Medium Voltage Working Clearance

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roncom0351

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Reference NEC table 110.34(A) 12,460 Volts

My employer is building a new sub-station for Cranes to unload ships. I have two Cooper Medium Switches that have a switch on both ends of the enclosure. When the enclosure is opened it exposes the switch terminals that have Cooper Dead Break Connectors installed on the 15Kv wire and attached to the switch terminals.

My CG and EC say that because the switch terminals have the Cooper Dead Break Connectors installed and that constitutes insulated (non-exposed) no live parts that, Condition 1 applies (1.5M 5') clearance between the connectors.

I say that Condition 3 (2.8m 9') applies because there are exposed parts on both sides of the space (both enclosure covers can be opened at the same time).

What is the intent of the NEC table and what is the working clearance required.

Thanks,

Ron C.
Senior Electrical Inspector
 

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This is certainly not condition 1. That condition is not just about visible live parts. It is also about no grounded surfaces. The enclosure itself counts as a grounded surface, and that would put you into condition 2 as a minimum. I am not sure I completely understand the circumstances of your installation. But if you can open two covers that are opposite to each other, and if by doing so you can see live parts, then you are in condition 3.
 
I moved this thread away from the Hazardous locations topic area, as that area involves explosive atmospheres and similar issues.
 
I began answering while Charlie relocated the thread.

Exposed (as applied to live parts) defined in Article 100 is a loaded definition and leaves too much for an AHJ to decide with the clause, "It is applied to parts that are not suitably guarded, isolated, or insulated." "Suitably" implies a judgment of some kind must be made. Ask how they disconnect the terminals. They don't just walk up and grab them. They use a hot stick as a minimum implying the switches are, "[c]apable of being inadvertently touched or approached nearer than a safe distance by a person". Insulating the terminals alone is irrelevant and it is definitely Condition 3.

That said, you failed to mention a critical fact; how is the system grounded? Table 110.34(A) is based on volts to ground, not line to line.
 
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Volts to Ground

Volts to Ground

This is a 3 phase switch and is 12,460 volts to ground. The switch itself is grounded by way of 350 MCM and a 10'x3/4" ground rod.

Thanks,

Ron
 
System grounding is typically derived at an upstream transformer or generator, not at the switches. The auxiliary equipment grounding conductor (at least I hope it's only an auxiliary) you described is irrelevant. I wish you'd said the system was ungrounded or corner grounded; nevertheless, you have Condition 3 by your description.

BTW, a 12,460V corner ground is unlikely, but I suppose it's possible.
 
Grounding

Grounding

Sorry............12470 volts 3 phase..........7200 volts to gnd.
Grounded at Substation transformer that is 10' away and auxiliary ground at switch enclosure.

Ron
 
Well now you know that, while it's Condition 3, a 6' clearance is sufficient - but not because the terminals were insulated.

EDIT ADD: You have learned a very valuable lesson and I bet you will always be aware of "voltage to ground" in the future ;) BTW, if the system were ungrounded or corner grounded (unlikely at 12,470V) it would be 12,470V to ground. [See Article 100 definition "Voltage to Ground."]
 
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roncom0351 said:
Never mind............I answered my own question. Yes it is 6' because it is 7200 volts to Gnd.

Ron
Click to expand...
Very good,; I like to see folks answer their own questions. That's how they learn best.

Don't know if you noticed my previous "EDIT ADD:" You have learned a very valuable lesson and I bet you will always be aware of "voltage to ground" in the future ;) BTW, if the system were ungrounded or corner grounded (unlikely at 12,470V) it would be 12,470V to ground. [See Article 100 definition "Voltage to Ground."]
 
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