Terminal Shrouds for Enclosed Disconnect

[petersonra]

You have two different hazards associated with electricity - electrocution and arc flash.

If you can somehow justify working live on a panel, the electrocution protection is relatively simple. Basically you wear the gloves and shoes, along with glasses and hat. It is not real hard to determine if you are exposed to an electrocution hazard either. If you are going close to an energized and exposed voltage over 50V, you are considered exposed and need to protect yourself from the hazard. There are very effective ways of reducing your exposure potential including use of guards such as finger safe design. People argue over them being imperfect, but they are pretty effective.

Personally, I think you are exposed if you are taking voltage measurements since your hands will be close to the exposed part of the probes being used unless the combination of the probes and the guards in use prevent this. Since the vast majority of the time you will be working on energized stuff will be for troubleshooting purposes, unless it is < 50V, by default you should probably consider yourself to be exposed to the hard of electrocution.

The arc flash hazard problem consists of two separate issues. One is how likely the hazard is to occur and the other is how severe it is likely to be if it occurs. You want to attack this problem from both sides. Guards can significantly reduce the risk of an arc flash incident happening. They do nothing to change the severity of a potential arc flash incident. There is a benefit in doing things that reduce the risk of an arc flash occurring even if it does not reduce the level of PPE required.

I am not convinced that just working within the arc flash boundary of a part like a switch that is completely enclosed and not being operated exposes one to an arc flash risk at all.

 

[GoldDigger]

But the OP's situation of the switch or disconnect being open implies that at some point it was being opened. I agree that once it has been opened there should not be a significant arc risk unless you manipulate something.

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[petersonra]

But the OP's situation of the switch or disconnect being open implies that at some point it was being opened. I agree that once it has been opened there should not be a significant arc risk unless you manipulate something.

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This statement from the OP suggested to me that he was more concerned about the live terminals after the switch was open and not about the risk of an arc flash caused by opening the switch.

The concern is that even with the disconnect in the off position, the line terminals are still hot.

Does anyone here think that just opening a small disconnect switch actually has a significant risk of arc flash? Especially an enclosed one?

History has shown that racking out large circuit breakers from switch gear does have such a risk but is there any evidence that suggests such a risk with merely operating enclosed disconnect switches, especially smaller ones?

In fact is there any history at all that suggests merely operating a large CB in a piece of switch gear involves a risk of an arc flash incident? If there is such a history, would it not be the case that a permanent arc flash boundary would have to be established around such a device and no one could ever get near the thing unless suited up since there is some chance of it tripping and causing an arc flash.

 

[kwired]

In fact is there any history at all that suggests merely operating a large CB in a piece of switch gear involves a risk of an arc flash incident? If there is such a history, would it not be the case that a permanent arc flash boundary would have to be established around such a device and no one could ever get near the thing unless suited up since there is some chance of it tripping and causing an arc flash.

That used to be the way I looked at it, and still do to some extent, but reading this forum has taught me that just because the disconnect is inside a dead front enclosure, it still could blow up in your face when you close a switch into a fault. I used to always think if it was inside an approved enclosure it was safe to operate without PPE.

Kind of scary when you think of how many non electrical professionals operate circuit breakers and switches that have no idea of the potential hazards, and even some electrical professionals for that matter.

Now the disconnecting means inside a control panel - at very least has increased arc flash potential should you decide to close the switch while the main cover is open, how much more would depend on the ability of any shield to contain the arc incident.

 

[petersonra]

That used to be the way I looked at it, and still do to some extent, but reading this forum has taught me that just because the disconnect is inside a dead front enclosure, it still could blow up in your face when you close a switch into a fault. I used to always think if it was inside an approved enclosure it was safe to operate without PPE.

Kind of scary when you think of how many non electrical professionals operate circuit breakers and switches that have no idea of the potential hazards, and even some electrical professionals for that matter.

Now the disconnecting means inside a control panel - at very least has increased arc flash potential should you decide to close the switch while the main cover is open, how much more would depend on the ability of any shield to contain the arc incident.

why would closing into a fault cause an arc flash at all? it is designed to trip on such a fault.

