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 Post subject: Arc-Flash on the secondary side of a transformers
PostPosted: Thu Jul 07, 2016 2:35 pm 

Joined: Wed Nov 11, 2015 9:18 am
Posts: 7
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.


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Thu Jul 07, 2016 4:02 pm 
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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.


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Mon Jul 11, 2016 7:15 am 
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I asked almost the same question as you've asked. However, I asked the people at NFPA. Instead of answering my question, they asked why I was working on energized transformers? I never got an answer.

However, my answer your question is we label almost everything including transformers down to as small as 15 KVA - with 480 volts present. That might seem like overkill, but the 480 volt source is still there and so we consider it a voltage and arc flash warning. BTW, that 125 KVA exception was deleted from from the 2012 70E Standard. Unfortunately, the 70E did not say it was deleted, they just deleted it. The only reason I knew about it was Mersen Fuse sent out a document on the changes in the Standard.

You might also ask why label down to that small equipment. Here's another reason. Years ago we had a client who wanted us to label everything in his plant. I told him about the exception and asked why he'd want this level of labeling? He said, "I never want to hear (following an accident) that the equipment was not labeled and so I didn't think there was any hazard." I've heard electricians say as much, after they were injured. "well it wasn't labeled, so I didn't think there was any problem." That's not something you want to experience as the person responsible for labeling.

Lastly, I'd suggest you think in terms of being called as a witness. So, you're on the witness stand and their attorney asks you "why did you label this transformer but not that transformer, after all, they both have the same voltages inside them." I'd want to have solid justification for that kind of decision.

On your question about the 2 second rule. It's really not a rule, but an accepted practice. We clearly state (in our reports) the labeling assumes the worker can safely get outside the arc flash boundary within 2 seconds or less. That if they cannot safely get outside that boundary within 2 seconds, the equipment must be turned off.


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Mon Jul 11, 2016 8:07 am 
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wilhendrix wrote:
BTW, that 125 KVA exception was deleted from from the 2012 70E Standard. Unfortunately, the 70E did not say it was deleted, they just deleted it.


The 125 kVA exception (no such size so 112.5) was from the 2002 Edition of IEEE 1584. Before the dividing lines of responsibility were drawn between IEEE and NFPA, there was overlap between calculations and work rules. i.e. NFPA 70E originally had Lee equations and added the 125 kVA wording. IEEE 1584 referenced PPE. Now the boundaries are pretty clearly set and IEEE is responsible for the calculation methods and NFPA is responsible for the work practices / PPE requirements etc.

The other thing that happened is when NFPA added the 125 kVA exception they copied it wrong from IEEE 1584 which applied it to transformers LESS than 240 volts implying 208. NFPA stated 240 volts OR LESS implying 240.

Mersen has conducted testing that indicates under certain conditions, an arc flash could be sustained at lower values of short circuit current (below what may be attained with a 112.5 kVA transformer)

Disclaimer: as always these are just my thoughts and not an official interpretation of any particular standards organization


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Mon Jul 11, 2016 9:11 am 

Joined: Wed Nov 11, 2015 9:18 am
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Yes, we also do the calculations currently on all transformers 15 kva and larger.
As far as the comments that are made as to why are you working on a transformer hot, I would ask how to perform an IR scan on the transformer without removing the cover and having it energized? Assuming there are no IR windows on it, and I haven't seen one yet with them.

I am just wondering, while I understand the 2-seconds is more of accepted practice, does that make the arc-flash incident any less hazardous just because a person can turn away in that period of time?


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Mon Jul 11, 2016 5:33 pm 
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My thoughts (without benefits of Jim's presence at the NFPA meetings) is this. NFPA 70E clearly defines a set of work rules but carefully skirts any kind of "arc flash calculation" of any kind. The one solitary exception is with the arc flash tables.

Once you leave the comfortable world of NFPA 70E then there are no less than 8 different calculation methods detailed in Annex D at least when the "125 kVA rule" was removed, and those are by no means the only ones out there. NFPA 70E does not actually discuss the merits of any particular calculation method, so each is given roughly equal weighting. It is an end user's decision as to what is the "best" method to use. That being the case, only one of the 8 methods documented in Annex D actually has the "125 kVA transformer exception". It would be inappropriate to use the 125 kVA "rule" for instance if IEEE C2 tables are being followed because IEEE C2 has no "exception". At best it has a minimum PPE for all tasks of 4 cal/cm2 with different PPE requirements for 4 cal/cm2 compared to the table method in 70E. So we would have a discrepancy if we're following 70E but the particular calculation method chosen doesn't have the "125 kVA rule". The alternative is that 70E would have to favor one particular calculation method over another. But since calculations are not part of the scope, it carefully avoids any specific method.

Similarly IEEE C2 has extensive tables including values for higher voltages (well over 15 kV). The values are calculated using ArcPro but IEEE C2 merely states that the calculations are done with a commercial software program without naming names. Unlike 70E the "low voltage" table in IEEE C2 has extensive footnotes on how the values were derived on MOST of the values. Some footnotes do handwaving referring to testing done at a utility which is PG&E but they don't call out names even here.


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 Post subject: Re: Arc-Flash on the secondary side of a transformers
PostPosted: Wed Jul 20, 2016 12:13 pm 
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I don't typically include transformer secondaries in the analysis nor label them. I cannot think of a reason to open an energized dry type transformer. Our company policy is "If it is not labeled then de-energize prior to opening."

I do calculate the energy on either side of our oil filled pad mounted transformers and provide a primary and a secondary side label. Most of those have the oil sample valve inside. We take samples every 2 years for analysis. Any new transformers we purchase have the sample valve on the outside with a lockable cover over them.

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