Arc-flash

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Arc-flash

  • Would you calculate the arc-flash level at the panel/receptacle?

    Votes: 13 86.7%

  • Would you 'assume' that the instantaneous MCB would reduce the available level to 0?

    Votes: 1 6.7%

  • Would you just take the cal level whatever is at the MCC?

    Votes: 0 0.0%

  • None of the above

    Votes: 1 6.7%
  • Total voters
    15
  • Poll closed .
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weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
So here is the question:
A 480V receptacle or panel is feed from a MCB located in an MCC. The MCB is usually in the range of 100A and appropriate wire size of about 150' length is the average feeder size.
1./ Would you calculate the arc-flash level at the panel/receptacle?
2./ Would you 'assume' that the instantaneous MCB would reduce the available level to 0?
3./ Would you just take the cal level whatever is at the MCC?
4./ ?????
 
weressl said:
So here is the question:
A 480V receptacle or panel is feed from a MCB located in an MCC. The MCB is usually in the range of 100A and appropriate wire size of about 150' length is the average feeder size.
1./ Would you calculate the arc-flash level at the panel/receptacle?
2./ Would you 'assume' that the instantaneous MCB would reduce the available level to 0?
3./ Would you just take the cal level whatever is at the MCC?
4./ ?????
Click to expand...

Interesting: no responses, no controversy?! I must be failing in my misson..........:p
 
weressl said:
Interesting: no responses, no controversy?! I must be failing in my misson..........:p
Click to expand...
Hey, people are voting. Do you want them to post too? :)
And among those who are voting there does not seem to be much controversy.
If you use a properly rated sealed load-breaking receptacle and plug combination, I would be less concerned about getting an arc flash rating done though.
A panel, h*** yes!
 
Wouldn't you calculate it at the point where the arc flash might occur?, otherwise you might end up with a higher rating without including the impedance of the feeders or branch circuit, to me the Cal level should be calculated at each point that someone would maintenance, trouble shoot or use the equipment while energized that has a risk of an arc flash, to use the cal level for the main panel would make no sense?:?
 
weressl said:
So how does a sealed load-breaking receptacle offers you more safety than an MCB or MCP behind the MCC's closed door?
Click to expand...

I wonder if it is the one that the receptacle slides out a little breaking a set of contacts that removes the power before you actually pull the plug out they were pin and sleeve we had some of these at a structural steel plant I worked at on our motor generator welders, but they would go bad all the time and single phase the motors, we then found a pin and sleeve receptacle that had a micro switch that controlled a contactor that wouldn't energized the receptacle until the plug was seated all the way in well after the contacts in the plug made contact with the receptacles contacts, did this because even though we had a disconnect at each receptacle the welders would still unplug the welder while power was on and one time it cause a small arc flash that burned the welders hands and arms.

The ones we have where I work now are all integrated into a safety switch with a mechanical interlock that wont let you plug in or unplug the cord until the disconnect is off, I never seen one like this before working here but it works great, I believe they are from Square D.

We had an outside contractor who needed to plug in his welder and he couldn't figure out why it wouldn't go in, I got called check it out turned the disconnect off and plugged in his welder his mouth just dropped, and there was a plaque that stated that to use the receptacle you must have the disconnect off, he made the commit "How stupid" and my reply was yes burned up hands and arms are stupid when you have a disconnect right there, and told him about what happened to the other welder at the other place, I told him at 480 volts it is not safe to ever plug or unplug a load from a receptacle, I wont even do it on ranges or dryers 240 volts as I had one blow up in my hands once as two of the plugs pins pulled out and was still making contact with the receptacle when they touched together.
 
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hurk27 said:
I wonder if it is the one that the receptacle slides out a little breaking a set of contacts that removes the power before you actually pull the plug out they were pin and sleeve we had some of these at a structural steel plant I worked at on our motor generator welders, but they would go bad all the time and single phase the motors, we then found a pin and sleeve receptacle that had a micro switch that controlled a contactor that wouldn't energized the receptacle until the plug was seated all the way in well after the contacts in the plug made contact with the receptacles contacts, did this because even though we had a disconnect at each receptacle the welders would still unplug the welder while power was on and one time it cause a small arc flash that burned the welders hands and arms.

The ones we have where I work now are all integrated into a safety switch with a mechanical interlock that wont let you plug in or unplug the cord until the disconnect is off, I never seen one like this before working here but it works great, I believe they are from Square D.

We had an outside contractor who needed to plug in his welder and he couldn't figure out why it wouldn't go in, I got called check it out turned the disconnect off and plugged in his welder his mouth just dropped, and there was a plaque that stated that to use the receptacle you must have the disconnect off, he made the commit "How stupid" and my reply was yes burned up hands and arms are stupid when you have a disconnect right there, and told him about what happened to the other welder at the other place, I told him at 480 volts it is not safe to ever plug or unplug a load from a receptacle, I wont even do it on ranges or dryers 240 volts as I had one blow up in my hands once as two of the plugs pins pulled out and was still making contact with the receptacle when they touched together.
Click to expand...

With the microswitch interlock the switching occurs remotely in the MCC, so the 'interaction' with the device does not represent a danger as the fault would occur remotely at the MCC. In other words the 'interaction' can not produce a local fault unless of course the interlock is faulty, in which case arc fault IS assured.

With the Square D intergated switch the circuit break would occur localy, at the swicth, so the arc-flash hazard is present, level must be determined and posted.
 
weressl said:
With the microswitch interlock the switching occurs remotely in the MCC, so the 'interaction' with the device does not represent a danger as the fault would occur remotely at the MCC. In other words the 'interaction' can not produce a local fault unless of course the interlock is faulty, in which case arc fault IS assured.

With the Square D intergated switch the circuit break would occur localy, at the swicth, so the arc-flash hazard is present, level must be determined and posted.
Click to expand...

So perhaps the working distance goes to 3 inches??
 
weressl said:
So how does a sealed load-breaking receptacle offers you more safety than an MCB or MCP behind the MCC's closed door?
Click to expand...
That is a good question but the ads for those products say that they do.
... Eliminate the need for arc flash protection boundaries and cumbersome PPE. Decontactors maintain a NFPA 70E "hazard risk category" equal to zero. ...
Click to expand...
 
weressl;1496117... yet the conscensus is that the NFPA 70E says so.....[/QUOTE said:
NFPA 70E makes no such claim. You are dealing with people's reluctance to follow the requirements of NFPA 70E that say a risk analysis must be performed.
Click to expand...
 
weressl said:
With the microswitch interlock the switching occurs remotely in the MCC, so the 'interaction' with the device does not represent a danger as the fault would occur remotely at the MCC. In other words the 'interaction' can not produce a local fault unless of course the interlock is faulty, in which case arc fault IS assured.

With the Square D integrated switch the circuit break would occur locally, at the switch, so the arc-flash hazard is present, level must be determined and posted.
Click to expand...

The integrated Square D ones we have remove the arc flash problem from using the receptacle but does not remove the problem from working on the disconnect as it will still require PPE, this is why it confuses me as to what operation of a disconnect can require PPE's but they design integrated receptacle/disconnects interlocked to make just the receptacle safer???? although I will say that the disconnects are made very strong and even the door is bolted shut, so maybe they can contain the arc flash?
 
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