I perform Arc Flash Studies. I had a recent interaction with one of the local Utility providers that I’ve recreated below for the enjoyment of anyone else doing the Studies. I replaced the Utility provider's name with "UTIL" and I've separated each correspondence with dashes.
I had to trim out some info because the file was too big to post.
Dear UTIL,
We are still requesting Power System Fault and Upstream protective device information for ….
We've been trying for eight months! I know that all it takes is for one knowledgeable person to spend ten minutes looking up the information but for some reason this isn't getting into the right hands.
Mr. Mayan,
I was able to track this request down. It was first set up incorrectly. XXXX is now working on it.
Part of the process is to verify …..data. This can take a few weeks.
ARC FLASH INFORMATION RECEIVED from field tech (MV system capacity in per unit, xfmr kVA and fusing info, but no xfmr impedance was provided
Thanks XXXX. But you've provided a range of Z for the 300 kVA service xfmr and I need to use the Z listed on the nameplate as specified in IEEE 1584-2013 section 5 Data Collection.

I understand the xfmr could be replaced at any time by one with an impedance (Z) in the range of what was provided, and I will use the lowest Z for the short-circuit evaluation, but the arc-flash calculations should be performed using the nameplate values. I can perform the calculation with reasonable accuracy without test data by using a typical X/R ratio for the measured Z that's on the nameplate. But I need the nameplate data.
If you won't or can't provide the actual nameplate impedance number I will model it using both extremes and present the worst-case, but given that the Study must be updated every 5 years, the actual is preferred and if it's changed it's caught on the next update.

Can you send me a picture of the nameplate, or alternatively just the percent impedance?


Hello again, (UTIL ENGINEER)

Let me address this one since I am an arc flash expert.

Transformers of this size do not come with test reports and it is not a simple process to retrieve nameplate data. The most conservative incident energy levels are achieved using impedance ranges. As you know, low fault current values could have a higher incident energy level than a high fault current because of the slower clearing times. Not only can the transformer be changed at any time, the source impedance changes often, and the protective device can change. I would be concerned about an Arc Flash calculation that uses precise values; however, since the PPE ranges are relatively wide, with only five categories between 0 to greater than 40 Cals/cm2, precision is not a requirement. If precision were necessary then changing out the transformer tomorrow could cause safety issues. With this said, if the PPE requirements appear unreasonable, then we should take a second look at the data and the calculations and I will be happy to assist you with this.

And I consider myself an expert at arc flash concepts as well; and an expert in the application of NFPA 70E, which is the basis for the requirement for arc-flash labels, as I'm an NFPA-Certified Electrical Safety Compliance Professional.

My calculations show a range of IE levels for the impedances you provided from less than 2 calories for the low-impedance transformer to 25 calories for the high-impedance transformer.

If you don't supply the nameplate Z then they are inconvenienced with an unrealistically high incident energy posting as I will be forced to use the worst-case scenario. And we all know you won't be buying a 6%Z 300 kVA transformer.

IEEE Standard 1584-2013 specifies the use of nameplate data so I adhere to that standard and advise accordingly. It is in the clients best interest to have an accurate representation of the hazard at their service-entrance switchboard. NFPA 70E requires that this information be verified every five years, so changes are caught.

I'm also not looking for test report data for a padmount transformer, although I know that information is available at the factory. The Army Corps of Engineers requires this for their Power Study submissions. The information I'm looking for is on the transformer nameplate: the impedance, %Z.

So will you provide the transformer nameplate information, or will our client's switchgear be labelled with a higher-than-likely incident energy level?

My apologies if you felt insulted. I stated I am an expert because I am the appropriate person to answer the question.

The question here is, who would be liable if UTIL provided (arc flash related) nameplate impedance on a transformer that could be changed out tomorrow, or the protection changed, or the source impedance changed? Providing nameplate impedance is risky for UTIL and the customer, which is why we provide a range. We work with hundreds of companies and consultants and no one else is asking for nameplate impedance.

After recently attending an IEEE PES seminar on Arc Flash, I feel even stronger that the customer should be given conservation PPE requirements. Customers do not understand the values consultants are providing to them, some consultants are not properly providing the variances / risk, and the customer is putting too much faith in these numbers. The drastic change in energy level quoted below is even more of a concern; if UTIL changed out the transformer tomorrow and the value could easily shoot from 2 cals to 25 cals; this is extremely dangerous.
No apology required YYY as I was not at all insulted. I was just making my credentials known as I've made this line of work my career passion.

And I come back even stronger with my insistence that accurate data must be used for the analysis.

I've been the engineer-of-record for my employer XXX Engineering for over 70 schools in STATE and COUNTY and an ever-increasing number in Other County, STATE and we've used nameplate data on every one of them. UTIL1 and even UTIL have provided us with this information, although with documented resistance from UTIL to spread the data-collection charges out to the Other County school system. We're getting nameplate information from John Doe at UTIL right now.

The intent of the standard is to represent the incident energy levels with accurate calculations using the best information available. If the system changes, 70E requires a re-check every 5 years so it will be caught. The intent is to be accurate in determination of the hazard.

The range of IE levels I quoted are for transformers that vary in impedance from 2% to 6%. You know we won't see that level of variance on any size transformer. If I have to use a library value it will be around 5.2%Z. UTIL probably buys more efficient transformers so it will likely be less. I just want the nameplate value because that's what the IEEE Standard 1584 calls for.

All I really need from UTIL is an impedance number between 2 and 6% that's written on the label on the transformer, with a UTIL heading on the communication. All UTIL will be held to is that the information they provided was accurate on the date of the communication. If anything changes it will be caught in 5 years because we'll be back to ask again. The customer is responsible for changes they initiate like addition of large motors that would affect arc flash levels.

Will UTIL provide this information to XXX Engineering for use in the development of arc flash levels? Is it necessary that client, your client, request this information from UTIL? ... as this is the way we have been performing Arc Flash Studies for them at multiple other facilities.

You could try putting this in as a separate request that is not related to Arc Flash. You will need to contact the customer service center and request name plate impedance on the transformer. They will set up a work request for this and you will be charged for the service.
Let me know if you need anything else from me.
Yes, that's exactly what UTILITY attempted to do with the XXXX County school system was to charge them for this service.
Your man was already at the transformer. Is it safe to assume that he did not photograph the nameplate?
Unfortunately the info you have provided shows primary fault current only and without the transformer impedance the secondary fault current is not able to be calculated.
Your information letter states: To obtain the maximum fault current as quoted to the electrical inspector please contact the local office. Which office is this? If we can get this information it will have included the transformer impedance and will suffice. Or is this just using the low-level 2% impedance number?
Please be advised that UTILITY is the only Utility I have encountered in my 30 years of doing this work that is requesting money to provide system information, for their own system.
Please advise as to how I can get a picture of the nameplate or just the transformer impedance percentage for their installed service.
Does your client YYYYY have better access to this information than their engineering representative?
Mr. Mayan,
I should have mentioned, the person who opens up the transformer is not the same person who checks the service for an Arc Flash request. It has to be a specially trained and qualified person to open up a hot transformer. This is very dangerous work.
If you need maximum fault current XXXXX can provide this value to you. He will use a combination of infinite bus impedance of the transformer and the size of the switchgear to calculate this value.
The CSC is the same place you called to request an arc flash study. You will not be making the same request with a transformer nameplate impedance so it needs to come in separately.
Hope this helps.