For those that own PPE for doing energized work.
Do you own just one set of PPE, is it the maximum Level available for the sites you are working on?
Do you fully dress, balaclava, arc flash gloves, or do you just put on the a PPE coat that you have no matter what the label calls for.

 

I have PPE that gets me up to an incident energy (IE) level around 60 calories. If I'm going into an application where the IE level is higher then I will vary the working distance from the energized bus to provide a reduction. If the 480V switchboard has a label of 60 calories, that's for a working distance of 18 inches. If you stay 36" away from the bus, you automatically reduce the IE exposure to 1/3 of the original, or in this case 20 calories. The arc flash label references the bus at the face of the gear, and when you open it up you can see the location of the bus and use that as the basis for the IE calculation. LV Switchboards use a working distance of 18 inches and LV Switchgear uses a working distance of 24" since the more expensive switchgear construction has a deeper bus.

I do fully dress for the exposure, even though it's extremely cumbersome. And if I'm just opening a panel I don't think there's much risk, but I still try to wear everything.

 

I was reading on an engineering web page and they have a Arc Flash Calculator, in the calculator they have a space for entering "ARCHING TIME". Did this site make a booboo or is ARCHING time an actual term. I googled ARCHING TIME and nothing came back for electrical.

 

Booboo Brian.
There's so much misinformation out there, and anyone can buy a Power Systems analysis package, enter the data, and press the button for arc flash calcs, or for equipment short-circuit evaluation.
In fact the thing I see that is most flawed in Power Studies is where equipment fails the equipment evaluation exercise because the button was pressed and it said it failed. It's incumbent on the practitioner to exhaust every possible option for passing the equipment, which really comes down to NEC 240.86, series ratings.
Another thing I like to say that is in days of old, when a Power Study consisted of Fault and Coordination, if your settings are implemented and there's a trip of a device and you're able to make adjustments that make things better, then you screwed up with your original settings. But nowadays folks are adjusting breakers for arc flash reduction so now there's a whole lot of subjectivity to the adjustments. We did a large project for Harvard and their requirement was that 8 calories was the maximum incident energy (IE) level they would allow. That's just not possible in some cases if you have a coordinated system, but with enough money there are ways to get close. In that case there was a section of the gear that was isolated and made inaccessible because the IE level was higher than 8 calories. ZSI, maintenance settings and bus diff are also critical for IE reduction. But of course bus diff is prevalent on systems above 1000 Volts because it's much more practical due to CT sizes.

Keep the questions coming

 

For those that do Arc Flash Studies,

I see labels on switchboards stating PPE Level lets say 40cal/cm2 on the Utility termination/CT Cabinet, the down stream of the Main they have the same rating.

Could it be possible downstream of the main CB/Fused Switch the level could be lower?
Is this because they only do a single set of calculations and use it for the complete switchboard.

Not knowing all the details of the application, I'd say that equipment downstream of the circuit breaker would benefit from the characteristic curve device of the breaker, but only if it's in another cabinet. But I'll also state that just about every Study I look at is flawed in some way. The worst are the ones that don't take into account the line side of a main breaker and report the load side incident energy level, which is reduced by the breaker's trip unit.
There's an expectation you develop that is based on the size of the transformer feeding the switchboard/switchgear you're in. A 2500 kVA, 480V transformer likely has a secondary IE rating of 130 calories. IEEE 1584-2018 has replaced the old 125 kVA limit for arc flash calcs with 2000 amps of bolted fault current. That is to say that if there's less than 2000 amps, the incident energy level is less than 1.2 calories. That translates roughly to a 30 kVA transformer at 480 Volts.
Another possible response to your ? : Could it be possible that downstream could be a lower PPE? Sure, depending on the distance and the response of the upstream protective device. It could also be higher due to a slower response by the ocpd given the additional resistance in the feeder.

 

Could it be possible that downstream could be a lower PPE? Sure, depending on the distance and the response of the upstream protective device. It could also be higher due to a slower response by the ocpd given the additional resistance in the feeder.

I read that some time ago and while I never really thought about it, it does make sense,. Another item I was reading about is the secondary of transformers could be higher, due to I assume to the larger OCPD?

I was involved with a project and to lower IE level it was suggested to install a transformer Delta/Wye 480 to 480/277 with impedance between 4% and 5.5%

 

{quote]Could it be possible that downstream could be a lower PPE? Sure, depending on the distance and the response of the upstream protective device. It could also be higher due to a slower response by the ocpd given the additional resistance in the feeder.

