Machine panel construction - isolation of 480v

[jackal405]

I am in the process of approving new machines for my facility. These machines have control panels with 480v coming in, mainly 24v wiring and controls. Allen-Bradley controller.
My question: What code or standard dictates how to isolate the 480v, so when maintenance members open the panel they do not need arc flash ppe, as they will only have access to the 24v circuitry?
Thanks in advance!

 

[Jraef]

I am in the process of approving new machines for my facility. These machines have control panels with 480v coming in, mainly 24v wiring and controls. Allen-Bradley controller.
My question: What code or standard dictates how to isolate the 480v, so when maintenance members open the panel they do not need arc flash ppe, as they will only have access to the 24v circuitry?
Thanks in advance!

If 480V is in the panel, they are exposed to arc flash risk*, even if the only place 480V exists is the line terminals of the disconnect switch. The only accepted way to totally eliminate the arc flash risk is to disconnect the 480V at its source before coming into the panel. Enclosure mfrs are now making this option available, but you can accomplish it by using an added disconnect switch outside of the panel in question.

Also discussed here:
http://forums.mikeholt.com/showthread.php?t=186967

*Keep in mind that arc flash risk is not the same as electrically safe. Opening a main disconnect switch, with proper barriers to avoid contact with live conductors, puts a panel in an electrically safe working condition. An arc flash however that takes place away from the work will exhibit blast pressure in all shared air spaces and arcs can flash over, even across open disconnect switches and circuit breakers, depending on conditions. So since PPE is intended to protect the worker from both conditions, arc flash rated PPE will be still be necessary if line voltage is brought into the panel.

 

[Aleman]

We have a couple of double panels fed by one disconnect. All 480V in the main along with drives, power supply etc. The other panel is control voltage only.
That allows work to be done on the control voltage panel without need for any PPE.

We also have one European made panel that is a double door, and the 480V side is completely shielded from the other control voltage side. There is an actual
metal barrier between the 2 sides. Whether or not that would allow a tech to be working in the control side without PPE I can't say.

 

[FionaZuppa]

what the heck is 24v controls doing inside 480v panel?
what type of service instructions does the equip maker provide?
unless that 480v is dead i would not go in to work on 24v controls!
any reason why the equip maker puts 24v controls inside the 480v panel?

 

[Jraef]

what the heck is 24v controls doing inside 480v panel?
what type of service instructions does the equip maker provide?
unless that 480v is dead i would not go in to work on 24v controls!
any reason why the equip maker puts 24v controls inside the 480v panel?

Control panel with motor starters, drives, transformers etc. using 480V and coils, PLCs, safety controls that use 24V.

For sure the trend now is to separate them, or to use Hoffman's parlance, "sequester" the line power from control power that needs to be accessed when troubleshooting. But unfortunately, lots of OEMs are either ignorant of the issues, or just don't care or won't bother to change because it might cost more. That then is fueled by buyers who don't make this issue clear in their specs and buy on lowest price. So if you come across this, raise some stink so that your engineering and purchasing people make it a priority.

A bit off topic, but the exact same thing happens with regard to SCCR labels now as well. No specs/requirement in the tender offer results in the orders going to low bidders who don't / won't deal with it and you end up with a 480V control panel that can only be connected to a system capable of 5kA Short Circuit Current, which is virtually impossible to attain in an industrial setting, yet it's the poor electrician that gets stuck with the problem.

 

[Besoeker]

what the heck is 24v controls doing inside 480v panel?
what type of service instructions does the equip maker provide?
unless that 480v is dead i would not go in to work on 24v controls!
any reason why the equip maker puts 24v controls inside the 480v panel?

Sobering video.

 

[Aleman]

what the heck is 24v controls doing inside 480v panel?
what type of service instructions does the equip maker provide?
unless that 480v is dead i would not go in to work on 24v controls!
any reason why the equip maker puts 24v controls inside the 480v panel?

Putting 24 controls in a 480V panel is certainly not a rare thing, in fact almost every panel I have worked with over the years has been done that way.
Lately, I am seeing some separation but it's not yet the norm. The equip maker provides required warning labels and the owner should have the panel
Arc Flash rated. If things are built right (low flash hazard level) then a service tech will be able to go in with the minimum level of PPE. Which really isn't
that bad. For most electrically oriented techs working in facilities, turning off entire systems is not a realistic option. Usually the 24 will be apart from
the line side, but still within the same cabinet. So I could be working on 24 controls but behind the next door may be 480V. The OP wanted to know
whether that would be ok to do without PPE, but as pointed out it is not ok to ignore the 480 even though you may not be directly working in that area of
the panel. In my opinion it's best to completely separate the 2 voltage levels into isolated cabinets. But that adds to both expense and required space. Which
I believe is why you have both 24 and 480 in the panels. Money and space.