 

[GoldDigger]

If the breaker is defective it could fail while opening in response to that fault. The breaker would be more likely to fail when opening a fault than during a normal manual opening operation under normal load. And the latter is considered an arc flash risk. We are looking at various degrees of probability here, not varying amounts of energy.

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[fifty60]

My main concern with the shrouds is the disclaimer from ABB:

"The issue here is not just if the spacing exists for the shrouds to fit but the bend radius for your cables and or how you will attach cables into the enclosure."

I want to make sure that if I do install the shrouds, it is not making the disconnect any LESS safe...even if it is only making it marginally safer...

 

[GoldDigger]

My main concern with the shrouds is the disclaimer from ABB:

"The issue here is not just if the spacing exists for the shrouds to fit but the bend radius for your cables and or how you will attach cables into the enclosure."

I want to make sure that if I do install the shrouds, it is not making the disconnect any LESS safe...even if it is only making it marginally safer...

It seems to me that all that they are saying is that although the shroud fits into the empty disconnect as they ship it, it may no longer fit once you have field wired the disconnect. No more or less than that.

 

[fifty60]

I do have bend radius considerations per the NEC right...creepage...clearance...

 

[petersonra]

I do have bend radius considerations per the NEC right...creepage...clearance...

I don't see how it is relevant as long as long as the shrouds are not in the bending space.

 

[fifty60]

So as long as the conductors are going straight up, and out of the enclosure...nothing in the NEC applies?

 

[petersonra]

So as long as the conductors are going straight up, and out of the enclosure...nothing in the NEC applies?

Nothing other than the conductors are allowed in the bending space, although this is not a hard and fast rule since it does not actually say that anywhere. You could certainly extend the bending space so the shrouds were not in it to be sure.

409.104(B) covers required bending space in industrial control panels. It refers you to 430.10(B). The same values can be found in UL508a table 25.1 so if you are building to UL508a, it is already covered.

 

[GoldDigger]

Nothing other than the conductors are allowed in the bending space, although this is not a hard and fast rule since it does not actually say that anywhere. You could certainly extend the bending space so the shrouds were not in it to be sure.

Since shroud is removable and would, I expect, be removed before doing anything with the conductors, I am not sure it would be seen as a violation even it there were a printed rule. :angel:

 

[Aleman]

If the breaker is defective it could fail while opening in response to that fault. The breaker would be more likely to fail when opening a fault than during a normal manual opening operation under normal load. And the latter is considered an arc flash risk. We are looking at various degrees of probability here, not varying amounts of energy.

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Yeah. Stuff like this can happen. Turned on breaker on a motor that had gone south. Contactor exploded. Never take any of this for granted.

 

[kwired]

why would closing into a fault cause an arc flash at all? it is designed to trip on such a fault.

You close a circuit that is faulted, you get an arc flash, unless there is little energy available from the supply to create an arc flash. Now the device is supposed to be selected to have a withstand rating equal or greater than what is available. But that doesn't mean there will not be any fireworks at all, and if you are lucky, all that happens to you is you have a need to get some clean underpants.

When I was in college we went to the Square D plant in Lincoln NE. Their primary product made there at that time was the QO series circuit breakers, not sure if they are still in production there or not that was back in about 1988 or 1989. One of the favorite parts of the plant for most of us was the testing lab. They demonstrated testing a three pole 10kA breaker from a source that was supposed to be able to deliver 10kA. I don't know what the trip setting was on the breaker, but am guessing it was probably 40-60 amps as the load side conductors were probably roughly 6 AWG, 30 amp or smaller would not have accepted that large of a conductor. They took a short lead from each pole and bolted them together to give a solid three phase fault, then plugged the breaker into a panel already mounted inside a big heavy walled enclosure and closed and latched the heavy door. They then energized it and it made a pretty loud bang, opened the heavy door a few seconds later and smoke rolled out of the enclosure. They then reset the breaker and it did physically reset, but it did not really look like you would want to use it again.

The AIC rating is for the equipment, it should not "explode" if applied within its rating, but there is no guarantee there will be no arc flash incident.