I read that some time ago and while I never really thought about it, it does make sense,. Another item I was reading about is the secondary of transformers could be higher, due to I assume to the larger OCPD?

I was involved with a project and to lower IE level it was suggested to install a transformer Delta/Wye 480 to 480/277 with impedance between 4% and 5.5%
[/QUOTE]
I'd really have to look at the application where a transformer would have reduced the IE level, unless of course it was smaller than the source transformer. Then it makes sense.

One thing about arc flash levels that is not necessarily intuitive is that at comparable power levels, low voltage IE levels will be much higher. Looking at the secondaries of two 3000 kVA transformers, one at 4160 volts and the other at 480 volts. Using the 2-second timeout for calculating the IE level, the 4160V secondary will have an IE level of around 4 calories, and the 480V secondary will be at around 166 calories. There are a couple things at work there, one of which is that the arcing fault for systems above 1000 Volts is around 95% of the bolted fault current level, so the ocpd trips quick. Whereas for low voltage systems <1000V, the arcing fault current is around 55% of the bolted fault current, so the ocpd trips slower thereby increasing the IE level. MV Systems also benefit from an arc flash standpoint because the working distance is 36" versus 18" for most LV applications, except switchgear which is 24".

The elephant in the room though is electric shock. The prevalence of electrical injuries is from shock and not arc flash, and shock protection is much more critical for higher voltage systems. Arc flash incidents are rare, and account for maybe 2-3% of electrical injuries. But I think the horrific nature of arc flash injuries has resulted in a knee-jerk response by NFPA 70E that says energized systems can't be worked on unless it's more dangerous to turn the circuit off, like for an operating room, or a hazardous waste pump motor.

But 70E is getting better. Recall that it was just two code cycles ago where you couldn't operate a breaker without suiting up to the IE level, or getting out of the arc flash boundary. That led to a prevalence of chicken switches and remote racking systems. With today's 70E, you can operate the breaker control switch in street clothes as long as the equipment is in Normal condition, i.e maintained, doors closed, normal load, no impending failure...

I tell you Brian the thing that irks me most is seeing breakers in critical applications that still have their factory default settings, 1.0 LTPU and everything else at minimum. I was in a 12 year-old government data center with all factory defaults on the breakers and no selectivity between 3200-amp mains and all of the distribution breakers. The maintenance manager told me they had just spent 100k on maintaining the breakers (PM he called it), when in actuality all they had done was performed IR scans.

It gives indication that there's lots of opportunity for electrical testing outfits.

 

I'd expect some of the trade journals would have some information written by some pretty proficient folks.

@paulengr would be a good source. I believe he's substation experienced?

It's just not something I really got into a lot. My employees did but it was never necessary for me to intervene. The public looks to our industry to keep them (and us) safe. Sounds like a great topic for ET Newsletter.

 

In the past two years I have renovated services and distribution gear for 12 or so privately owned, municipal water treatment plants. The worldwide parent company had contracted with Square D to perform the arc flash and circuit breaker coordination studies at all of the plants. Quite a few had equipment that exceeded the 40 cal. limits required NFPA 70E. A few of the larger services (800a to 2000a) had to have the gear that was only three to four years old replaced. Back to downstream equipment. If one piece of gear (the main) doesn't have a complete isolating steel barrier from any of the downstream equipment, the same arc flash rating progresses throughout the connected equipment. On a couple of the sites, we had to move the main breaker section (2000a) few inches away from the transfer switch and MCC line-up which was all bussed together through open channels in the factory gear. We removed the factory bussing and Square D supplied the two new end plates that we had to install in the new gap that we made. The Square D engineers wouldn't accept installing nipples in the new 2" gap. Not enough gap to suppress an arc flash. We had to install back to back sweeps with 2' nipples up towards the ceiling and back down.This created 5 to 6 foot nipples. According to Square D this reduced the available arc flash in the transfer switch and MCC sections to acceptable levels. Another site required a new 1200a main breaker be installed outside of the existing service building because of area constraints. The new main breaker contained a maintenance selector switch and processor the let you switch the breaker time setting down to minimum temporarily from outside of the equipment.This would reduce the arc flash while working in any of the downsteram equipment. I never took pics of these. If I get a chance in the near future I'll try to take pictures of a couple of these and attach.

