I've had this question about padmount transformer arc flash protection for awhile and I haven't had anyone able to answer the question. Here it is....

Typically anytime you are working on the secondary side of a transformer, and more specifically for my question a padmount transformer, there is a large amount of incident energy available and therefore the arc flash boundary is fairly large. If you are tasked with verifying the absence of voltage at a padmount transformer with a 480v secondary you will have to open the door to the secondary which exposes you to the exposed parts of the secondary terminations.

NFPA 70E 120.2 (A) says "Electrical conductors and circuit parts shall not be considered to be in an electrically safe work condition until all of the requirements of Article 120 have been met."

NFPA 70E Table 130.5 (C) Estimate of Likelihood of Occurrence of Arc Flash Incident

- This table list the likelihood of an occurrence of an arc flash as "yes" for two applicable scenarios when working on a padmount transformer:

1. Opening hinged doors or covers to or removal of bolted covers (to expose bare, energized electrical conductors and circuit parts).

2. Application of temporary protective grounding equipment, after voltage test.

Depending on the transformer's incident energy a 40 cal, 65 cal, or even greater PPE would not be sufficient.
Some would say go downstream of this transformer and check on the load side of an overcurrent protection device and then return to the transformer to install the personal protective grounds. Even then you still can't be sure that the conductors are de-energized.

Ultimately the question is how can you work safely on the secondary side of a transformer to verify the absence of voltage when the arc flash boundary could possibly be 10'+?

Any thoughts/insights and experience on this would be appreciated. Thanks.

Use a tic tracer mounted on a four foot insulated hot stick. That will increase your distance to about 6 feet. Use 40 cal suit. Have done the calcs but this is our practice.

Hello,
How do you disconnect the primary of the padmount? We have put our mains on air switches, therefore we use a tic tracer to verify primary voltage and after primary disconnection, verify the absence of voltage on the conduits coming from the air switch to the transformer, i.e. no exposed conductors. If we have to disconnect at the transformer to keep the other systems live, we only have a 100 CAL suit and use a shotgun to disconnect LBOR switch in transformer and verify the secondary voltage at the system downstream. Yes, you cannot be sure the conductors are "dead" at transformer until you secondary test before installing grounds, but I think your downstream "verification" is sufficient if you have verified the presence of voltage and the same absence of voltage at the same location. Unless you have a secondary voltage source as with a generator, your verification should be fine on a downstream verification point.

We handle this a different way. 1st, all of our padmount transformers have exterior hinged doors with no interior covers. When you open the right side door, you are looking at the LV spade type bushings. When we perform our Activity Hazard Analysis, we recognize that under normal circumstances, this act of opening a hinged enclosure presents no hazard.

So, we are able to to open these doors without any PPE to make visual inspections. Our primary side (left side door) is dead front because we use those molded rubber T bodies for terminations. To do LOTO at this point, we would put on our PPE for the primary side which is much less than that of the secondary side (usually less than 4 cal/cm2).

We would open the switches on the primary side, note that the transformer is no longer "humming" and then use the tick tracer to verify no voltage on the secondary side. Then, we would use our regular voltmeter to verify the secondary is truly secured. After that, we verify the voltmeter is still working (portable test source) then remove PPE and proceed. But again, the first step is to open the hinged door and no ppe is required to open a hinged door. A bolted cover is different. Also, if we suspect there is a problem behind the hinged door, we have a whole different procedure for that which includes going upstream and securing power ahead of the transformer.

A couple of thoughts on this:

1. A padmount transformer is utility like equipment and most likely will be owned by the utility. If customer owned, it is still considered utility by OSHA and would fall under OSHA 1910.269 rules.

2. What gap spacing was used to calculate the incident energy level? Typically the gap for the secondary spades in a padmount are outside the IEEE 1584 values.

3. Based on 1 & 2 above, the proper place to look for PPE for a padmount transformer would be the NESC (National Electric Safety Code). The table in this document, has a 4 cal/cm^2 clothing system for padmounts up to 600V. This is based on actual testing done for arc flash in a padmount transformer.

