arc flash "Dangerous" levels...

[lielec11]

If, based on an arc flash risk assessment performed via the incident energy analysis method (aka SKM, ETAP, EasyPower, etc.) the incident energy is considered "dangerous", or over 40cal/cm2, does that mean under no circumstances you can operate that device? So if you have a $60,000 circuit breaker you're not allowed to open or close it because the arc flash hazard is greater than 40cal/cm2?

 

[petersonra]

no.

it can still be operated if there is no risk of damage to human beings in the area. a remote operator is one way.

it kind of depends on what you mean by operate too. a MCCB in good condition that is entirely enclosed represents a low enough risk that NFPA70E might well allow one to operate it.

 

[lielec11]

no.

it can still be operated if there is no risk of damage to human beings in the area. a remote operator is one way.

it kind of depends on what you mean by operate too. a MCCB in good condition that is entirely enclosed represents a low enough risk that NFPA70E might well allow one to operate it.

I mean open/close the breaker... open/close a plug-in busway switch. and i don't mean remotely via a remote control panel. i mean can someone in full PPE stand in front and throw the switch, etc? where does 70E discuss this?

 

[jim dungar]

I mean open/close the breaker... open/close a plug-in busway switch. and i don't mean remotely via a remote control panel. i mean can someone in full PPE stand in front and throw the switch, etc? where does 70E discuss this?

That is a decision your company needs to make.

All that OSHA and NFPA70E require, is for you to perform a risk analysis and then adopt appropriate work practices, including PPE.
Knowing what danger is present (the AF incident energy) is only part of the issue, you also need to address the probability of an arc occurring.

Just remember, 'possible' and 'probable' are not equal, (just ask all the people that bought lottery tickets)

 

[petersonra]

I mean open/close the breaker... open/close a plug-in busway switch. and i don't mean remotely via a remote control panel. i mean can someone in full PPE stand in front and throw the switch, etc? where does 70E discuss this?

If there is a risk that exceeds 40 cal, then there is no amount of PPE that makes it "safe".

If the risk is so low as to be all but nonexistent, no PPE is required at all.

 

[Paul1955]

I mean open/close the breaker... open/close a plug-in busway switch. and i don't mean remotely via a remote control panel. i mean can someone in full PPE stand in front and throw the switch, etc? where does 70E discuss this?

I would say 130.2(A)(4) covers this situation: (4) Normal Operation. Normal operation of electric equipment shall be permitted where all of the following conditions are satisfied:
(1) The equipment is properly installed.
(2) The equipment is properly maintained.
(3) The equipment doors are closed and secured.
(4) All equipment covers are in place and secured.
(5) There is no evidence of impending failure.

 

[lielec11]

So let me ask a potential "dumb" follow up question. If I'm in front of a piece of switchgear and operating a breaker or switch with all doors closed, does that affect the level of incident energy compared to if the enclosure was open? I know the calculations don't take into consideration switchgear construction (i.e. metal enclosed vs. metal clad) but common sense would say there is less incident energy if the arc is behind a close cabinet. Thoughts?

 

[Paul1955]

I recently had a client whose corporate chief electrical steward required me to design a "remote breaker control switch" after the AF study was completed. Essentially a 25 ft SOOW cord, with an open-close switch mounted in a single gang deep outlet box and a twist lock plug on the end. Mounted a twist lock receptacle on each main breaker & the tie-breaker. The local electricians plug into whichever breaker they want to control, walk out the door of the equipment room, and push the switch. The chief steward didn't want his people to open or close the breaker while standing in front of the switchgear, even when suited up.

 

[petersonra]

So let me ask a potential "dumb" follow up question. If I'm in front of a piece of switchgear and operating a breaker or switch with all doors closed, does that affect the level of incident energy compared to if the enclosure was open? I know the calculations don't take into consideration switchgear construction (i.e. metal enclosed vs. metal clad) but common sense would say there is less incident energy if the arc is behind a close cabinet. Thoughts?

the amount of IE does not change. what changes is that when the door is open there is a higher risk.