 

Why was 36" picked as the working distance for determine the IE, why not closer that is more realistic?

As for Arc Flash compared to Shock. Sadly I have experienced both to my body. The burns were painful but I knew I was going to be OK, the last shock I received shook me to my core and affected my confidence for some time.

 

There was an episode of the NFPA Podcast ("The Mysteries of Electrical Injuries, Published February 8, 2021) and a kind of PTSD after an electrical shock is, apparently, a very common side effect. A psychologist specializing in this said some guys have panic reactions to changing light bulbs afterward.

I hope you have recovered and glad you survived!

I had a pretty bad arc flash/blast last year that, only because of dumb luck, left me with a "suntan" on one side of my face but otherwise unharmed. It took me a while to mentally get past that incident.

 

For those that do Arc Flash Studies,

I see labels on switchboards stating PPE Level lets say 40cal/cm2 on the Utility termination/CT Cabinet, the down stream of the Main they have the same rating.

Could it be possible downstream of the main CB/Fused Switch the level could be lower?
Is this because they only do a single set of calculations and use it for the complete switchboard.

On a switchboard in many cases it jumps the breaker. Going forward main lug only is preferred.

 

I was told flash boundary is the distance the maximum calculated flash event by a phase to phase fault in the specific measured equipment could produce that would give you 2nd degree burns with no PPE. Now if you don’t have all the covers secured in place on said equipment then it’s further.


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For those that do Arc Flash Studies,

I see labels on switchboards stating PPE Level lets say 40cal/cm2 on the Utility termination/CT Cabinet, the down stream of the Main they have the same rating.

Could it be possible downstream of the main CB/Fused Switch the level could be lower?
Is this because they only do a single set of calculations and use it for the complete switchboard.

I worked at a mine and had a 40 cal suit. Most all of our medium voltage disconnects were 40 cal. and most of the equipment in the shovels were 40 cal if energized. We used 10 gauge steel boxes with a vacuum breaker in them for turning the power on and off. These could be 1/2 mile away from the shovel. I started modifying the boxes and put 30 foot cords on them so you could get out of the area where the doors could come off. These boxes are 10 feet long x 4 feet wide and about 4 feet high. Double doors on both sides. Ends were solid. Initially the remote cord idea was rejected because like most miners they did not think of it. The only people that supported me were HR the safety department could see nothing wrong with bending over and touching a switch on the box to open and close the breaker.

I wore my FR shirt, pants and underwear inside the 40 Cal suit. Having seen a couple of boxes grenade I never argued about wearing the suit. Even in 100 degree weather. The mine furnished our pants and shirts so all I had to wash was the underwear. Needless to say you do not wash FR with everything else. I used to air dry my Carhartt long sleeved T shirts and under butt wear.
Emerson used to have an arc fault video all about clothing. It is not something that I ever wanted to be involved in. Learning to hold your breath and turning your head became second nature. I flatly refused to open any of the cabinets on a shovel energized, all 40 cal. Most of the handles were metal and about 3 feet long, and in the middle of a door that opens. Really hard to get out of the blast area. S&C put the handles on the side so you could get out of direct fire. Yes I have seen a S&C cabinet bulge like an M80 in a trash can. Never saw any less values and never saw any higher so I suspected they just bought a bunch of tags and put them on. After all they were miles away when the magic was released.

There was not a single electrical panel that did not have a arc flash sticker. I once got a work order on nights to re identify the circuit breakers. Took an average of 20 minutes to fill out the LOTO form and get started. Most of the big stuff over 50 amps was correct, the 20 amp breakers seemed to change every couple of years. Gotta love the mechanics and welders that were there. Being alone it was impossible find anything with a meter. So I would take the cover off and turn the panel back on and trace the circuits one at a time, 42 each panel. If operations was quiet I could get one panel a 12 hour shift. We labeled the receipts as well.


The only idiot thing that they did was require a 12 gauge extension cord with 15 amp plugs on the end. They called it a 20 amp cord. After 4 years of trying to get them to see the cord was really 15 amp I surrendered and let them have their identification. I never worked with management that had their heads where the sun does not shine like the mine. As long as we were loading trucks they really did not care what we did.

 

I cant even find my safety glasses. But it was interesting to note some higher possible fault energy with lower voltage.