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 Post subject: 480v risks
PostPosted: Thu Jan 16, 2014 9:43 pm 

Joined: Thu Jan 16, 2014 9:30 pm
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I have a 1-man shop with a 150A 480V 3PH electrical service. Most of my equipment is connected via twist-lock plugs into 20A and 30A receptacles. Two questions:

1. I've never had any arc events when plugging or unplugging unloaded equipment into/from energized outlets. Is data available that quantifies the risk of arc flash with unloaded connection on these low current connections?

2. I've seen suggestions that 480V receptacles in metal boxes be wrapped with electrical tape to avoid arcing between the terminal screws and the grounded box, but I've also been told that it is just a waste of tape. Is there a real danger and is electrical tape sufficient to mitigate that danger?

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PostPosted: Mon Jan 20, 2014 7:10 am 
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Joined: Fri Dec 06, 2013 9:30 am
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Location: Pittsburgh, PA
For properly installed receptacles, the gap between the terminals and the metal box will be adequate to prevent arcing, unless you have a large voltage surge for some reason. Adding tape isn't really necessary, but it also wouldn't hurt. If you are removing the plugs with the equipment turned off, especially induction motors that are not still spinning, the chances of an arc occuring are pretty slim. I don't have any statistics to back this up; this comment is only based on my 15 years of professional experience and arc flash incident reports from MSHA and OSHA that I have read. If you are interested in a higher degree of safety, Meltric makes receptacles that are meant to reduce the possibility of arcing from plugs being removed under load. These receptacles have internal contacts that break before the plug is removed. But if you are unplugging equipment that is not loaded, you shouldn't need to upgrade to these more costly receptacles.

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PostPosted: Mon Jan 20, 2014 10:33 am 
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1. If you have to wrap the plug with electrical tape then you need to replace the defective plug. This should not be necessary. What is necessary is to "routinely" inspect the plug and cord every time before ou use them. A quick glance is all that is needed. If something is out of place, frayed, broken, bent, etc., replace it. Your life is not worth the money saved. While you are at it, make it a point to operate all the breakers in your panel once a year. This drastically increases the odds that if something happens that they will work (redistributes the lubrication). Breakers this size rarely do fail but this simple step drastically reduces the odds of failure by eliminating one of the major causes of breaker failures.
2. The major danger as mentioned with plugs is if you have it under load. The plug is not rated as a load break device. Neither are any of the 120 V plugs on anything else you own. What happens when you unplug it under load is that it forms an arc. Since electricity can go through air at least for a short period of time but air is something of an insulator, there is a lot of heat generated. This can easily melt and vaporize plastic and metal. As the voltage goes up, this effect becomes much more drastic. How much damage is caused depends partly on whether there is an inductive load (motor) attached and how fast the plug is pulled out. You probably have heard of cases where slowly turning a light switch off causes a lot of crackling and arcing in the switch and can destroy the switch or cause a fire. It may not happen the first, second, or third time you do it but eventually the plug can and will explode. I knew an operator in a mine in Northern Kentucky who was responsible for setting up and running several large fans. He always went to the box with a disconnect on it and just pulled the plug out instead of using the handle mounted directly above the plug on the box. That was the way the man before him did it to "save time". He told me that we had to stop buying cheap plugs because this was the third time in 2 years that a plug exploded on him when he yanked it out. He had done ithe same thing for over 10 years before one sent him to the hospital.
3. If on the other hand the plug is in good shape and used properly, I have not seen cases of arc flashes from this. Lots of plants use 480 V welders, hoists, and fans extensively and the cords and plugs see extremely heavy use. The biggest problem is damaged cords and bent/broken pins, not arc flash. I can't recall any case of an exploding plug except for the case I mentioned of an operator who was doing just the opposite of what you described.
4. This may not be much consolation but if you are working with 20-30 A receptacles and have similar sized (20-30 A) molded case breakers, another poster had previously described finding that it is extremely rare to find a case where a life threatening injury due to arc flash is even possible at 480 V because the breaker sizes are typically small and the breakers are very fast. It would take some paperwork and time to figure out the exact rating of your system but chances are it is similar.

All that being said, let's step back for a minute and consider what is needed to make an arc flash survivable even if the plug does explode. The exact PPE I would recommend for anyone at work or at home for this situation is the same: wear "traditional" shop wear: long shirt, long pants, all leather work gloves, leather work boots/shoes, ear plugs, and safety glasses, when inserting or removing plugs. If you have a class E hard hat handy (not a cheap "bump cap" or steel), by all means use it. If nothing else, keep a long "shop coat" next to the door at the shop and put it on when doing this kind of work. Whatever you do, all of the clothing should have only natural fibers listed on the label except underwear and socks (waistbands) such as cotton, wool, silk, or leather. Denim (except "skinny jeans") almost always qualifies. Everything else these days more and more seems to be some kind of blend whether it is from those "UA" guys, or polypropylene long underwear, or "work dry" shirts, fancy boot socks, or "mechanics" gloves. So it might take some fishing in the closet on your part or a trip to the store.

If you do everything as described as far as exercising breakers and inspecting the cords and plugs every time, you've already improved the odds to where you are about 100 times more likely to be injured in a car accident. If you also wear all the clothing as I described, then the odds of survival even if the plug explodes increase another 10 times.

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PostPosted: Mon Jan 27, 2014 1:28 pm 

Joined: Mon Nov 19, 2007 5:25 am
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Location: Titusville, Fl.
Seen electrical tape being used since making up the receptacle can chaff the wiring insulation, but if proper workmanship applies upon installation this should not be an issue/requirement , instead just an extra step for good practice. If not installed properly, I seen faults occur w/n the receptacle body which somewhat captures a considerable amount of the energy released. Otherwise, what's mentioned above in terms of likelihood to occur, and the severity of the incident should be considered for this type operation....

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