?

• Yes
• No
• Doesn't apply

Kind of an open ended question isn't it? Miscoordinate, not operate at all, blow up, other failure, etc. etc. I voted yes, I'm sure we have had all of this.

Thanks for the comment! The question was intentionally "open ended" with regard to the term "mis-operate". Since the magnitude of incident energy is a function of the arc flash duration, any type of mis-operation can have a serious affect on Ei. If a device operates slower than predicted or not at all, the incident energy would be much worse. Great case for reviewing maintenance practices.

I can also hear someone saying mis-coordination can improve things if the protective device upstream from the one used for the calculation operates faster. (HOWVER - NOT Justification for miscoordinating the electrical system )

Bottom line, I believe most in the industry have had a device not perform as expected. There have been a few papers written about this as well.

Don't forget that mis-operations aren't just failures to operate as expected during fault conditions, but include failures to stay secure as expected during non-fault conditions. Might not have the IE implications, but still not a good thing.

Funny you should bring this up. Last week my son was installing a ceiling fan and before he began, went to turn off the breaker (he learned well )

He said as he was switching the breaker off it popped of the bus and let out a bit of a spark - still under load. Yes, faults aren't necessary to have a problem.

You can't seriously ask this. Misoperation in service with a breaker is somewhat rare but definitely still happens. Biggest issue for us seems to be with the larger ones with the external power supply, whether capacitive trip or batteries. Seems to be about 20 years when failures occur, sooner with batteries unless properly maintained. With a fuse as they age they tend to trip early. Never had a fuse trip late (yet), although it probably has happened. I can't think of misoperation of a breaker though that is newer than about 10 years old except for recent vintage GE Wavepros when the contact tips fall off after a few months of operation (not making this up).

I have. 69 kV primary power fuses. When the arc quenching chemical inside the tube absorbs enough moisture, it holds the element in place so that it fails to separate. After the element vaporizes, the now charred tube continues to carry the current. Nice bright white glow from those things.

I have not seen a fuse clear late either, unless you count the customers for that replaced some 480V fuses with 32V fuses. That failed spectacularly.

I have also seen people replace ABC (240VAC) fuses with automotive (32V) fuses in small DC Drives. That always results in the power cube failing

The one thing that always puzzles me with fuses is the interrupting rating on an expulsion fuse...huh? If you exceed the interrupting tating, it blows apart...which is exactly what it's supposed to do in the first place!

Unless the plasma generated by the arc continues to carry the current until interruption by another device allows the plasma to dissipate.