NFPA-70E
NFPA-70E
(OP)
Can anyone direct me to resources on Arc-Flash for DC. We have someone who uses 250VDC for overhead cranes. They would like some more infomation on how to comply with OSHA. They believe that NFPA-70E is gear towards AC and batteries. They are generating their own DC.






RE: NFPA-70E
RE: NFPA-70E
The requirements of NFPA-70E are certainly based on the assumption of ac current, but it doesn't say that dc systems pose no arc-flash risk. In lieu of anything else, the task-based tables in Article 130 could serve as the basis for an arc-flash safety program.
These old crane systems with 250 Vdc running down the wall are not really addressed specifically, but no way is OSHA going to give you a pass if there's an accident.
You have to remember that NFPA-70E is a consensus standard and is not directly enforced by anyone. It's a guide that OSHA accepts, but in some cases, you will need to develop your own safety requirements.
RE: NFPA-70E
RE: NFPA-70E
that voltage means AC and DC. Which means the standard applies to AC and DC.
The calculations are all based upon RMS values. This means the calculations do not care if it is AC or DC. What this will do is give you conservative values for you arc flash calculations.
RE: NFPA-70E
Sorry, I can't really agree with that assessment. One issue is what is the minimum voltage that can sustain an arc. Because ac goes through zero current twice a cycle, the dynamics of ac arcs are much different than dc.
I don't know of any test basis to say that using the equations in NFPA 70E or IEEE-1584 will yield conservative results for dc. For a 250 V dc arcing fault, I would think the chances of the dc arc being sustained are higher than for 240V ac.
The IEEE-1584 standard is specifically intended for ac systems. NFPA-70E doesn't exclude dc from arc-flash consideration, but it doesn't give a calculation method that I'm aware of.
RE: NFPA-70E
The NFPA 90E calculations for arcing time are based on the overcurrent protective devices. This is what determines the arcing time.
RE: NFPA-70E
However, I also agree with dpc that the dynamics of an AC and DC arc are much different and there are no test results to show that DC would be more conservitive, so here is what I did.
Went back to the shop, got an AC and DC hipot up to 50kV and started drawing arcs to ground, the 50kV AC jumped much further than the 50kV DC. Now, the voltage and humidity were the same, but this was just a quick little experiment, this test is in no way proof that AC arcs eaiser than DC but it makes me lean towards that theory.
At the NETA (PowerTest) conference, the safety panel discussion will include this discussion with NFPA 70E and IEEE 1584 commitee members, I will post anything relavent from that discussion.
Scott Peterson
Training Manager
Power Plus Engineering
www.epowerplus.com
RE: NFPA-70E
AC arcs typically extinguish at a zero crossing.
My point is that most people consider the chances of actually sustaining an arc at 208V or even 240V to be very low. In most of the testing, arcs were not sustained at these voltages. But I suspect the results would be different for dc.
RE: NFPA-70E
RE: NFPA-70E
RE: NFPA-70E
RE: NFPA-70E
Maybe the high dV/dT of the AC compared to the DC in the air around the end of your probe contributes to the easier breakdown of the air during the AC tests.
RE: NFPA-70E
RE: NFPA-70E
on DC where there is a flash hazard.
RE: NFPA-70E
RE: NFPA-70E
RE: NFPA-70E
We are considering going back to Ralph Lee's equations and using the maximum power transfer equations. This occurs when the arc voltage equals 1/2 the source voltage and the current equals 1/2 the source short circuit capacitiy. For the time, if a cable is involved, we would use the time needed to fuse the cable at the applied current.
ex. 240V dc, 60kA short circuit capacity, 1 second to fuse the supply cable equals 3.6MJ.
Comments?
RE: NFPA-70E
Iar = (Vs - Var) / V * Isc - Arcing current as s function of short circuit current and arcing voltage,
where, Var = Uc + Upc * L,
Uc is the sum of the anodic and cathodic drops, It has been found to be:
1. 32 V for Al-Al and Cu-Steel
2. 36 V for Cu-Cu
3. 30 V for Steel-Steel
Upc is positive column voltage gradient has been reported to be between 16 and 24 V/cm
L - gap between contacts in cm
Arcing power Par = Var * Iar, W
Arcing energy Ear = Par * t, Joule
where t - arcing time determined using fuse time-current curves (ac) and the dc circuit time constant,
In case of open arc assuming all the energy is converted into thermal (arc) energy:
Incident energy Einc = Ear / 4piR2, joule / cm2
R being working distance in cm2
Comments?
RE: NFPA-70E
Current IEEE equations are not valid for any conditions beyond they are specifically indicated in the standards, which is 3 Phase AC systems with lot of other restrictions.
RE: NFPA-70E