arc flash risk

[the blur]

Is the arc flash risk less if a service entrance panel is unloaded ? by unloaded I mean turning off all of the loads, HVAC's, sub panels, etc, etc. or would I presume currently the arc flash risk is higher with a few hundred amps flowing through the panel ?????

I'm not talking about OSHA rules, or PPE ratings, just wondering the science behind arc flash.

 

[Volta]

Not that I can think of, other than rotating machinery loads that could contribute to the energy otherwise available from the line conductors.

 

[zog]

Loading has no effect on arc flash, you still have the same available fault surrent and will have the same clearing time weather the loads are on or off.

 

[charlie b]

Sorry Zog, but Volta has it right. Any motors that are running at the time of the fault will add to the total current at the point of the fault. Those motor contributions are included in the calculation of short circuit current, and the calculation of arc flash energy. To address the original question, whether the board is loaded or unloaded is not, by itself, relevant. What is relevant is the amount of that load that comes from large motors.

 

[the blur]

So then it would make sense to shed any MOTOR loads off the panel, correct ???

 

[petersonra]

So then it would make sense to shed any MOTOR loads off the panel, correct ???

As a practical matter, probably not that much difference. But there is some difference.

You could look at the calculations and see what it is, but it is not likely going to change anything substantially in how you have to do stuff.

 

[the blur]

So then how do you reduce the risk of AF ?
Saying your trouble shooting a problem, you must work on live equipment.
You shed all the loads, your not dropping any tools. your careful about removing covers, especially the corners.

The spacesuit only increases risk of being clumsey and making a mistake, so we'll leave this for another discussion.

There must be certain steps you can take to minimize the risk of AF.

 

[WorkSafe]

So then how do you reduce the risk of AF ?
Saying your trouble shooting a problem, you must work on live equipment.
You shed all the loads, your not dropping any tools. your careful about removing covers, especially the corners.

The spacesuit only increases risk of being clumsey and making a mistake, so we'll leave this for another discussion.

There must be certain steps you can take to minimize the risk of AF.

You have to perform an arc flash analysis in order to figure out what Cal/PPE you would be using. You can't start troubleshooting equipment without knowing what level of PPE is required just on the assumption you will "be careful."

A well thoughtout design of the equipment is one way of reducing arc flash. Just my opinion.

 

[the blur]

How does doing the calculation reduce the risk of an AF ?
Were not talking about PPE.

 

[petersonra]

How does doing the calculation reduce the risk of an AF ?
Were not talking about PPE.

It does not reduce the risk, but it does tell you what the level of risk is.

You can use it to make decisions about potential changes that might reduce the risk though.

 

[charlie b]

A calculation cannot reduce the probability that an arc flash event might occur. But the calculation will tell you the amount of energy that could be released during an arc flash event. If that number is high enough, then no amount of PPE would be enough to protect the worker, in which case live work must not be attempted. You need to know where you stand, before you begin this work.

 

[WorkSafe]

How does doing the calculation reduce the risk of an AF ?
Were not talking about PPE.

Read the rest of my post. I said better designed equipment could reduce the chance, then again, I'm not a engineer. Sounds like the OP is just talking about existing equipment.

 

[zog]

Sorry Zog, but Volta has it right. Any motors that are running at the time of the fault will add to the total current at the point of the fault. Those motor contributions are included in the calculation of short circuit current, and the calculation of arc flash energy. To address the original question, whether the board is loaded or unloaded is not, by itself, relevant. What is relevant is the amount of that load that comes from large motors.

Yes I know motor contribution is a factor, done plenty of studies, but motor contribution may actually lower arc flash hazards depending on the effect of clearing time.

But I was not refering to motor loads (I should have added that), my point was for a panel you still have an arc flash hazard regarless of there being load or not.

 

[zog]

How does doing the calculation reduce the risk of an AF ?
Were not talking about PPE.

You were talking about PPE, stating a moonsuit increases the risk, and I agree, which is why you do a study to select the right PPE. Working in coveralls and a face shield is a lot easier than a moon suit, given it is the right PPE to wear.

 

[handy10]

Is the arc flash risk less if a service entrance panel is unloaded ? by unloaded I mean turning off all of the loads, HVAC's, sub panels, etc, etc. or would I presume currently the arc flash risk is higher with a few hundred amps flowing through the panel ?????

I'm not talking about OSHA rules, or PPE ratings, just wondering the science behind arc flash.

