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 Post subject: Yet another question about <240 V systemsPosted: Fri Dec 02, 2011 2:22 pm

Joined: Tue Mar 02, 2010 1:50 pm
Posts: 26
I've been searching around for the answer and I'm sure it obvious to some but it has aluded me, although I'm sure its on the forum somewhere. NFPA has a task list with HRC for <240V. IEEE 1584 is only for system down to 208V (although even it says its VERY conservative at that voltage). There's also the 125kVA exemption. My question is this: For 208 V three phase systems and 120/240 V single phase systems, if the transformer is less than 125 kVA can I say the HRC is zero using the exemption in 1584?
For systems less than 208V, can I also, based on 1584, assume HRC is zero?
Does this also apply to DC (in other words, at 125DC do arcs sustain themselves?)

Just trying to figure out when I can label cat 0 and when I have to rely on the task list.

Thanks,

Casey

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 Post subject: Posted: Mon Jan 09, 2012 11:22 pm

Joined: Wed Aug 31, 2011 3:33 pm
Posts: 14
Location: Denver, CO
208V 3-ph

I would say no you can't assume a HRC of zero since NFPA 70E defines this type of equipment as a hazard risk category of 1.

Further, NFPA 70E 2012 has published hazard risk categories for DC systems and equipment while we are waiting an update from IEEE 1584 with DC arc-flash calculation methods.

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 Post subject: Posted: Tue Jan 10, 2012 4:24 pm

Joined: Fri Jan 06, 2012 12:46 pm
Posts: 3
I have work quite a bit with DC motors and DC does arc incredibly especially if there is some inductance in the system. Look at the Dynamic braking relay on an old DC motor system, and compare it to the same Ampacity (frame size) in an AC system.
DC never crosses zero to extinguish the arc, so gap between the contacts is the key along with magnets to draw the arc into a longer path.
Remeber in an inductive circuit it tries to maintian the flow of current, voltage is the wildcard it will reach very high levels. DC Motor circuits are inductive.
Anyway that is my worthless 2 cents

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