Ok. I'm investigating an arc flash accident.

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In the above image. The breakers encountered a first short which deposited carbon into the plastic enclosure and lugs. During a second event. The electrician just touch the right most live terminal into the right most lugs. It suddenly arc flashed causing the damage as well as hurting his arms.

What I want to know is. If there is merely carbon deposited in the breakers. Is it enough to cause arc flash between the two live terminals. It's like the carbon initiated the arc flash?

The transformers is an open delta 3 phase composing of two 75kVA transformers serving 240volts to an office building (the red and black line). It's connected directly by short wires to the service meter and directly the breakers. When the arc flash occurred, there was no overcurrent devices upstream of it. The transformer breaker didn't trip.

Just tell me if mere carbon deposit in the breakers can initiate arc flash. Notice the middle plastic is not damaged. It's as if the arc flash jumped in front of it between the terminals, is this possible? Note the smaller wire is not connected to the right most live wire. The smaller wire is just the ground hidden at the back of the live wire.

posted yesterday:

To clarify my questions. What I was asking was supposed just for sake of discussion the drop thing is not a tool but a thin layer of carbon (again just for sake of discussion or better yet let's say the switch gear or whole facility is turn off and you put a thin layer of carbon between the conductors ). Since a thin layer of carbon is a conductor.. and this bridges the gaps between the two terminals or huge wires with large incident energy, would this initiate an arc flash or should the thing drop be a tool that can pass through huge current? Or can a thin layer of carbon enough to cause chain reaction (or negative resistance) effect that would build up the arc plasma from miniscule until the textbook arc flash occurs that throw the person across the room?

original message:

In panels or switchgears where a tool dropped can initiate a major arc flash. I'd like to understand the behavior if carbonized particles (say from previous short) get into the conductors causing a short. What quantity before they can initiate and chain react into a major arc flash?

Imagine there is little amount of carbonized particles between the two phases or conductors. Once it conducts. It can vaporize immediately. Would this be enough to establish a major arc flash. What is the carbon deposit threshold before this can occur?

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    \$\begingroup\$ Don't think of this type of explosion as being like a gas explosion, where it's fairly well defined at atmospheric pressure at what concentrations there is opportunity to ignite. Different gases, and different particle impurities will affect the resistance of the gas and also the way it behaves when it becomes part of an arc. If air is being used as an insulator, any effect on its resistance puts you at risk of an arc beginning. The voltage and ampacity of the system, type of current and any inductive/capacitive factors will all have an effect on the circumstances necessary to create or \$\endgroup\$ – K H Feb 22 at 1:12
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    \$\begingroup\$ quench an arc. The moment an arc has begun, the resulting plasma has vastly different properties from non-plasma gas, temperature and pressure change, and because plasma is a better conductor, the arc easily sustains itself, although they tend to move upwards in the middle due to thermals/air pressure, lengthening the arc and sometimes quenching it, although when this principle is used intentionally(see arc horn), I think there is usually a motion of physical separation. Other than the chance of the arc quenching, by design or otherwise, once it begins, it can easily reach the capacity of \$\endgroup\$ – K H Feb 22 at 1:28
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    \$\begingroup\$ the power source, so it's more that the particles can increase until the point an arc begins, but after that point, whether you have a "major" arc flash will be mostly determined by the source(and possibly quenching properties of the design). The carbonized particles may serve to help initiate the heat required to produce the plasma(although this is just a function of voltage and resistance). \$\endgroup\$ – K H Feb 22 at 1:29

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