rmgbob wrote:

I am curious on how others that are performing arc-flash assessments are handling the arc-flash on the secondary side of dry type transformers? I believe that IEEE 1584 states something to the affect that on a transformer of less than 240v and less than 125 kva (for a three phase system), the calculations can be cut off at 2-seconds. So let me ask:

1. Are you cutting off the calculations for these systems at 2-seconds?

1. If there is a panel on the secondary side of the transformer, are you cutting off the calculations at that panel at 2-seconds also?

3. What are you doing with the calculations for transformers over 125 kva?

Thanks for your input.

You are mixing two entirely different statements within IEEE 1584. The first is that a general recommendation was made to cut off arc flash calculations at 2 seconds except in cases where the worker cannot escape such as within an underground vault. This applies universally to all voltages and currents that IEEE 1584 encompasses and is being used even outside the valid range for IEEE 1584.

The second statement is that 3 phase circuits with a voltage of 208 VAC or less and fed from a single transformer rated 125 kVA or less "need not be considered" which has been taken to mean that the incident energy is under 1.2 cal/cm2. This section of IEEE 1584 is under review and will be revised in the next version (whenever that is), but for now this is what we have for a "low voltage rule".

Given that we're within the transformer enclosure or the termination compartment(s) in most designs the only overcurrent protection provided is from the overcurrent protective device on the primary side. Because a fault is scaled with the turns ratio of the transformer and because the primary side protection usually has to be set very high to avoid tripping due to magnetization currents, this usually means that there is little or no protection on the secondary side and incident energy values will be quite high relative to the primary side except with small transformers where the transformer impedance acts as an effective current limiter.

All this applies for over 250 VAC. Below 250 VAC we have the basic problem that the IEEE 1584 model is based on a single point at 208 VAC which is the only point where they were able to achieve stable arcing and thus it is questionable whether or not the incident energy values are even valid at that point.

The problem is a matter of physics. At steady state, the temperature at the arc core which is only a couple millimeters in diameter is 20,000 K. Essentially all of this energy is absorbed by the surrounding air. Excited electrons within the surrounding air then emit less energetic photons which are in turn reabsorbed again although the absorption rate is less than 100%, and this trend continues in a cascade as the energy spreads out away from the arc. As the temperature of the air increases it loses its insulative properties and by the time it reaches a few thousand degrees, it is essentially a conductor. Since substantially all the heat transfer is radiative in nature, the heat transfer rate is propertional to the 4th power of the temperature delta so it is extremely efficient. The net effect is that when the arc stops (after a zero crossing) the air begins to rapidly cool down and as it does so it becomes more and more insulative again, raising the minimum voltage to form an arc. If the air cools down enough, the arc never restrikes. Above around 250 V, arcs are self-sustaining but below that point conditions have to be right to form a self-sustaining arc...generally they are just going to go out. Even if they don't go out, the fact that the arc is only conducting for a fraction of the power cycle (the arc does not restrike until it reaches the breakdown voltage of the air...which is temperature dependent), contributes to a much lower time to heat up the surrounding air and works against good, strong self-sustaining, stable arcing below 250 V.

IEEE 1584 is based on STABLE arcs. Unstable arcs so far have been unpredictable so they are not included in the IEEE 1584 data set. We do not yet have an equation for "arcing time" for the unstable case. So the only practical way to analyze this situation is to use known test data or standards to handle the "under 250 V" case.

IEEE C2 gives a flat 4 cal/cm2 value for 250 VAC or less, single or 3 phase equipment based on testing conducted by some utilities as well as EPRI. Test data on a 130 VDC system which has been widely reported at 20 kA and 1/4" arc gap showed that it would only arc for 0.85 seconds which works out to very close to 1.2 cal/cm2, which would seem to show that the vast majority of 120 VAC or 125 VDC or less circuits are also similarly unlikely to be a hazard. So taking this all into consideration I have not seen evidence that at 125 V or less with 20 kA or less of available fault current that there is a significant hazard. From 125 VAC to 208 VAC fed by a 125 kVA or less transformer for the 3 phase case (and can be extrapolated to single phase), we can also assume it is 1.2 cal/cm2 or less based on the current (subject to change) IEEE 1584 recommendation. At above 20 kA of available short circuit current or from 208 VAC to 250 VAC I would then defer to the IEEE C2 recommendation of 4 cal/cm2. This approach then is mostly standards based and the closest you can get to a realistic value for the case of under 250 V. Above 250 V and below 15 kV, IEEE 1584 empirical equation is about as good as it gets for 3 phase AC. For single phase AC it is not as severe as the 3 phase case but without sufficient test data the 3 phase result should be used. Note that some have recommended dividing the 3 phase case by 3 but there is ample evidence that this is a bad assumption and should not be done. For DC Ammerman's equation is currently the best we have available...there simply isn't any test data. Above 15 kV, OSHA among others recommends using ArcPro from Kinetrics. Absent test data above this voltage that's probably the best we can do at this point.