Transit System DC Arc Flash
Transit System DC Arc Flash
(OP)
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?
Can anyone direct me to resources that could help us determine the level of PPE required?






RE: Transit System DC Arc Flash
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.
RE: Transit System DC Arc Flash
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.
RE: Transit System DC Arc Flash
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?
RE: Transit System DC Arc Flash
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.