NFPA-70E

[IASMike]

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.

 

[cuky2000]

There are no guidance for DC arc calculations, except DOE Handbook

 

[dpc]

NFPA-70E applies to all electrical systems except for exceptions listed in Article 90. The problem is that there are no recognized standards for calculating arc energy for dc systems, as cuky mentions.

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.

 

[Zogzog]

The IEEE 1584 group is planning on doing some research on DC arcs soon, when they have the budget

 

[IASMike]

The NFPA 70E Handbook specifically says in the definitions
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.

 

[dpc]

IASMike,

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.

 

[IASMike]

Sorry, I disagree ac will jump a large gap than dc, but dc will maintain the arc longer. The zero crossing has no effect on the arc.

The NFPA 90E calculations for arcing time are based on the overcurrent protective devices. This is what determines the arcing time.

 

[Zogzog]

I am with IASMike on this one, the amount of fault current determines if an arc is self sustaining, if the arc will sustain itself, then the OCPD clearing time comes into play.

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

http://www.epowerplus.com

 

[dpc]

The zero crossing has a big effect on sustaining an arc. Take a look at dc ratings versus ac rating for contacts and breakers that have dc ratings.

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.

 

[stevenal]

NFPA 70E uses the methods of calculation outlined in IEEE 1584, the scope of which specifically excludes single phase AC as well as DC systems.

 

[davidbeach]

Of course a 50kV rms AC source would initiate an arc over a larger air gap than a 50kV DC source. You would need to compare AC peak voltage to DC voltage.

 

[Zogzog]

I used 35kv AC to get a 50kV peak and its a hipot, not exact science. I knew someone would have something to say about my test, let me guess, you work in a lab or something.

 

[davidbeach]

Nope, no lab work. I do find your 35kV rms AC vs. 50kV DC to be much more meaningful than 50kV rms AC vs. 50kV DC.

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.

 

[Zogzog]

Wouldn't the same concept applies to arcs, thus the point of the discussion (and experiment).

 

[IASMike]

NFPA 70E does not exclude single phase from its calculation methods. In Annex D.8 says if you use this method for single phase it will give conservative results. At this time since there is no better method of calculating Incident Energy.So this is the method you have to use
on DC where there is a flash hazard.

 

[stevenal]

The Annex is not part of the requirement. The same annex section references the IEEE standard which disclaims any use for single phase AC or DC systems. Also note that D.8.1 limits its use to 50 and 60 Hz systems.

 

[Zogzog]

I think the point IAS Mike is tyring to make is using either equations for DC is better than nothing. I agree.

 

[lou1952]

NFPA 70E paragraph 310.5(C)states "THe requirements of 130.3, Flash Hazard Analysis, shall not apply to electrolytic cell line work zones. IEEE 1584 paragraph 1.2 states that "Single-phase ac systems and dc systems are not included in this guide." So what to do?

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?

 

[mykh]

Isc = Vs / Ri - Short circuit current as a function of source voltage and internal resistance

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?

 

[rbulsara]

I am with dpc. IEEE 1584 equations are not meant to be used for any voltage or any conditions. They are curve fit equations, to fit the curve plotted by data collection of some tests results in very controlled conditions using AC source. It does not imply using them on DC systems or even single phase systems.

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.