 

[GoldDigger]

the amount of IE does not change. what changes is that when the door is open there is a higher risk.

I would say rather that there is a higher exposure to (some portion of) that incident energy.
To me risk would relate to the likelihood of an incident happening in the first place, and having the doors or covers open or closed while operating a breaker does not change that.
It surely does change the likelihood of dropping a wrench across live surfaces when working hot!

 

[lielec11]

I think you're both saying the same thing in different ways. The IE is calculated at any point on an energized conductor or circuit part correct? The way I'm seeing it now is that it's the same thing as a stick of dynamite; the amount of energy available in the dynamite is the same whether you're standing on it or you're behind a 12" thick brick wall.

 

[petersonra]

I think you're both saying the same thing in different ways. The IE is calculated at any point on an energized conductor or circuit part correct? The way I'm seeing it now is that it's the same thing as a stick of dynamite; the amount of energy available in the dynamite is the same whether you're standing on it or you're behind a 12" thick brick wall.

Not a bad analogy.

 

[jeremy.zinkofsky]

The category ratings do not factor in any arc flash resistance or withstand of the electrical enclosure unless it's design has been approved for such use. If a piece of equipment is categorized as dangerous, it means that no one is allowed to operate it (at all) when energized. However, you can still mitigate the risk to personnel proceduraly. Take a 4160v switchgear at a substation for example. It is most likely fed from a high voltage circuit breaker in a switchyard or some other high voltage equipment. The incident energies at these pieces of equipment are usually less than at medium voltage equipment. Furthermore they are usually remotely operated. If you de-energize the feed to the dangerous equipment then you are free to do whatever you want to that equipment without wearing any PPE.

Of course you don't have to wear any PPE at all if you are so inclined. But if something happens not only will you probably die, the company that owns that equipment will be held liable. So if there is no remote operation or maintenance mode capability of the equipment, it is best to just de-energize everything to be safe.

 

[MD84]

I agree with a lot of the information here but I do question the open vs closed enclosure topic.

I agree that the risk of causing an incident is reduced when doors and panels are closed. However should an incident occur a closed box could present an increased arc blast due to pressure building to a higher degree before exploding the enclosure and releasing the energy.

Incident energy classified as dangerous will surely blow open the doors to a panel or enclosure in a violent manner.

I am not suggesting opening doors to reduce risk or hazard.

I worked at a company where we operated medium voltage equipment classified as dangerous. We used a third party analysis which ultimately allowed us to operate the equipment. Multiple layers of redundant safety measures were utilized to mitigate the hazard.

Increasing distance from the calculated point will reduce incident energy inversely proportional to the square of the distance.

Although NFPA 70E allowed local operation, one vendor I have worked with devised a string pull mechanism to open breakers remotely that did not utilize remote operators.

I do not have a copy of 70E in front of me but I believe that if one is not manipulating equipment in such a manner to cause an arc flash it would not be required to wear ppe. This does not prohibit the use of ppe. I would say that operation of a breaker qualifies as an operation that could cause an arc flash. What constitutes an operation that could cause an arc flash is subject to interpretation.

 

[MD84]

The category ratings do not factor in any arc flash resistance or withstand of the electrical enclosure unless it's design has been approved for such use. If a piece of equipment is categorized as dangerous, it means that no one is allowed to operate it (at all) when energized. However, you can still mitigate the risk to personnel proceduraly. Take a 4160v switchgear at a substation for example. It is most likely fed from a high voltage circuit breaker in a switchyard or some other high voltage equipment. The incident energies at these pieces of equipment are usually less than at medium voltage equipment. Furthermore they are usually remotely operated. If you de-energize the feed to the dangerous equipment then you are free to do whatever you want to that equipment without wearing any PPE.

Of course you don't have to wear any PPE at all if you are so inclined. But if something happens not only will you probably die, the company that owns that equipment will be held liable. So if there is no remote operation or maintenance mode capability of the equipment, it is best to just de-energize everything to be safe.

