This week’s question continues the series of questions about the confidence in the ability of various devices to perform as expected when it comes to an arc flash.

This week’s question is about older low voltage circuit breakers. Old is probably a relative term but let’s think over 20 years old.

The duration of an arc flash is generally considered to be based on the clearing time of an upstream protective device. The incident energy is directly proportional to the arc duration. Typically, the device’s time-current curve is used to determine the clearing time as part of an arc flash study and is frequently taken as absolute.

Of course, it is also based on condition of maintenance – feel free to comment about this issue or anything else you may think of.

Here is this week’s question:

Would you trust that a Low Voltage circuit breaker’s clearing time will be as predicted?
-100% Total Trust (a life may depend on it)
-Moderate/Reasonable Trust (nothing is perfect)
-Low Level of Trust
-Not at all

If the breaker is maintained then I’d be confident, but I think the ‘nothing is perfect’ option loads or skews the question, in the sense that of course nothing is perfect;. Perhaps ‘a life depends on it’ does too as in any arc flash calc we are looking at potential injury or worse, all of which can effect the quality of a person’s life.
‘100% total trust’ is an interesting place to go, it requires total trust in many inputs from data collection to maintenance to manufacturers tables to software. The 2002 IEEE 1584 standard was based on 300 tests and the 2018 about 1900. Results are interpolated. SKM software does not even (yet) have the same enclosure options.
Therefore, I find it difficult to vote on one option versus another. On an earlier question on maintenance switches I have the same concern. Would it make sense to revise the question for example replace ‘total trust’ with ‘highest confidence’?

The brand of breaker would considerably influence my answer to this question. In our region, one particular brand of breakers has a particularly negative reputation and are typically referred to as "firestarters". My own experience is that when these breakers age they have wildly varying trip points (in one case the pole fuse blew and the breaker never did trip.) In another case, the old analog circuits in the trip relay failed, reducing the breaker to functioning as a manual switch.

Great idea. My wording did seem to make it an absolute - which it is not. Thanks!