NFPA 70E Clarification for UPS work 2

[raithrovers1]

I am trying to come up with a definitive answer to what PPE is required for my technicians to use when taking voltage measurements on a 480VAC input UPS less than 100KVA with a nominal 120VDC buss. Looking at the NFPA 70E tables it states that that panelboards or other electrical equipment rated >240Vac and up to 600VAc require arc flash category 2 PPE. There is then a note at the bottom of the table that says "for equipment rated 600VAC or less, and protected by upstream current limiting fuses or current limiting circuit breakers sized at 200A or less, the arc flash PPE category can be reduced by 1".
My conclusion is that my technicians will only need level 1 PPE when taking measurements inside a 480Vin, 120VDC buss 100KVA or less UPS. My only concern is what constitutes a current limiting circuit breaker. This seems to be a UL classification but I can't find a definitive answer.
Any help as always will be appreciated.

UPS Service Manager

http://www.nolanpower.com

 

[dpc]

You need to read the limitations stated in NFPA 70E for use of the tables. The tables can be used only up to a maximum level of fault current and are based on a maximum assumed fault clearing time.

The size of the UPS is not relevant since the potential arcing current is determined by the source.

Current-limiting circuit breakers are NOT common. The vast majority are non-current limting (per the UL definition of current-limiting). So if the upstream device is a molded-case circuit breaker, you must assume it is non-current limiting unless you are certain it is current-limiting.

If you are going to use the tables, I'd stay on the conservative side unless you are extremely familiar with the electrical system involved and have the necessary information to reduce the PPE requirements. The differences between PPE Category #1 and #2 are not that great, except for the addition of the balaclava.

 

[jraef]

So you have a UPS that supplies 84VAC on the output? I say that because you've said, several times, that you have a 120VDC bus, which would only support an AC output of roughly 84VAC. If by that you meant it is a 120VAC output, that would mean the DC bus is roughly 150VDC. But I'd seriously doubt that as well because if it is 100kVA, that would imply a UPS capable of 833A output, not something to trifle with.

So for clarity, I would be more inclined to believe that you actually have a 3 phase UPS that puts out 208Y120V, where you have a Line to Neutral voltage of 120VAC, but the Line-to-Line voltage will be 208VAC. In that case, your bus will be roughly 300VDC, assuming a 208VAC input and if the input is 230VAC, the bus will be 330VDC. The point of this is that 300VDC capable of 100kVA is still a LOT of potential "incident energy" from an Arc Flash standpoint. The PPE that is required by your workers when exposed to this energy is based on the actual energy potential as calculated by a thorough study, not on an arbitrary number that someone assumes, in this case incorrectly. It is the lives and health of people involved here. You need to hire a qualified electrical engineer to perform a Hazard Risk Assessment for your equipment and then from that, determine the proper PPE that your technicians need to use.

One other thing: Technically, if your end customer has higher PPE requirements when your technicians are at their facility, your people MUST follow those requirements, not yours. Facility owners are not "off the hook" for their own Safety Procedure Requirements simply by hiring or assigning a task to someone else.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[raithrovers1]

