Separating high and Low Voltages in an enclosure...

dmmons

Member
D
Join Date
Sep 2011
Location
South Jersey
Posts
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I have heard a lot about separating the high voltage (>50V) from the lower control voltage in an electrical control panel, but have never run across one or had to do one myself... until now.

The customer requires the high voltage to be either "shielded" so the enclosure can be opened under power for testing/repair without the technician being exposed to dangerous voltages, or to put all of the high voltage in a separate enclosure.

I plan to speak to our enclosure vendor Monday for his thoughts, but was curious about what the members here have done in the past.

If you have any pics you are willing to share, that would be great too.

Thanks
 
you can use stand-offs and plexiglass to cover the >50 volts (which by the way is low voltage >1000V (600VAC) is medium Voltage and greater than 35000 is High voltage.
We have used this successfully in the past. If you hinge it use a limit switch to initiate the trip coil of a shunt trip circuit breaker used for the "high Voltage feed" that way it cannot be easily bypassed when opened. Any question ask.
 
As designers and owners become more aware of arc flash hazards you will see more of this requirement. More than just voltage level goes into arc flash hazard ratings - it can get way too complex for me to deal with.

I actually like the idea of putting anything over 120 VAC in a separate enclosure, including the control power transformer. I've done the Plexiglas system, and used other kinds of shielding. The problem is you never know when a maintenance guy in a hurry will leave the shielding off. I personally feel that a separate enclosure is better. Then you can sticker the bejeezus out of it so only qualified guys (in theory) will go into it.
 
I just have to add ... for no real benefit at all ... I have a cabinet designed, stamped and built by professionals that has 480-120 vac , and 24 and 12 vdc next to each other and the PLC . Thats bad enough, but down in the corner is a pressure transmitter with the fluid piping. If the piping were to spring a leak only a metal splash shield would prevent it from hosing the entire cabinet.
 
There has been a huge push over here to isolate the LV (1000~50V) from the ELV (less than 50v) voltage normally 24v ac or dc.
Mainly due to the licensing regulations that permit access to electrical enclosures.
The idea is to allow Graduate engineers to access control panels with no risk - so they can blame others when their changes cause major issues...đź’€

Sorry a pet gripe of mine:beerchug:
 
I feel the same after 32 years of doing everything from medium to low voltage testing, live I might add, safety experts who have no idea about my job or how to perform it, are now trying to tell me how to do my job. I have never had an accident, and don't need someone telling me how to be safe. And I resent the interference.
 
Yep
I have a HV operators ticket - and it is regularly renewed.
but it is something that you don't want to be doing too often
 
It is my opinion, if you don't know how to do it safely, or have doubts, then stay out of the cabinet for everyone's safety.
 
oddly enough it is complacency that causes more accidents with HV.
Although there are people who see RED lights in switch yards and assume the power is off so they attempt to steal the copper.
makes a nice shadow and carcass :eek:
 
I want to join in on the rant :)
Out job is tough enough, without trying to make everything "Idiot Proof". Every time we do they just make better idiots.
Max is correct: The best way to handle this is to put a sticker on the cabinet that says "Stay out unless you know what you are doing!!"
I have had it up to here (Hands above my head) with all this safety **** that comes from people that don't know a damn thing about safety.
 
I have to disagree here. Just because someone has 30+ years working on this stuff does not make them immune to the laws of physics. There are plenty of experienced people who have been injured or even killed while working on equipment they've dealt with for years.

It can be a moment's inattention or a slip of a screwdriver that falls just where it does the most damage. Even if you never make a mistake, a fault that puts voltage where it shouldn't be won't care that you know what you're doing.
 
like I said Complacency is the most common cause Timbert
and in other cases it is a total mental blank
this has happened , someone climbs onto a 220kv trans that was under access the day before, not realizing that the access permit signs are on the adjacent transformer. And that one has 'LIVE' posted on it.
This was a very old horror story - I believe long remedied, but it can still happen.
But note I am not talking about accidents where some poor person comes into contact with supply.
workers in a cherry picker or the guy moving a metal structure with a tractor, very hard to prevent.

I dont want to be rude - "Complacency" - acknowledges years of experience an overconfidence as a cause.
 
Last edited:
Under LLoyds we use separate starterboxes (sometimes just two doors above each other with one having the actual starters and safeties in and the other the controls. On the starters have the lights and the pushbuttons all on <50V.
plexiglass is just to add, as any engineer needs to measure he will remove the plex. We shield the big rails, and drill some holes in it to have measuring points.
The switches are always shielded enough for touching.
Regulations say they must switch off before opening the door, how to control the system, so evry engineer has a wrench and knows how to override this.
 

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