Therefore, a 4 cal/cm^2 clothing system would be the appropriate system to use and is used by utilities nationwide.

That is why NFPA 70E 130.7(C)(1) Informational note was added for the 2021 edition.
It was added for this common situation.

Where the estimated incident energy exposure is great than the arc rating of commercially available arc rated PPE, then for the purpose of testing for absence of voltage, the following examples of risk reduction methods could be used to reduce the likelihood of occupancy of an arcing event or the severity or exposure.

Then 4 methods are listed such as non-contact proximate voltage test instrument. Basically, this amounts to a preliminary check to pre-verify before getting up close for the contact test.

Thanks for the input. How do you comply with the below section of NFPA 70E with just a "tic tracer"?

NFPA 70E 120.5(7):
"Use an adequately rated portable test instrument to test each phase conductor or circuit part to verify it is de-energized. Test each phase conductor or circuit part both phase-to-phase and phase-to-ground...."

Thanks for the response. Please see my questions in red below.



A couple of thoughts on this:

1. A padmount transformer is utility like equipment and most likely will be owned by the utility. If customer owned, it is still considered utility by OSHA and would fall under OSHA 1910.269 rules.

- So you would say don't follow the rules of NFPA 70E?
- When should we start and stop using NFPA 70E?

2. What gap spacing was used to calculate the incident energy level? Typically the gap for the secondary spades in a padmount are outside the IEEE 1584 values.

- I am talking generally about pad-mount transformers so I can't speak to specific calculations.
- I am not very experienced in IEEE 1584. Are you saying there are not accurate values of incident energy for the secondary side of a padmount
transformer?

3. Based on 1 & 2 above, the proper place to look for PPE for a padmount transformer would be the NESC (National Electric Safety Code). The table in this document, has a 4 cal/cm^2 clothing system for padmounts up to 600V. This is based on actual testing done for arc flash in a padmount transformer.

- I have read the paper "Arc Flash Testing of Typical 480v Utility Equipment" by Eblen and Short, which I believe is where the NESC is getting the 4
cal/cm^2 rating.
- With this in mind why do most padmount transformers typically have "Dangerous" arc flash labels? Is this not correct?

Therefore, a 4 cal/cm^2 clothing system would be the appropriate system to use and is used by utilities nationwide.[/quote

Thanks for the response. See my questions below in red.

The scope of NFPA 70E Scope exclude installations under the exclusive control of an electric utility. In the NFPA 70E Handbook, there is a note for an OSHA Connection that states OSHA note to 29 CFR 1910.269(a)(1)(i)(A) considers equivalent generation, transmission, and distribution installations of industrial establishments that follow OSHA's rules for utilities to effectively be utility companies.

So I would say that the line of demarcation would typically what it would be if the utility owned the padmount. In my experience with utility owned padmounts, the customer would provide the secondary cable and terminations but the utility would make the connection so they work under utility rules such as OSHA and NESC.



I am saying that the typical spacing used in IEEE 1584 for 480V is much smaller than the actual spacing in a padmount transformer. So the values will be conservative. This was a concern for utilities and there was actual testing done to determine an incident energy value.



Are these "Dangerous" labels that you are referring to stock, generic labels or do they have specific incident energy levels on them? If stock, it would be the same as the High Voltage labels one sees on 480V equipment where technically it is not high voltage but a good way to scare unqualified people away from messing with the equipment.
If the label is the result of an actual study, I would say that the wrong gap spacing, enclosure size, perhaps even infinite bus bolted vs actual available fault current used, 2 sec clearing time, etc. The study would have to be reviewed to determine why that specific value was used. Additionally, since the padmount would be under OSHA 1910.269, no label should be applied as OSHA is not in favor of labeled utility transmission, substation and distribution equipment.

The danger signs I'm familiar with are those required by NESC. It requires us to place a safety sign where it is visible once the first barrier is opened. We are then referred to the ANSI Z535 series. The danger sign is appropriate because the next act exposes live parts in excess of 600 V. A warning sign would be appropriate on the exterior. This is not about IE or PPE, it is about the shock hazard and unauthorized access.