This topic surface from time to time.

The calculated incident energy for equipment located on the secondary side of a LARGE transformer can be quite large. (i.e. not referring to small lighting transformers) This is usually because the transformer's primary protective device is used to define the arc duration which can be quite long.

There continues to be concern about how to handle such a situation. Energized work should be kept to a minimum however establishing an electrically safe work condition often requires operation (interaction) of the protective device where the high incident energy exists.

The option of opening the primary device is not always possible since this may be under utility control or part of a medium voltage system where outside personnel are required.

Here is this week's question.

How should equipment on a transformer's secondary with large incident energy be treated?

• Do not operate
• Open the primary device (even if outside personnel are required)
• Remote operation
• Wear appropriate (high arc rated) PPE and operate the device
• Something else

I'm sure there may be lots of different opinions and ideas for this one. Your thoughts are always welcome and encouraged!

I voted for Something Else as large incident energy was not defined. I have seen facilities where 12 cal/cm^2 is considered large and others where large does not enter into the picture unless it is >40 cal/cm^2. Therefore, some facilities will use remote operation or open the primary device while others will suit up in PPE and operate it directly.

We have several stations along our pipeline that the secondary of our 115kv/2.4kv primary transformers has 20cal/cm^2 or above. We decided to modify our 2400 HVL switches for remote operation. The manufacturers representative met us at our stations and spec'd a motor operator and fuse kits to modify the switches. We ran the control power outside the arc flash boundary of the HVL switches and put them into a weatherproof box with indicator lights. The techs operate the switches and then can verify the switches closed properly without having to don PPE. The fuse kits were spec'd so that they have a fast enough clearing time to lower the incident energy at our equipment that these switches provide power.

Why would you have to modify your HVL switches? There are portable remote operators for HVL switches that require no modification to the switch and are easily moved from one switch to another.

In my situation I've been finding that above about 1500 kVA @ 480 V on the secondary, it is pretty difficult at best to even get arc flash values reasonable with a breaker and frame sizes are in the 2000 A or larger category so these breakers ar none too fast either. At higher voltages of course the kVA threshold moves up.

The solution I've been finding that works well is to install a breaker on the primary side of the transformer with CT's on both primary and secondary sides, using a multifunction relay. The price is higher than a fused setup but on the other hand no fuse replacement is needed and also it eliminates the need for a breaker/disconnect on the secondary side since it creates a "virtual breaker" arrangement. Although MV equipment is generally more expensive than LV, as long as it stays under the "magic" threshold of around 600 A, price is not too exhorbitant.

Just for the sake of argument, I would like to bring up NFPA 70E, 130.7, Informational Note No. 2: "It is the collective experience of the Technical Committee on Electrical Safety in the Workplace that normal operation of enclosed electrical equipment, operating at 600 volts or less, that has been properly maintained by qualified persons is not likely to expose the employee to an electrical hazard."

Now, assuming that the equipment on the secondary side is enclosed, has been maintained, and is 600V or less. Why would any extra measures be required to operate, regardless of the incident energy?

Because 70E says in numerous places that you need to analyze the HAZARD but does not mention the likelihood or risk AT ALL anywhere, except in the definitions. So one is directed to analyze the hazard and then...nothing. Under the definition of an arc flash hazard in Article 100, an arc flash hazard exists "provided a person is interacting with the equipment in such a manner that could cause an electric arc." Now if it ended there, we'd have a vague definition but it doesn't. Informational note 2 in the same section says to refer to table 130.7(C)(15)(a) and (B) for examples of activites that could pose an arc flash hazard. In those tables, specific activities that are listed include operating circuit breakrs and disconnects, performing IR scans, and reading a meter while operating a meter stick. Frankly I completely disagree with this interpretation, and it does rely on informational notes, but one is not directed to perform a risk assessment. All of the arc flash hazard analysis methods described in Annex D do not even mention a bit about the likelihood of a hazard. So from a strict interpretation point of view, whether or not I agree with the informational note or anything else, it is pretty cut and dry to me that at least in its current form, 70E makes everything except just walking by an arc flash hazard and by the way the definitions are constructed, an arc flash risk.

Second, the construction of the hazard analysis prior to the 2012 edition is that there was a paragraph located in section 110 which specifically directed one to use Article 120 for de-energized work and making equipment de-energized, and Article 130 for energized work. Not withstanding this construction, one was still required to analyze the hazards which were located in sections of 130. Further, the EEWP was evoked only when crossing the limited approach boundary while the 2012 edition eliminated this confusion by invoking the EEWP requirement when either the arc flash hazard boundary OR the limited approach boundary was crossed. This creates a further problem because at some point one has to actually de-energize a system to even evoke article 120 and article 130 now makes this an EEWP-required task whereas before by going directly to Article 120 via Article 110, we bypassed the EEWP requirement. It is my conclusion that this is the reason for the new exception under 130.2 but so far no one can articulate exactly what the exception is saying. Try it yourself by reading it and trying to discern what it means. Either way, now we end up at the crux of the matter. Regardless of how I attempt to de-energize at some point I am going to be doing energized work in order to de-energize by opening some kind of disconnect hardware, and further I am doing de-energized work by testing for the absence of voltage. In either case, I'm directed to analyze the hazards. Both operation of disconnects and voltage testing clearly fall under work outlined in Tables 130.7(C)(15)(a) and (b) and one has to take appropriate precautions (don PPE) to perform these tasks.

Stepping back for a moment and looking at what is happening though and how ridiculous this is. To even get to the point of doing proper maintenance one has to be able to actually operate the disconnect device and test for voltage. This is pretty much prevented or severely hampered by the hazard analysis and/or EEWP steps. So the knee-jerk reaction is not to do maintenance on the equipment....which actually increases the hazard and makes it so that the informational note 2 in section 130.7 can't even be invoked in the first place because equipment is not being properly maintained. Second, in many cases, even the EEWP rule itself is patently ridiculous. If we have the common situation of a breaker on the secondary side of a transformer feeding some other equipment lineup, regardless of the nature of what it is, we have two choices. We can either de-energize the breaker on the secondary side of the transformer which is going to be labelled with some sort of obscenely high incident energy rating, or we can work live on the equipment downstream of that breaker even if the work is something relatively hazardous such as removing an MCC bucket or a drawout breaker from the cell, or even the simple act of removing a molded case breaker from the bus of a panelboard. Assuming that the feeder breaker is in another enclosure some place else, the incident energy within the MCC/switchgear/panelboard is likely to be low since the transformer secondary protection will trip in the event of an arc flash to limit the arcing time. But since all of the activities I just described are clearly "working live", one is directed to instead operate the breaker/disconnect on the secondary side of the transformer which is inherently a far more risky operation than "working live". Thus the greater hazard rule can and should be evoked but instead one is directed to operate the breaker, especially if we are in the current situation where operating a breaker in normally operating and maintained equipment is considered an arc flash hazard, especially when considering that anything over 40 cal/cm^2 requires "heightened awareness". See informational note #3 in 130.7.

And no, I'm not advocating taking more risks than are necessary. What I'm advocating is that we can all agree on what are tolerable risks and which ones are simply intolerable. What we have to today as far as a Code goes, if strictly read, creates all kinds of silly situations which is exactly where Jim's question leads to.