Arc Flash

[cb3]

I'm an Electrical Engineer studying up on Arc Flash. It seems to me based on NFPE 70E that the PPE (Personal Protective Equipment like fire suits) requirements are only if someone has exposed live conductors. The Arc Flash boundary does not seem to exist if there are no exposed live conductors. (see definition of arch flash boundary in NFPE 70E)

To me this means that if someone deenergizes (lockout/tagout and grounds) a 480V MCC or switchgear, for example, then works on the equipment with no PPE. Later, they finish the work, close up the MCC, and are ready to close the main breaker on the MCC/switchgear. Do they need to have PPE to close that main breaker right in front of the MCC/switchgear?

I?m concerned with the literatures ambiguity/vagueness stating that they don?t, but safety says they do. I imagine a case where they close onto a fault in the MCC/switchgear. When they close the breaker effectively an explosion occurs in the panel and that flimsy cover isn?t going to help that guy from getting severe burns.

I think that none is necessary to be compliant with NFPA 70E but one should for safety anyway.

 

[bphgravity]

Keep in mind that all NFPA standards and codes contain minimum provisions for safety and should not be considered good design.

 

[mo2004]

I am an electrical engineer currently desgin to elaminate ARC fLASH. The rule of thumb is if there is as ource of electrical energy inside the enclosure regradless of the main disconnect of the specfic enclosure it is considerd to be live and arc flash applies.

 

[zog]

You are correct in your thinking and rightly confused with the definition. That definition is being changed for the 2009 70E. I have discussed this with the 70E commitee and they say the arc flash boundary applies whenever you "interact" with the equipment, regardless of the live parts being exposed or not.

In other words, the boundary applies to operate the switch/breaker but not to just walk past the equipment.

 

[Pierre C Belarge]

Keep in mind that all NFPA standards and codes contain minimum provisions for safety and should not be considered good design.

Bryan
Would it be more appropriate to state "may not be considered" good design as compared to what you stated? There are some instances when the minimum of the NEC is good design...

 

[jtinge]

To follow up on what Zog said, the definition for Flash Hazard was proposed to be changed as follows:

Flash Hazard. A dangerous condition associated with the possible release of energy caused by an electric arc.

FPN No. 1: A flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc.

Per the FPN, the hazard is considered to exist if energized parts are exposed or a person is interacting with the equipment IN SUCH A MANNER TO CAUSE AN ELECTRIC ARC. Not all interaction with energized equipment will cause an electic arc. Need to be careful how this is interpreted. Racking a breaker certainly would fit the bill, but I don't think resetting a breaker would.

 

[jim dungar]

Racking a breaker certainly would fit the bill, but I don't think resetting a breaker would.

Closing a breaker onto a short circuit might cause. So it depends on why the breaker is being reset.

 

[zog]

Right, depends on why the breaker is being reset. IAW OSHA 1910.342. I have responded to dozens of arc flash accidents this year following improper reseting of a breaker into a fault.

 

[brian john]

As Zog said...

The electrician spent time in the hospital and missed several weeks work after closing this into a fault.

I have seen several were burns (and worse) were the result of improper closing of a CB or bolted pressure switch after a fault.

 

[zog]

Here is one from this summer, breaker tripped (50 relay) and was reset, a few seconds later....

 

[WDeanN]

Zog - Wow!
Just out of curiosity, though, what does the front of the breaker compartment look like?

 

[zog]

Slight buckling of the doors but they held, here is another one from earlier this year (I could post these all day)

 

[weressl]

Right, depends on why the breaker is being reset. IAW OSHA 1910.342. I have responded to dozens of arc flash accidents this year following improper reseting of a breaker into a fault.

I think we are barking up to the wrong tree here.

Even if you are closing onto a fault, the breaker should open and safely interrupt the fault IF the device is properly rated. To me the incidents indicate that we do not have properly rated breakers in place and nobody is paying attention to the fault levels. Incidentaly how many people are aware that MCB's are designed to interrupt two(1?) major faults after wich they should be replaced?

I believe THIS to be the core issue and that OSHA needs to step up in enforcing the short circuit and protection coordination requirements and compliance with those.

Another issue is obsolete equipment in place which have outlived it's design life and no longer safe to operate.

 

[zog]

" Incidentaly how many people are aware that MCB's are designed to interrupt two(1?) major faults after wich they should be replaced?"

I would guess less than 1% know this. I cover this topic in my substation maintenence training xourses and almost every student is shocked to hear this, its right there in the operation and maintenence manual.

 

[zog]

"Another issue is obsolete equipment in place which have outlived it's design life and no longer safe to operate"

I have to disagree with you on this one. I will take a properly maintained 35 year old AK-2 or K-1600 over any of this cheap plastic stuff the OEM's are making these days.

 

[Pierre C Belarge]

"Another issue is obsolete equipment in place which have outlived it's design life and no longer safe to operate"

I have to disagree with you on this one. I will take a properly maintained 35 year old AK-2 or K-1600 over any of this cheap plastic stuff the OEM's are making these days.

I do not believe a properly maintained device would be considered obsolete. If during the maintenance program they thought it was obsolete, they would probably replace it.

 

[weressl]

I do not believe a properly maintained device would be considered obsolete. If during the maintenance program they thought it was obsolete, they would probably replace it.

Discussing this during our maintenance approach planning, we decided to approach the manufacturers. Asking about common elelctrical equipment they informed us that the design life of the equipment is for 25 years WITH proper maintenance. After which time the manufacturer would and can not say that the equipment will perform to its original specification, including safety features and interrupting rating. One of the exceptions to this was power transformers.

 

[Pierre C Belarge]

I am curious...
Does the manufacturer state the 25 years life span is for older equipment?
What about equipment designed and installed presently?

 

[coulter]

... I will take a properly maintained 35 year old AK-2 or K-1600 over any of this cheap plastic stuff the OEM's are making these days.

Interesting choice for comparison.

An older, high end, metal clad compared with a current production molded case. Also interesting you apparently like the older mechanical protective relays installed on 35 year old metal clad. Can't say I do.

I don't see your point.

My choice would be new cheap plastic stuff over 35 year old cheap plastic stuff. Or newer high end metal clad with electronic trip units over 35 year old metal clad with mechanical trip units.

weressl -
I trhink you have a good point. When the old stuff doesn't meet spec any more, then it's time to rebuild or replace.

carl

 

[weressl]

I am curious...
Does the manufacturer state the 25 years life span is for older equipment?
What about equipment designed and installed presently?

They made no distinction. After the equipment thermally aged there is no way of predicting its performance. In system breakers where interruption of full fault current is allowed ~200 times +, the performance may be tested with full fault current but the repeatability remains in question. If you look at failure curves it exponentially increases with age and the required maintenance is proportionately increased. On the practical side: unless your setup is such that you can take your equipment down for maintenance at will, the equipment availability becomes so cumbersome that it hinders your operation and that adds cost. So eventually it becomes economically the wiser choice to replace it anyway.