24 vdc control - 480 vac power single enclosure -work in control side while energized

[Dan Kissel]

We want to design a multi- door control panel that has the 480 segregated to one side of the control panel behind one of the doors and use the other door for access to the 24 vdc control circuits.

What kind of barrier do we need between the left and right side of the backplane so that we can keep the 480 door closed but be able to work without PPE on the 24 vdc side. Cat 5 cables and some relay contacts need to run between the two sides of the enclosure.

Assume there is 10 cals of energy at the panel's disconnect.

 

[GoldDigger]

It is not hard to make a barrier to protect against voltage exposure.
It is potentially very hard to make a barrier to contain an arc flash in one section from exploding out another section. The arc gas pressure can exert a lot of force!
You might try the logic that you can prevent what you are doing on the lower voltage side from causing an arc event on the higher voltage side, but I am not sure what level of engineering and testing OSHA would require to accept that.

 

[Dan Kissel]

Thank you for your reply. Enclosures are not rated for calorie level so I don't believe that should be an issue. The panels will have a sufficient SCCR.

 

[wbdvt]

Thank you for your reply. Enclosures are not rated for calorie level so I don't believe that should be an issue. The panels will have a sufficient SCCR.

I am confused on your statement about not rated for calorie level. The calorie (incident energy) level is determine by analysis of the system the panel is installed in so there is no rating that can be applied to the cabinet. The incident energy level at that cabinet would determine the level of PPE required to work in the cabinet.

How do you know the panels will have sufficient SCCR? Are you going to design and test the panels to have a 65kA rating? I would think that rating would be sufficient for a vast majority of facilities.

 

[petersonra]

if it was me, I would get two separate enclosures and just butt them up against each other. Then you don't have to worry about it.

 

[Jraef]

if it was me, I would get two separate enclosures and just butt them up against each other. Then you don't have to worry about it.

How do you pass conductors from one to the other without also allowing blast pressure / plasma flash through the same openings?

There are lots of options available now from the major enclosure manufacturers to address this, using what can be generically referred to as "power isolation enclosures" where the 480V main disconnect is in a separate structure, either inside of the main cabinet or on the side, but the power passes through the walls via a terminal block with connectors on both sides, but no air passing through a hole. So with that, you can open the main disconnect and although its line terminals will still be hot, they are sequestered in their own separate cabinet.

Hoffman's version

Saginaw's version

 

[petersonra]

If you are not working on the side that has the arc blast potential how is there any risk of Arc blast in the control cabinet? In any case how far away do you have to be for the risk to be limited enough that it would be safe? A mile?

 

[Dan Kissel]

With reference to my question. The 480 side will have motor starters, VFDs, etc. The two cabinet solution is really just a "punt" on the issue. There would be conduit gong between the two cabinets. When the 480 door is closed, there is very little chance of a fault or shock hazard happening because of an accidental tool drop or wring problem.

I want one enclosure so that I can test it before the panel is shipped. I don't want the double cost of handling, re-connecting and then retesting. Is a simple barrier with holes as needed sufficient to protect me while I work in the 24 vdc side without any PPE?

 

[petersonra]

With reference to my question. The 480 side will have motor starters, VFDs, etc. The two cabinet solution is really just a "punt" on the issue. There would be conduit gong between the two cabinets. When the 480 door is closed, there is very little chance of a fault or shock hazard happening because of an accidental tool drop or wring problem.

I want one enclosure so that I can test it before the panel is shipped. I don't want the double cost of handling, re-connecting and then retesting. Is a simple barrier with holes as needed sufficient to protect me while I work in the 24 vdc side without any PPE?

I don't see how it is a "punt" You can ship it connected together as a single unit.

I am not a fan of the DIY barriers. Most people make them so they are not all the "safe". If the 480 and 110 Volt stuff is completely contained within a separate enclosure it is hard to be a whole lot safer when working with the 24 VDC stuff as long as you do not open the door to the 480 Volt side. As long as the door to the 480 Volt enclosure is closed, I don't see how working in the 24 VDC introduces a risk more than if you just walk by the 480 V enclosure.

 

[Jraef]

If you are not working on the side that has the arc blast potential how is there any risk of Arc blast in the control cabinet? In any case how far away do you have to be for the risk to be limited enough that it would be safe? A mile?

Plasma doesn't know it is not supposed to travel...

How big is the opening between the cabinets? Is 1" OK? If so, how about 4"? Then 8"? Where is the line drawn? The answer is, where the blast energy that gets through is in excess of 1.2 cal/cm². But how is one supposed to know how much energy will pass through an opening, and who is going to take responsibility for that number?

I know for sure in MCCs, ALL air spaces are considered "common". So when you open one bucket door, even if the stabs are REMOVED from the bus (as some mfrs now offer), technically you are still in the "Arc Flash Boundary" because an arc flash in another section somewhere could potentially blow out into your face.

 

[petersonra]

I think the question that has to be asked is if there is a risk of arc flash from working in another cabinet how is that any different than the risk of arc flash just from walking in front of the cabinet? The cabinet does not contain the arc flash anyway so any openings between the cabinets are not especially significant.

 

[JP79]

I too would be interested in further discussion on this point. I am faced with a similar request of a customer and need to determine a safe solution that meets any relevant codes. I have the same basic opinion as jreaf, that a simple divider just would not protect from a potential blast. Regardless of the cause, dropped tool, cross wire, dust kicked up by a draft, an arc flash in the unprotected face of an unsuspecting electrician will surely work it's way through the easiest path to escape.

The question is then, what is safe enough?

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[wireman]

Look into Hoffman’s sequestor line of enclosures. That be an option.

 

[JP79]

Look into Hoffman’s sequestor line of enclosures. That be an option.

Thanks for the reply wireman, but I am really hoping to answer the question of safety in keeping to the original posters question and separation inside of a single cabinet. I did review your suggestion and the ones earlier and neither will work well in my situation.

I am trying to focus on this because of my customers push for a separation and my bosses acceptance of that solution. I would like to refute their decision with some code examples that they can't argue against. Right now they are just looking at the dollars which is a poor deciding factor.

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[Khecksher]

Major problem is, can you show the AHJ or OSHA, any testing and documentation that the hazard is mitigated between the potential arc flash side and the lower voltage side? This requires a cabinet that has been fully tested and qualified by an appropriate testing authority (either CE or UL), like other's have said, their is no 'DIY' solution. So, unless you are willing to submit to UL for testing and have listed properly (which is going to cost way more than purchasing an enclosure that has already been certified and tested for such), you would be better off purchasing a cabinet certified and listed for such. Plus in most states you as the provider of the product to the end customer, will be liable for a minimum of 10 years for any liability issues (called in most states the "Statute of Repose"). This is not legal advice, but I wouldn't want to be liable for that product without proper testing for 10 years myself.