Working live on the 24vdc side of a 480 vac enclosure.

[Dan Kissel]

We build control panels that are supplied to control 480 vac motors and other devices. We use 24 vdc for control circuits. The panels will be installed where there is normally over 1.2 cal but less than 25 cals of incident energy available. If make our enclosures a 2 door and separate the 24 from the 480, with the 24 behind one door and the 480 behind the other door, can the 24 vdc side be worked on hot if the door on the 480 is closed. Would a partition make a difference? Can I work on the 24 vdc side if I am outside the arc flash boundary? I realize we could build two separate cabinets but the customer is in a very competitive industry and is asking why can't he work on the 24vdc side.

 

[wbdvt]

From your sketch it looks like the 25 cal/cm2 is at the circuit breaker mounted on the side of the cabinet and I would assume that value is on the line side of the breaker. The incident energy is most likely a lot less on the load side of the breaker. Therefore if this breaker can be isolated from the 480V in the cabinet (nipple with firestop??) then the lower value can be used for the entire cabinet.

Another item to consider is shock. Most likely the people servicing the 24V side are PLC techs who may or may not have had training in the hazards of 480V so there needs to be something in place to prevent inadvertent contact with the 480V.

Of course the exact incident energy values will not be known unless they are modeled at the location where this cabinet is being installed. Could be higher than 25 cal/cm2.

 

[petersonra]

The key would appear to be whether the tech is exposed to voltages in excess of 50 volts or not.

Some people think you can accomplish that with partitions of various types. I think that is a reasonable idea but I would want to know a lot more about the partition before I would say yes or no. If the partition essentially made it into two enclosures it would not appear to anything different than two separate enclosures butted up against each other. Personally, I would bet buying two enclosures will cost less anyway. Enclosures are pretty much sold by the pound and you can't weld in a partition for less than paying the manufacturer for an extra enclosure wall.

 

[Electric-Light]

The key would appear to be whether the tech is exposed to voltages in excess of 50 volts or not.

Some people think you can accomplish that with partitions of various types. I think that is a reasonable idea but I would want to know a lot more about the partition before I would say yes or no. If the partition essentially made it into two enclosures it would not appear to anything different than two separate enclosures butted up against each other. Personally, I would bet buying two enclosures will cost less anyway. Enclosures are pretty much sold by the pound and you can't weld in a partition for less than paying the manufacturer for an extra enclosure wall.

Having two doors that opens into one big room with no wall in between is the same thing as having one big door. The prudent thing to do is put a barrier to avoid accidental contact into line side. Not just for safety but for control side protection too.

 

[iwire]

This is basicly the design of pad mount transformers.

Two doors, you have to open the first door to the low voltage side before you can open the second door to the high voltage side.

There is a partition in between.

 

[petersonra]

Having two doors that opens into one big room with no wall in between is the same thing as having one big door. The prudent thing to do is put a barrier to avoid accidental contact into line side. Not just for safety but for control side protection too.

isn't that pretty much what I wrote?

my only concern is that I would want to make it so the partition effectively creates a complete enclosure. i would not want something like a piece of plexiglass added that separates the two but potentially would allow for a nut or bolt that flies off to enter the 480 V side.