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 Post subject: Arc Gap in ArcProPosted: Thu Jun 19, 2014 9:44 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
I have been reviewing a report done for a utility company done previous to the new OSHA regulations for utilities was issued. In this report, which was done mainly for substation work at 115kV, the arc gap chosen was the height of the bushing supporting the switch plus 15%. This was apparently an attempt to be conservative and this is for gang operated switches that are operated from the ground.

Now the instructions in ArcPro state that the arc gap is the distance between two electrodes but these would have to be close enough for an arc to occur at a given voltage. OSHA has provided at table on selecting a reasonable arc gap which is based on the dielectric breakdown of air. Therefore for the system I am reviewing a 115kV bushing is 38.5" and with the 15% adder, the arc gap used was 44.28". Using the OSHA method, this arc gap would be 11.5". However, the preamble discussion seemed to center around lineman during work on conductors (wire). Table 5 references the class of equipment as a single conductor in air which seemed to be alluding back to lineman but a switch is a conductor. In this case it is a gang operated air break switch in a substation, so what is the arc gap in this situation?

I have some thoughts but am curious to hear other opinions.

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 Post subject: Re: Arc Gap in ArcProPosted: Mon Jun 23, 2014 10:32 am
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Note 1 of NESC Table 410-3 says that the arc gap is calculated by using the phase-to-ground voltage of the circuit and dividing by 10, based on the dielectric strength of air being 10 kV per inch.

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 Post subject: Re: Arc Gap in ArcProPosted: Mon Jun 23, 2014 10:51 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
Yes, I guess where the confusion exists is that there is a lot of discussion in the OSHA preamble regarding conductor and live line tool work, so people seem to think that this solely relates to overhead lines not totally applicable to substations where the bus (conductor) is firmly supported and not moveable like an overhead conductor.

So apparently, this company used the height of the bushing +15% for the arc gap. It would seem that there would not be any flashover until the lowest dielectric strength was breached. I would think this is 11.5"

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 Post subject: Re: Arc Gap in ArcProPosted: Mon Jun 23, 2014 10:31 pm
 Sparks Level

Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
A bit off-topic: 10kV per inch? That is 4kV per centimeter. Here they say 1kV per centimeter in air to be safe, this would equal 2.54kV per inch (it is a rule of thumb).

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 Post subject: Re: Arc Gap in ArcProPosted: Tue Jun 24, 2014 4:14 am
 Plasma Level

Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
Luc, yes dielectric strength of air is approximately 10kV for every 25.4 mm (1 inch) as stated in the recently released OSHA 1910.269 regulations

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