Perhaps this is not the place to ask about how an arc flash occurs, but I will anyway. If there is no current flowing (that is what I take is meant by a completely unloaded panel), how can a flash occur? Is not the flash caused by opening a circuit when current is flowing?

 

[charlie b]

No. The arc flash event begins with a short circuit of some type within the panel that is open for maintenance. It does not matter if the panel is loaded at that moment. Perhaps a tool accidentally contacts a live bus bar, creating an arc. That heats the air within the immediate area to such a high temperature that it ionizes. The ionized air becomes itself a short circuit path, so that current can flow from a Phase A bus bar to a Phase B bus bar right through the ionized air. That will continue to heat the air, and creates a flash of light as well as a pressure wave that can shock, blind, and push the worker away from the panel with such violence that he can be injured or killed by his body striking the opposite wall.

 

[charlie b]

Is not the flash caused by opening a circuit when current is flowing?

If the load is inductive in nature, such as a motor, then when the breaker opens it will draw an arc across the breaker's contacts. However, breakers are designed to safely take that amount of arcing. Large breakers (i.e., switchgear drawout breakers) have specially designed arc chutes to disperse the arc, and to allow it to dissipate, without causing any damage. Smaller breakers would be serving smaller loads that would draw smaller arcs, so the contacts are built to withstand the arc.

 

[Jraef]

Perhaps this is not the place to ask about how an arc flash occurs, but I will anyway. If there is no current flowing (that is what I take is meant by a completely unloaded panel), how can a flash occur? Is not the flash caused by opening a circuit when current is flowing?

The one and only time I was subjected to an arc flash was when I was commissioning a 500HP motor starter. The installing contractor had pulled the cables into the enclosure with the starter removed so he would have plenty of room to work, then tested the cables before installing the starter. But to get the starter panel back in, he had to pull the cables back into the conduits. He installed the starter panel, then terminated the cables again and I was called in to commission it. I did a visual inspection prior to energizing it and as I was closing the enclosure double door, the guy in the switchgear room down the hall jumped the gun a bit and hit the Close button on the power breaker feeding this unit. Unfortunately when the installer had pulled those cables back into the conduit for mounting the starter panel then bak out again as he terminated them on the starter disconnect, he didn't re-test the cables. Turns out he must have nicked the insulation and when the guy closed the up-stream feeder power breaker, the cable faulted to gound and vaporized, with me still closing the enclosure door. Lucky for me the breaker door interlock had just engaged, so the force of the flash was slightly diverted up and out the center crack, but I was standing off to the right just enough not to get badly burned. Everyone got excited about why it flashed and why I had closed the starter disconnect prematurely, until I pointed out that the starter disconnect was still OPEN! In other words there was ZERO "load" on the circuit when the arc flash occurred. Load has little or nothing to do with it.

Arc flash has only to do with the AVAILABLE energy in the circuit, not what is being utilized. Motor loads may contribute to INCREASE the available energy by a small amount, but removing them, or anything else for that amtter, does nothing to reduce the POTENTIAL energy that is inherent in just having the conductors present and connected.

 

[Volta]

No. The arc flash event begins with a short circuit of some type within the panel that is open for maintenance. It does not matter if the panel is loaded at that moment. Perhaps a tool accidentally contacts a live bus bar, creating an arc. That heats the air within the immediate area to such a high temperature that it ionizes. The ionized air becomes itself a short circuit path, so that current can flow from a Phase A bus bar to a Phase B bus bar right through the ionized air. That will continue to heat the air, and creates a flash of light as well as a pressure wave that can shock, blind, and push the worker away from the panel with such violence that he can be injured or killed by his body striking the opposite wall.

All true.

It is also worth noting is the fact that the fault can also occur when the enclosure is closed, when seemingly simple interaction with the equipment happens. Just operating a disconnect, whether opening or closing, produces a chance of equipment failure, such as a weak structural insulator breaking, a clip coming loose, a mounting bolt falling, or a hundred other things.

It does not need to be anything a worker does "wrong", just being in the wrong place at the wrong time.

 

[zog]

All true.

It is also worth noting is the fact that the fault can also occur when the enclosure is closed, when seemingly simple interaction with the equipment happens. Just operating a disconnect, whether opening or closing, produces a chance of equipment failure, such as a weak structural insulator breaking, a clip coming loose, a mounting bolt falling, or a hundred other things.

It does not need to be anything a worker does "wrong", just being in the wrong place at the wrong time.

Right, perhaps the most common misconception I see is people believing enclosures (Excluding arc rated ones) are dsigned or tested to contain an arc flash. They are not and very often they won't.