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 Post subject: Servicing UL Listed Appliances
PostPosted: Tue Jun 19, 2012 5:18 pm 
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Joined: Tue Jun 19, 2012 4:05 pm
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You folks are way ahead of me and I am struggling to catch up. Please excuse me while I think out loud.
A fellow asked some similar questions here before but I don't think he really got a good answer --
(thread 1084) and I can't post it.

I install and repair UL listed electric pottery kilns for a living. It's my little niche and I work for myself -- it's part-time, but growing each year. I need a way to test these appliances under load and they can't have more than a 3% voltage drop.

I work at maybe 200 different sites a year. Residential, Commercial and Schools -- no Industrial. Some of these sites have conducted arc flash studies and as evidence of that, I see stickers, which I assume are for my benefit, telling me about high voltage and arc flash hazards, but that is not where I spend most of my time.

Where I spend my time is on a branch circuit, 208-240vac no more than 80 amps, sometimes 3 and sometimes single phase. There are no stickers there on my 60 amp switch box. I don't know how to do a study, so I don't know why. I assume it is because there is little to no arc flash danger, but if I go to a similar site where no study has been done, the table leaves me zero leeway and it appears that I would have to suit up in an FR coverall and wear a shield before I put my probes in the the switch box.

** My first question is if an arc flash study has been done and there is no sticker on the box I want to open, what exactly does that mean to me?

** Keeping within the spirit of 70E of attempting to work on de-enegized equipment and circuits, my second question is if if were to use a knife switch to de-energize the appliance (making an air gap) then clipped my probes on inside the appliance using insulated alligator clips, then energizing and testing the voltage while standing aside, would that meet the arc flash requirements of 70E? Then I would de-energize, move the clips, step aside and energize again while watching the meter. Would this excuse me from the face shield and FR clothing that 70E requires for testing a 240vac and below circuit? How would I meet the "Live-Dead-Live" safety requirement if I did that?

My goal is to keep within 70E recommendations, but do so at the least expense and bother. I realize that I am very small potatoes, but I have engineers to satisfy on some jobs, who may not be as familiar with 70E as you folks are and are unaware of the low voltage 50 - 240 volt testing controversy.


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PostPosted: Thu Jun 21, 2012 4:02 pm 
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Eric Vey wrote:
** My first question is if an arc flash study has been done and there is no sticker on the box I want to open, what exactly does that mean to me?


Depends. Their analysis will determine what that means. In one operation, no sticker meant that it is less than 1.2 cal/cm^2...as long as you are wearing non-melting clothing (cotton, wool, etc.), you will survive it if something goes badly. Others have you follow whatever the nearest upstream label says, but there are issues with this approach as well in that the further you get from the transformer, the longer it takes for a circuit breaker or fuse to trip, which actually usually raises the arc flash energy that is released.

Quote:
** Keeping within the spirit of 70E of attempting to work on de-enegized equipment and circuits, my second question is if if were to use a knife switch to de-energize the appliance (making an air gap) then clipped my probes on inside the appliance using insulated alligator clips, then energizing and testing the voltage while standing aside, would that meet the arc flash requirements of 70E? Then I would de-energize, move the clips, step aside and energize again while watching the meter. Would this excuse me from the face shield and FR clothing that 70E requires for testing a 240vac and below circuit? How would I meet the "Live-Dead-Live" safety requirement if I did that?


Not exactly. As you know, you assume everything is energized until tested dead. So until you've completed the test and proven the circuit dead including disabling and locking it out, it is still treated as potentially energized. Second comes the question of the possibility of an arcing fault. During the act of switching off the power, there is again a chance that an arcing fault could happen. 70E is pretty clear that they rate this as "H/RC 0" in the tables for the case you are describing for the switching activity only. Local engineering analysis may say otherwise. Then we get to the issue of the task of testing for voltage. If you are using insulated tools such as meter probes with a meter rated for the condition (the Category rating on the meter meets the transient overvoltage potential) then you have to look at your probes. If the amount of exposed metal is long enough that it can in any way bridge across phases, then there is an arc flash hazard so suit up. Otherwise, you have to consider any way possible to create an arcing fault. Some folks knee jerk this but obviously with 120/240 V equipment this is nearly impossible to do with insulated probe tips with only the barest amount uncovered.

This is where 70E really falls down at. In IEEE 516 which is what 70E refers to for shock protection among other things, they describe 4 different work methods: live line, bare hands (bird on a wire scenario with high voltage overhead lines), using insulated tools, using insulated gloves, and working with de-energized equipment. 70E glosses over the insulated tool method or barely mentions it even though it is hands down the best way outside of de-energized work to handle shock hazards because you are literally at arms length away from the hazard.

Quote:
My goal is to keep within 70E recommendations, but do so at the least expense and bother. I realize that I am very small potatoes, but I have engineers to satisfy on some jobs, who may not be as familiar with 70E as you folks are and are unaware of the low voltage 50 - 240 volt testing controversy.


There's controversy, but not the type you might be aware of. Right now IEEE 1584 which is the standard most engineers use for calculating arc flash hazards says that on 120/240 V systems, the hazard is likely to be a non-issue. 70E used to say that a calculation is not necessary but did not come right out and say that there is no appreciable hazard, leaving those that didn't actually read IEEE 1584 hanging. In the mean time, more testing and experiments have been done and some are now suggesting that contrary to popular belief, you can get an arc flash of a significant size below 240 VAC. So the end result is that in the very near future, things are probably going to get even more muddy.


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