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125V DC panel shutdown

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blayton1212

Member
Location
Troy NY
I work in a large industrial environment, while working on any of the AC we always lockout and shutdown the Power. Currently we are installing a /grid tied solar farm and we need to feed the metering control panel in our Substation with 125VDC. I am Told we Do Not need to shutdown the power ( 125VDC ) to power the panel, and this because its DC ? I am not sure where to verify this ?
 
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Timbert

Member
Location
Makawao, Hawaii
To put it bluntly, whoever told you that is an idiot. DC can certainly kill.

That said, generally speaking voltages under 50 Vac or 120 Vdc are considered "safe" extra-low voltages (IEC 60364-4-41). NFPA 70e lists DC voltages less than 100 volts as not an electrical hazard i.e. you probably will not be electrocuted by accidentally contacting them. That however is not the whole story. Since you tied to a PV system, and this would include large DC batteries, there is the risk of arc flash. You definitely can still kill yourself from an arc flash on such a system.

Now you say that this is for a metering station, which is probably connected with small, long wires so arc flash may not be realistic at that particular point in the system.

I would point to NFPA 70e 130.2(A)(2) which clearly states that energized electrical work shall only be done on voltage levels less than 50 volts (AC or DC is not specified). For a 125 Vdc system the restricted boundary is listed as: "avoid contact." So, a qualified electrical worker can do certain work in a panel with 125 Vdc if safe work practices and PPE are utilized.

What I don't understand is why the 125 Vdc can not be safely isolated for this work. Surely, there is a disconnect leading to the metering equipment.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
This misconceived notion MAY have sprung from someone's experiences with 125VDC being used in switchboards and power control equipment. When the 125VDC circuit is used to TRIP AND CLOSE high powered breakers and switches and if the equipment is in service while it is being worked on, the 125VDC is REQUIRED to be on all the time. This falls under one of those "worse if it is turned off" situations allowable by OSHA, however it has nothing to do with it being DC.

If your 125VDC circuit is not being used in critical up-time equipment, you need to do LO/TO on it just like anything else.
 
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blayton1212

Member
Location
Troy NY
Worse if turned off !

Worse if turned off !

Jraef said:
This misconceived notion MAY have sprung from someone's experiences with 125VDC being used in switchboards and power control equipment. When the 125VDC circuit is used to TRIP AND CLOSE high powered breakers and switches and if the equipment is in service while it is being worked on, the 125VDC is REQUIRED to be on all the time. This falls under one of those "worse if it is turned off" situations allowable by OSHA, however it has nothing to do with it being DC.

If your 125VDC circuit is not being used in critical up-time equipment, you need to do LO/TO on it just like anything else.
Click to expand...

Thanks,
your answer was exactly the reason we are going to work live, and for those of us that have been around a while it brings back memories. Remember, don't take any flash photography while the work is being completed.
 
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
blayton1212 said:
Thanks,
your answer was exactly the reason we are going to work live, and for those of us that have been around a while it brings back memories. Remember, don't take any flash photography while the work is being completed.
Click to expand...
The most hated person in a power room is the one that takes flash pics
without announcing it. The blue flash looks exactly like an arc flash :rant:
 
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