Transit System DC Arc Flash 1

[ahazlewood]

Our safety people are grappling with the PPE required to work on our third and fourth rail dc power systems used in rapid transit systems. The fault levels in the dc switchgear is "typically" around 30,000 Adc with a normal operating voltage of 750 Vdc. In the substations the clearing times for a dc switchgear fault would be dependant on the ac circuit breaker feeding the transformer-rectifier and would typically be in the order of 100 to 150 msec.

Can anyone direct me to resources that could help us determine the level of PPE required?

 

[dpc]

AFAIK, there are no widely accepted arc-flash calculation methods for dc systems. It's a big glaring gap in the current standards.

You might check with US Navy. They have done a lot of work on dc system safety because of the big battery banks on their submarines.

 

[Zogzog]

DPC is right, currently there is not much for DC arc flash calculations. The concensus among the 70E and 1584 guys is that the incident energies for DC arcs are comparable or less that that of AC arcs with the same clearing times and fault currents, so for now they recommend using the AC equations.

Based on the info you provided in your post you are within the limitations of the 70E tables (270kA-cycles) so I would start there for your PPE question.

 

[mls1]

Zogzog, why is the concensus that DC energy would be less than or equal to AC? I would think the lack of zero crossing would make it much more difficult to extinguish therefor resulting in wider potential arc gaps.

AC/DC breakers are rated much lower for DC than AC. While I expect this is due mostly to the ability to interrupt the arc and the mechanical stresses is it possible the thermal levels are higher as well?

 

[Zogzog]

mls1, you still calculate the DC S/C values using the DC equations. (SKM has a DC S/C module). Then you use the DC S/C to calculate clearing times and the Ei.

I would tend to agree with you about the zero crossing. Playing around with AC and DC hipots in the shop it seems that DC will arc easier than AC, but I have asked this question to soem of the 70E guys and they are saying to use the AC arc flash equations. Maybe that ius because there is not any other alternative at this time.