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 Post subject: Does assymetrical fault current contribute to arc flash?Posted: Wed Mar 09, 2016 8:32 am
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
A simple question: Does assymetrical fault current contribute to arc flash?

In a nonfaulted system, the X/R ratio causes the current to lag (typically) or lead the voltage. Thus the energy stored in the system impedance is alternately charged and then drained 120 times per second. If a bolted fault occurs, the energy appears as a DC exponentially decaying component whose waveform is:

The assymetrical fault current is %DC = 100*exp(-t/tau)
Tau = (X/R)/(2*pi*F
Where F is frequency (Hertz) and X and R are the circuit inductance (in ohms) and resistance

Although the component is dependent on X/R for most systems the asymmetrical fault component disappears within a few (2-3) cycles. The assymetrical component does not always occur and depends on the phase angle when a fault occurs. The minimum value is at a zero crossing at which point it is zero. The maximum value is 1.414 times the RMS fault current since the peak symmetrical current is 1.414.

During the first half cycle it would be expected that except for a lower current due to arc resistance, the fault current would not be significantly different from the bolted fault case. However we are assured that the arc will extinguish because even if the fault occurred at a fault maximum and X/R is very large so that decay time is slow, the symmetrical component will still reach a minimum which will exceed the asymmetrical component (because it is a decaying DC term) and force the arc to extinguish. Very large X/R may cause the voltage to be sufficient to cause an immediate restriking in the next half cycle but the arc must necessarily be extinguished every half cycle.

After the first half cycle, the phase angle that the arc restrikes may slightly decrease due to temperature but the dominant factor should be the X/R ratio alone. The initial phase angle no longer has any influence at all on the arc characteristics. It would also seem that rather than a term that decays over several cycles as per the bolted fault case, the charge on each half cycle should be a constant dependent only on X/R, not any initial phase angles. At best X/R may slightly increase incident energy but since the total energy transferred is merely shifted in time, it should have little influence on incident energy except this. If we consider the effect of the arcing fault similar to a triac-based light dimmer, increasing X/R reduces the "off" time but since the total energy transfer is held from one cycle to the next, incident energy is fixed and would only affect the likelihood that an arc does not restrike (arc stability) since a longer "off time" due to a low X/R ratio) causes more cooling in the vicinity of the arc column and thus increases the minimum voltage for a restrike.

Thus my general thought is that we should not be modelling asymmetrical fault current as a contributor to incident energy because there isn't one. At most X/R may eventually lead to a variable for predicting arc stability.

Any thoughts on this line of thinking? Am I out to lunch here? If so, why?

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 Post subject: Re: Does assymetrical fault current contribute to arc flash?Posted: Mon Mar 14, 2016 5:10 am
 Sparks Level

Joined: Sun Jan 04, 2009 5:00 pm
Posts: 286
Location: Louisville, KY
All fabric that has an arc rating is tested with asymmetry both ASTM F1959 and IEC 61482-1-1. We believed in the early days (1990's) that that would give us the worst case scenario due to the current spike. I did a study in 2013 to see if this was correct since we had done substantial work on higher currents and found that protection levels went up with this higher heat flux BUT we did find that fabric's breakopen response was lower with higher current so we left in the asymmetry. I'm not sure about 1584 testing but I would guess that they left in the asymmetry also. I'm sure Jim can answer that part. But all clothing is tested with asymmetry (just in case).

Hugh Hoagland
ArcWear/e-Hazard

 Last edited by wbd on Mon Mar 14, 2016 5:23 am, edited 1 time in total. deleted personal contact info

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 Post subject: Re: Does assymetrical fault current contribute to arc flash?Posted: Mon Mar 14, 2016 8:33 am
 Sparks Level

Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
"but since the total energy transferred is merely shifted in time,"
I am not sure that this is really true, especially when stated this way
"but since the total energy transfer is held from one cycle to the next"
It sort of seems like you make this assumption, and then get this same assumption back[ phrased differently] as your result.

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 Post subject: Re: Does assymetrical fault current contribute to arc flash?Posted: Mon Mar 14, 2016 7:03 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
In a bolted fault condition, both a DC and AC current can exist simultaneously, superimposed upon one another. The size of the assymetrical fault current is dependent on phase angle and X/R. It can persist for a few cycles depending on X/R. Whether the current passes through zero or not is immaterial because superposition applies.

In an arcing fault with an infinite X/R (purely reactive, current lags voltage by 90 degrees. At the current zero crossing, the voltage will be a peak but opposite sign. If the arc is self-sustaining (and at that point it certainly would be almost without regard for system voltage), an arc will restrike but flowing in the opposite direction. By nature there is no longer any stored charge and it is only during the arc that charge builds up enough to sustain arcing again until the next voltage peak. As we decrease X/R the effect is that the phase angle for the current zero shifts down from 90 degrees lagging. When the voltage dips below the minimum restrike voltage, current flow stops altogether until the restrike voltage is high enough.

It's the nonlinearity of the arcing fault that confounds a simple superposition treatment. But after walking through it in my head, I can see a case made where X/R can in fact increase incident energy by as much as doubling it. But initial phase angle is not a factor as it is with assymetrical fault current.

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 Post subject: Re: Does assymetrical fault current contribute to arc flash?Posted: Mon Mar 28, 2016 7:06 pm
 Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 178
Paul
I am thinking there was a paper written on this but I cannot find it. It seems to me the DC will contribute but only to some degree. If I am not mistaken, the maximum can only occur in one phase (fault at the zero crossing) but not in the other phases, the other phases will have some degree since they will be some where between zero and ninety degrees (not half because of the exponential).

If I run across that paper I will let you know.

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