Have you ever had an overcurrent device fail to operate? i.e. not open

• Yes
• No

On several occasions. Fortunately the upstream device trips but then it has us all questioning where the fault was since the device closest to the fault did not trip like logic would indicate.

For anything other than fuses (and that is very rare except expulsion types with the spring), is this even a valid question? Some designs are more reliable than others but all fail. Some relay engineers even purposely plan backup protection.

Great comment! Everything can fail sometime - especially when you need it most (one of those laws from someone like Murphy). I believe it is a valid question and I was fishing to see how many have actually experienced a failure. We'll see how close to 100% this gets.
Back in my utility protection days, we were always planning backup, 1st and second contingency etc.

Fortunately the several failures that I have had were caught during routing testing.

I have seen several fuse failures - without exception they were all due to the wrong voltage fuse being used to replace a blown fuse.

I have seen more small DC Drives with exploded power cubes than I can even count - all failing as a result of 240VAC ABC-type fuses being replaced with automotive fuses.

And in one case someone replaced some 480-volt FNQ fuses with 32-volt rated fuses in a drive cabinet. The 32-volt fuses exploded when an across-the-line starter was engaged.

Other than that, I once saw a circuit breaker with a welded phase

I once considered eutectic overloads near perfect. At a recent site they had problems with them packed full of dust and not tripping. Prior to that most failures I came across were repetitive resetting to the point where the solder was depleted. At this point fuses, vaccuum contactors (under 20-25 years old), molded/insulated case breakers and electronic and eutectic overloads are very high reliability for me, from a safety perspective. That is, even devices such as fuses can give false negatives (trip when it shouldn't) those are acceptable as long as the false negative rate is not excessive. Things like force guided relays, ice cube relays, drawout breakers, air contactors fall in the less than desirable reliability category.

For some of those (force guided relays and air contactors), designing for higher machine risk involves redundancy and a check after release that the component did really release.

We have never had one fail while in-service, it failed during calibration checks, lucky find. The relays may have misoperated, but not failed.

The worst one was a C/B impressively failed. It seems it was extremely overdutied.

What timing. Can we also consider a failure as the device operating when it wasn't suppose to? Sure is a hot topic with the super bowl outage right now. The stories are sounding like it was a relay problem, perhaps mis-coordination.

Run across more than a few Siemens 480v, 800A, 1200A and 4000A rated power breakers failed to operate with direct injection testing due to bad trip units. Examining the trip unit internals found popped 15VDC capacitors on the circuit boards. Had a few GE 480V power breaker that wouldn't close after testing due to jammed trip linkages. Witnessed the aftermath of a few 480V MCCB explosions in I-line panels mainly due to being oil soaked and AIC underrated.

Follow-up question of the week:
Have you ever failed to specify an overcurrent coordination study, and then left the equipment set at the factory default (low), and then caused a significant electrical outage of the Super Bowl, and then blamed the outage on the equipment not functioning as expected?

• yes
• no

The equipment did not function as expected, it did however operate as it was designed to function.

So, is this a problem stemming from incompetance. a lack of education (knowledge), or from improper set up (training)?

A 2007 IEEE paper addressing the reliability and integrity of low voltage overcurrent protective devices reviewed various surveys. It was found that nearly one�third of all circuit breakers failed while in service and thus would not have been identified unless proper maintenance was performed. In addition, 16% of all circuit breakers failed or were damaged while opening.

What is becoming more common these days is vacuum bottle failures, VCB's became popular in the 80's and bottles were designed with a 20-30 year expected life span, when they fail it is usually pretty spetactular.

http://www.vacuuminterrupterfailures.com/