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 Post subject: Dead Front Pad Mounted Switchgear
PostPosted: Wed Dec 24, 2014 5:33 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 872
Location: Rutland, VT
I am curious about how people are handling dead front pad mounted switchgear such as that found on utility underground distribution system. I haven't had to analyze one yet but the thought came to me as I was driving around in a city and saw some.

Typically these can come as either a switch unit only, with fuses, or combination. The switch is usually operated from a handle with doors closed but switch mechanism is actually enclosed behind termination panel. Opening the doors will typically provide access to load break elbows, fault indicators, fuses, etc.

So, my questions:
1. Would operating the switch require AR PPE?
2. Would disconnecting the load break elbow require AR PPE as the switch would be visually verified to be open? If so, would AR PPE be based on a single phase arc fault?

I look forward to hearing everyone's opinion and/or experience with these.

Thank you

Barry Donovan, P.E.

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 Post subject: Re: Dead Front Pad Mounted Switchgear
PostPosted: Mon Dec 29, 2014 10:38 am 

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
Here is how I see it,

1. Operating the switch would not required AR PPE, as long as there is not any evidence of impending failure (per the examples in OSHA 1910.269, Appendix E, Table 1). At my utility, this question is not really relevent, since the folks operating the switches are also the one's working the primary in other ways and they will already be dressed in AR PPE. They will have a minimum level of arc rated clothing all of the time, so in the event there is some type of impending failure that goes undetected, they will be wearing what they need anyway.

2. For disconnecting the load break elbow, I would require AR PPE. Even with the open switch, if it is not grounded, it's should be considered energized and therefore AR PPE should be worn. A single phase arc exposure makes the most sense for a deadfront installation.

At my utility we also have livefront switchgear and livefront transformers, so they are the limiting factor for what level of PPE should be worn, and a three-phase arc exposure is used. OSHA 1910.269 Appendix E, Table 3, does not even address the recommended calculation method for single phase arc exposures for enclosures, which to me says that OSHA prefers you to assume a three phase arc exposure. That make senses for a broad industry suggestion, but if I only had deadfront on my system, then I would probably use ARCPRO single phase values and the conversion factor for an enclosure, despite the lack of reference by Table 3.

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 Post subject: Re: Dead Front Pad Mounted Switchgear
PostPosted: Tue Dec 30, 2014 5:33 pm 
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Location: North Carolina
Tests on 600 V class metal enclosed equipment (MCC's in particular) even ungrounded have shown propagation of the arc into 3 phase arcing faults in 1-2 cycles. So although metal clad and some PT fuse compartments may fpr instance be different, except for propagation from one section to the next, single phase meta; enclosed equipment may as well be three phase. This argument goes for load break elbows. By design a phase-to-phase fault would require a lot of destruction, very long arcs, etc. In addition by design they are ;imited to phase-ground faults. So I would start with single phase phase-to-ground faults with elbows. There were early documented cases of elbows exploding in cold environments due to the way the rubber deforms during disconnection. There is also one recorded case by OSHA where two elbows in poor condition became an arc flash in an underground vault but the case sounds like maintenance issues. I could not discern a root cause from the OSHA summary. We started using elbows about 4 years ago so I have been watching for any signs of issues since there is no detailed historical data despite oever a decade of use but so far all imdications point towards extremely safe operation, equivalent to cutouts.

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