This is preferred but not always practical.

Equipment classified as dangerous does not prohibit operation of that equipment. The equipment can be operated if an analysis is performed and operation is allowed following certain mitigation steps.

 

[jim dungar]

Incident energy classified as dangerous will surely blow open the doors to a panel or enclosure in a violent manner.

This is absolutely false.

Arc blast is primarily contingent on the amount of current flowing through the arc. As an event it occurs and is over before protective devices even becomes to react to the fault.
Arc fault incident energy is based on the length of time the current flows.

Anecdotally I have heard of many more people being burned but I have never heard of someone being crushed to death during any arcing fault.

It is too bad we cannot calculate arc blast, I believe there are many instances where doors are blown open, and people knocked down, even when the fault is <8cal/cm^2 of incident energy. Likewise, I am sure there are many 'Dangerous' locations, like transformers secondaries, that have relatively passive arcing events that happen to last for a long time.

 

[GoldDigger]

"Relatively passive". Gotta love that term! Can I use it too?
I will match it up " venting with flame" instead of burning and "spontaneous rapid disassembly" instead of explosion. (Both battery manufacturer terms.)

 

[Fulthrotl]

If there is a risk that exceeds 40 cal, then there is no amount of PPE that makes it "safe".

anecdotally, when i did my arc flash training, the person giving the test explained that
while you can get PPE up to 100 cal suits, any blast over 40 cal, the pressure wave was
most likely fatal, so buying suits over that level was just making a more miserable suit
to work in, without any benefit.

and the unapproved but simple "remote operator" fashioned with available materials,
(that 8' long 2x4 laying there) can go a long way towards reducing the energy.

I2R losses can be your friend for once.

 

[mull982]

This is absolutely false.

Arc blast is primarily contingent on the amount of current flowing through the arc. As an event it occurs and is over before protective devices even becomes to react to the fault.
Arc fault incident energy is based on the length of time the current flows.

Anecdotally I have heard of many more people being burned but I have never heard of someone being crushed to death during any arcing fault.

It is too bad we cannot calculate arc blast, I believe there are many instances where doors are blown open, and people knocked down, even when the fault is <8cal/cm^2 of incident energy. Likewise, I am sure there are many 'Dangerous' locations, like transformers secondaries, that have relatively passive arcing events that happen to last for a long time.

I believe the current IEEE1584 and NFPA 70E standards and equations for calculating Incident Energy only calculate the amount of thermal energy (heat) associated with Arc Flash but do not calculate or identify the energy associated with blast pressure.

Perhaps further testing and later versions of these standards will address the blast pressure associated with Arc Flash. I have also seen testing where the blast pressures were extremely high for incident energy below <8cal/cm^2

 

[MD84]

This is absolutely false.

Arc blast is primarily contingent on the amount of current flowing through the arc. As an event it occurs and is over before protective devices even becomes to react to the fault.
Arc fault incident energy is based on the length of time the current flows.

Anecdotally I have heard of many more people being burned but I have never heard of someone being crushed to death during any arcing fault.

No. Arc flash incident energy is determined by the distance to the fault, the magnitude of available bolted fault current, and the clearing time of the protective device. Arc flash incident energy is not only based on the length of time current flows. If that were true then for a given distance and duration a 5kA fault would have the same incident energy as a 50kA fault. This is just not true.

I will admit that my statement could have been worded more accurately. It is not always the case that classified dangerous equipment will surely and violently blow the doors off. This is only my real world experience.

I do agree that an arc blast would be determined by the current of the arc. A dangerous location generally has more current available then a location with lower incident energy. I would say that incident energy and arc blast levels are positively correlated. Therefore a location classified as dangerous would be more likely to explode violently than one with a lower incident energy.

How many catastrophic equipment failures or arc flash incidents have you witnessed where the incident energy was >40cal/cm^2 and the equipment doors/enclosure did not blow open?