When I shutdown the main disconnect

BachPhi

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Join Date
Dec 2007
Location
Los Alamos
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Do I expect to have all energy inside the enclosure to be off?
I currently have a main enclosure with two legs before the disconnect that go to a circuit breaker , then to a 24VDC Power Supply , that interlock to other enclosures with yellow wire.
I am seeking your opinion on this. TIA.
 
When I open a main disconnect, I expect that the only things in the enclosure that will be live is the cable to the incoming lugs.

I'm not enough of an NFPA79/UL508 expert to know if feeding a device with uninterrupted 480V AC can be legally done with yellow tape, or markings.

Ideally the disconnect, the circuit breaker, and the 24V PSU would be in their own enclosure.

My company uses a voltage indicator to remind people that the incoming lugs are still live: http://www.graceport.com/safeside/r-3w

If I had a good reason to build an enclosure like this, I would put one of the flashing-lamp voltage monitors right at the 24V PSU, and label the heck out of that circuit.
 
We have A LOT of panels that tie into two incoming phases above the disconnect to power a utility transformer. This transformer drives a lightning circuit for the panel as well as 120 volt utility power for tooling and laptop chargers. These panels should follow all NFPA/NEC ruling, but I need to look up to verify. There are LOTS of labels and placards showing the live wires.

These panels also have interlocks for emergency circuits that will be hot even if the panel disconnect is thrown. Once again all wire are labeled as well as placards.

With as many panels that we have spread all throughout the country, I’d say it’s probably a fairly common practice. I’d like to see more people chime in and hear what they have seen.
 
Commonly we have 120vac and 24vdc control in our 480 power cabinets.
When the disconnect is shut off the 480 and the 120 supplies are disabled.
There are lights in the cabinet that let you know the 24vdc is live. The PLC Remote IO is on the 24vdc circuit from the Control Panel. I normally do not want to power down the 24vdc, it will cause faults in the PLC, plus I use it to notify the PLC that the power panel is off.
Also .....NFPA allows this. Actually NFPA considers less than 50v to be "low voltage", so there are little regulations.
 
It is not off until the disconnect is open and verified open and the power in the panel is verified off.

You have no idea how many panels I have run into that one leg of the disconnect dose not open.
Then you have the foreign voltage from another panel to consider. They are supposed to be marked with a label on the outside of the panel but sometimes they just don’t get put on or they peel off over time. Code calls for the foreign voltage wires to be yellow or orange, orange in recent years but that doesn’t mean that somebody didn’t just use whatever wire they had handy in the old days we used black wire for just about everything.
I have had cases where the panel was improperly installed the handle went down to safe position but the disconnect inside remained closed. Not external lights on the panel so from the outside it looked like it was off. I couldn’t walk away from it in that condition so I had to delay their lunch until it was rendered safe.
And then you have the UPS system supplying that are becoming more popular supplying power if the mains are shut off. Be sure to check everything twice.

Then you have the case where somebody wires the disconnect upside down. Power entering the bottom of the disconnect. That actually happened about a year ago not far from me. A crew foreman went into a power distribution building on a large site to do some prep work for the next day’s work I think this was on 13K line. His crew found him the next morning just a foot sticking out the door. The report in the news said the disconnect was wired upside down ( feed on the bottom )
I could list many other examples where things could go wrong.

Different sites have different rules some say safe is levels below 50V to ground others require 0 potential where to considered safe even 5 volts is not allowed
I have been on other sites that after the panel is energized the controls guys were not allowed to open the panel at all even with the disconnect open, we had to send a request to installation electricians to have them open it for us to look only they had to do all the work in the panel.
Take the time to understand the site rules and follow them.
Take nothing for granted it’s not off until you yourself verify it’s off then check it again.
Electricity doesn’t give you many second chances.
I like and recommend the grace port devices that were mentioned before but they are only a first step. You must follow up with a known good volt meter.
 
Yes...of course.....ALWAYS verify with voltmeter.
I learned the hard way...I got the **** knocked out of me on a fused disconnect with a broken phase that did not disconnect when the switch was off.
 
I have several clients who require that enclosure lighting and a GPO be fed from the line side of the main switch. What we typically do is:
1. Big, obvious red and white label above the main switch "NOT ALL CIRCUITS ARE ISOLATED BY THIS SWITCH"
2. Use double insulated cables directly from the line side of the main switch onto two-pole RCD's which are protected by a perspex cover or similar, and with another big red and white label "FED FROM LINE SIDE OF MAIN SWITCH"
Of course, we in upside down land do a lot of things different to the 'muricans, so if I were the OP, I would absolutely not take my advice on anything related to electrical compliance.
 
Yes, there can be circuits that don't de-energize and are fed from another source. Yellow seems to be a common color - although I believe NFPA 79 calls for Orange and Orange/white.

There should be a sticker on the front of the cabinet that says something to the effect of: "XXX color wires in this cabinet are not de-energized by this disconnect".
 
as Rson pointed out, there can be other power sources when encountering yellow / orange wire and there is supposed to be a sticker indicating more than one power source.

if there is a vfd in the panel, wait several minutes to allow the internal power to dissipate.

BEFORE verifying with a volt meter, verify the volt meter is working with a known power source first !!!

james
 

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