It is somewhat about risk.
Yes a breaker installation that violates NEC 110.9 needs to be corrected. But how fast and how much danger does it represent?


A breaker's AIC rating is determined by the line side available bolted fault current. Any live work being done on the breaker's load side terminals would definitely provide an extreme danger to the worker.

But circuit impedance downstream of the 'overdutied' breaker will likely legitimately result in lower bolted fault current which may be below the AIC of the breaker. Absolutely NEC 110.9 does not allow us to consider the 'through fault current', so the breaker must be replaced.
But how quickly and what danger does it create by being left in place, should the power be disconnected immediately? Should a blast zone be established? What about the women and children?

In reality the most likely fault to occur will be an L-G arcing fault, unless work is being done on the conductors downstream. Why is it improper to not acknowledge this risk and schedule replacement of the device the same as any other NEC violation. Why can't the ESWPbe written so that live work cannot be done in the panel that contains the overdutied breaker, including provision that the circuit may not be rewired or devices replaced until the overdutied breaker is replaced?

So the question is: does an underrated circuit breaker present a greater or different hazard than an arc flash at the same point in the system? Different, I suppose so, but does it increase the arc flash hazard?

Here's an example: suppose you have an MCC with several underrated circuit breakers installed in it. The maximum calculated fault current is 36,000 amps, but the circuit breakers are only rated at 35,000 AIC. Suppose one of these breakers had a 36,000 amp fault and suppose it failed and ruptured. Does the arc flash energy change? If so, does it go up or does it go down? Certainly, there could be an injury because the breaker ruptured because of ejected matter; but does that fall under the arc flash and shock harazd warning requirements of NFPA 70E? If so, is that part of the labeling requirement?

Isn't it our job to notify the owner of the issues and then install regular warning labels that identify the arc flash and shock hazards per NFPA 70E?

You now have two separate locations where arcs are occurring, so you have increased the hazard. The worker at the downstream fault location has increased exposure due to the longer exposure time waiting for the backup device to clear. Some of the energy is diverted to the overdutied arcing MCC breaker, but I know of no good way to estimate it. And then you have the worker walking by the overdutied MCC breaker. Since he was not contemplating any interaction, he is not wearing PPE will be exposed accordingly.