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Control Circuits on MV Switchgear
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Author:  diakonos1984 [ Fri Aug 31, 2012 7:37 am ]
Post subject:  Control Circuits on MV Switchgear
Just noticed something in everyone's favorite Table 130.7(C)(15)(a) that I hadn't noticed before...

Under Metal clad switchgear, 1 kV through 38 kV (Page 70E-36 of the 2012 edition), we have two items:

Quote:
Work on control circuits with energized electrical conductors and circuit parts 120V or below, exposed: HRC 2
Work on control circuits with energized electrical conductors and circuit parts >120V, exposed: HRC 4



Now, coming from the perspective of a service company that does a lot of relay/controls troubleshooting at facilities where arc flash studies have NOT been performed, what were the intentions of NFPA, and what are the ramifications of these two items?

Example:
Traditional GE PowerVac gear with relays and meters mounted on the door, 125Vdc control power, and 3-phase 120Vac ±5% PT circuit. When you open the door you expose the dead front of the breaker, but the only exposed energized conductors are the control power and the PT bus.
Do they really intend for me to suit up to HRC 4 to pull the 5A 120Vac PT circuit fuses?

Before I had seen the lines referenced above, my reasoning would have been:
1) There are no MV conductors exposed, nor being interacted with, therefore arc flash protection is based on the control and PT circuits alone.
2) There is no arc flash protection specified for DC circuits.
3) Under Panelboards or other equipment rated 240V and below (page 33) it lists

Quote:
CB or fused switch operation with covers off: HRC 0
Work on energized electrical conductors and circuit parts, including voltage testing: HRC 1
Work on energized electrical conductors and circuit parts of utilization equipment fed directly by a branch circuit of the panelboard: HRC 1


4) So whether I'm dealing with insulated fuse blocks, or open fuse blocks where I have to use an insulated fuse puller, my HRC requirement is no higher than 1.

What is your take on the above?

Thanks,
Nathaniel
Author:  K. Jackson [ Sun Sep 02, 2012 2:09 pm ]
Post subject: 
2012 Edition of 70E page 38, Table 130.7(C)(15)(b) DC Tables >100V <240V.
Work on energized electrical conductors... HRC 1
The HRC goes up with short circuit current but I can't imagine much SC current for your situation.
Author:  jghrist [ Mon Sep 03, 2012 1:16 pm ]
Post subject: 
If an arc flash study had been performed, you would most likely be faced with a label that is based on a MV arc incident energy. You won't find a label specifically for the control compartment.

Do you use the MV PPE requirement from the label? Do you use the the 70E task tables for LV AC and DC? I think that the problem is not only the lack of an arc flash study, but the lack of a hazard/risk analysis and a well thought out electrical safety procedure that covers your situation.
Author:  321Liftoff [ Tue Sep 04, 2012 9:38 am ]
Post subject: 
Ah and therein lies the significance of an Electrical Safety Program (ESP) per NFPA 70E Art 110.3, that drives the procedures, hazard/risk assessments an ultimately the corresponding arc flash analysis and labeling... I realize this doesn't answer the original question, so is anyone willing to call the NFPA and discuss with them the intent of Table 130.7(C)(15)(a) - Metal clad switchgear, 1 kV through 38 kV - work on control circuits, and it's inconsistency with - Panel boards or other equipment rated 240V and below?
+1 800 344-3555 or[font=Tahoma]+1 617 770-3000 NOTE: I’d be willing to contact them, if a question is generated for them to respond to (hence they prefer a yes/no type question).[/font]
Or, is a ROP in order – though may take time for a response.
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