It is widely claimed that most arc flash events are caused by humans via mistakes, incorrect maintenance or installation etc.

NFPA 70E 101.1(F)(2) Human Error: goes as far as to include

… shall address the potential for human error and its negative consequences…

However, some arc flash events have occurred without us humans involved.

Here is this week’s question:

What do you believe is the main cause of arc flash?
Humans, operation, maintenance, installation
Equipment

Stories are always encouraged!

All the arc flash events, at companies i have worked for, have been human error. I know of some fires and other damage from lack of maintenance on equipment but no people were anywhere near. So the question is, If no one is in the arc flash zone, when an arc flash happens, is it an arc flash event?

While I know of insulator flash over and termination failures, however you asked for the main cause which, in my experience, is humans putting their hand/tools were they shouldn't.

In my discussions on this subject I use two terms. Arcing Fault and Arc flash event. All arc flash events are the result of an arcing fault, all arcing faults are not arc flash events. An arc flash event is an event where a human is involved and at risk of exposure to the harmful effects of the arcing fault. If no human is involved then it is an arcing fault with a potential series of negative impact on equipment condition, proper operation of the equipment, etc. The IEEE calculations in IEEE 1584 are not designed to estimate those impacts ore, necessarily relevant parameters. The Iarc calculation may be of value to determine what protective setting should be, though there are also other minimum arcing current calculations, for example, for arcing ground fault currents (see IEEE paper by PK Sen & K Malmedal).
In many cases the benefit of solutions implemented to reduce the incidence energy of an arc flash event will also provide value in the case of an arcing fault not associated with human exposure, I.e. an arcing fault that is not an arc flash event.

Hi Jim, thanks for asking this question. Years ago, I took an arc flash course from AVO. In the course I believe the instructor (Dennis) said that 71% of the arc flash accidents were caused by equipment failures, 2% by animals and 27% by humans. That surprised me. I felt the majority of accidents would be human caused. Since that time, I've come to believe that a large portion of arc flash accidents are caused by equipment failures. For example, the person who was killed when he opened an energized VFD cabinet, or the person who was injured when a fuse holder exploded in his hand, or the person who pulled a Square D QMB disconnect out of an energized gear and was burned because one of the buses stuck to a phase and shorted.

I think many of us believe any accident we're not involved in was because those involved made mistakes or were not qualified. I don't know that I agree. I've found that many of the human caused events involved highly qualified, but older electricians. I think you'll find some older electricians believe this "I've always done it this way, why should I change?" Unfortunately, I think that's a common attitude. Speaking for myself (I'm 71+) I look at some of stuff we used to do and have come to realize how very lucky I was to get away, mostly uninjured. My point is if we get that mechanical stuff sometimes breaks at the wrong time, even the very best electrician could be in front of the stuff when it breaks. In the end, wearing PPE is pretty smart.

In my opinion these are events caused by human failure; a decision was made to work on energized equipment. More details on the VFD incident would help, but the other two events would fall under maintenance, repair, or installation.

Thanks Bill, Hi everyone. Great discussion. FYI, Bill and I go back a ways and I wrote an article about an incident that he was familiar with where the equipment was placed in an electrically safe work condition and the switch failed upon re-energizing.

The article is at: Failure While Switching

Thanks for all the comments. JBD's comments about not working energized is spot on! However, the case above illustrates how failures occur when "doing the right thing" so PPE is an important part of establishing the electrically safe work condition as well as re-energizing (Captain Obvious, here ).

The 2021 Edition of NFPA 70E will now include operating a switch, C/B etc. AFTER installation or maintenance as a risk of an arc flash hazard when using Table 130.5(C) Estimate of the Likelihood of Occurrence of an Arc Flash Incident for ac and dc Systems.