Thanks for the replies.
DPC: Thanks you for clarifying the current limiting breaker. The reason for the this is that our company does not have a minimum guideline at this time and rely on the customers requiremnts. We completely understand that all of our customers have more stringent guidelines and they will trump ours. This is more for when they are working at the warehouse or training. We do not want to go overly excessive as some of our customers require level 3 PPE for working in UPS systems. It would also be interesting to know if the protection of the UPS input breaker can be classed as upstream protection as all of our work will be downstream of this.
Jraef: I was being a bit too generalist in my description. The UPS's we work on are industrial 3 phase 480VAac input, 120V nominal DC buss (sometimes 240VDC if required) with either a 120Vac single phase output or a 208/120Vac 3 phase output. I said up to 100KVA but I meant anything under the protection of a 200A breaker on the input. The majority of systems we work on are 50KVA or less. Has anyone heard of an arc flash study on a UPS? because we have been to hundreds of facilities where the UPS does not have an arc flash sticker on it.
Here is an example:
A 15KVA UPS with a 480VAc input (30.5Aac), 120VDC nominal Buss (114.9ADC) and a 120Vac out/bypass (125Aac). The batteries are Enersys 3CC-5M (60 cells) which have an short circuit rating of ~1kA.
All three supplies are protected by breakers on the UPS itself (and of course the upstream protection). In this instance it would seem that level 1 PPE is adequate. As I typed this I realize that the energy from the charger is not calculated in this example. Does the DC available from the charger have to be added to the battery short circuit current? This charger is 100Adc so it would still not move it up to the level 2 PPE for this example.
I agree that Level 2 PPE would be the best minimum to have as this really only adds the balaclava. I just need to be able to explain all of this to our techs so they understand why it was chosen. This is part of the OSHA requirement that employees are trained correctly.


UPS Service Manager

http://www.nolanpower.com

 

[jraef]

If your input is 480V, unless there is a transformer inside, the DC bus will be around 650VDC. The DC bus is always at the PEAK voltage of the system,so when you say "480VAC" or "120VAC" that is the RMS (root mean squared) voltage, a form of averaging over time. The peak voltage is actually around 141% higher, and that is the DC bus voltage (for the most part) of anything rectified from AC to DC, such as in a UPS. What you do with the DC to make it back into AC is the magic part of it, but regardless of the value you end up with, the DC bus is always the AC x 1.41 (or divided by .707).

NFPA70E is not a "law", it is a system of guidelines for establishing a safe electrical workplace, which IS mandated by Federal law and enforced by OSHA. What OSHA actually says is that YOU WILL have an Electrical Safety Program, you will review it periodically, and your employees will be qualified on it and periodically re-qualified on it. OSHA is light on specifics, but highly suggests that your program should look some thing like what is described in NFPA70E. Looked at another way, and in the light of how OSHA works, IF there is an electrical accident involving employee injury or death and there is an investigation by OSHA, the first question will be "What is your electrical safety program?". If your answer is "What?" or "huh?" or anything like that, it gets really ugly really fast, including, but not limited to, the filing of CRIMINAL charges for all managers and supervisors involved if they find gross negligence. If on the other hand your answer is "We follow a program that is based on NFPA70E", then the rest of the conversation is going to be about details and execution.

So does NFPA70E specifically address working on UPS systems? No, that's not what it is about. It is about working SAFELY on live equipment, ANY live equipment. Part of that practice of working safely is to establish what that means for any given task. SOME common tasks are laid out for you in the NFPA70E documents to help streamline the process, but just because it is not specifically mentioned does nothing toward relieving you of responsibility for creating a safe workplace. Your responsibility as an employer includes the process of IDENTIFYING the risks associated with a task, and there are very specific processes for identifying those risks as it pertains to Arc Flash energy (which by the way is not the ONLY risk). Some of those pre-defined tasks have tables (as mentioned earlier) that assign risks to them. If your tasks do not fit those, you must do a Hazard Risk Assessment to determine them and the procedures and PPE associated with them.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[raithrovers1]

jraef: Thanks again for another great reply. I have been in the industrial UPS market for so long that I take for granted that everyone knows the distinction between a true industrial UPS and a commercial UPS. The main difference is that an industrial UPS will ALWAYS have an isolation transformer on the input (and output). This in turn usually steps the secondary down to create a lower voltage DC bus to allow easy maintenance on the battery. In the US the industrial standard used to be a 120VDC nominal battery. Unfortunately, commercial UPS systems started creeping into the industrial market with no input isolation and higher DC buss systems and batteries. When these UPS's are replaced when the fail prematurely in industrial environments, the customer usually wants to keep the battery and therefore there is now a demand for higher DC buss voltages on industrial UPS systems. In truth, engineers could split a 480VDC nominal battery into 4 strings in parallel to accommodate the lower and safer DC buss voltage and retain the autonomy.