I'm getting some machines in that are plug and play.. they have 480Volt twist lock plugs. The concept is to allow one machine to be unplugged and a different machine plugged in its place. Here is my question.. is it safe to unplug the units with 480 Volt at the plug? Seems to be a no brainer to me that an arc flash potential exist. Am I over cautious?
480 Volt twist lock plug
- Thread starter cornbread
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luckylerado
Senior Member
- Location
- Lake of the Ozarks
I think so if you are using the appropriate size and configuration of cord and plug and also not plugging and unplugging while the equipment is on.
kwired
Electron manager
- Location
- NE Nebraska
There is arc flash risk, how much depends on several factors and won't necessarily the same at every receptacle.
A 20 amp receptacle should have less incident energy available at the receptacle then there is at the service simply because the resistance of the branch circuit conductors will have a limiting effect, the longer those conductors the more limiting effect there is. There can still be a pretty significant incident energy available though, it just will be less then upstream at the feeder or service level.
Is still a good practice to not plug items in under load.
Some receptacle/ plug designs may offer better protection then others as well.
Some places you do see a disconnect ahead of the receptacle and the proper procedure is supposed to be to turn the disconnect off before inserting or removing the plug.
A 20 amp receptacle should have less incident energy available at the receptacle then there is at the service simply because the resistance of the branch circuit conductors will have a limiting effect, the longer those conductors the more limiting effect there is. There can still be a pretty significant incident energy available though, it just will be less then upstream at the feeder or service level.
Is still a good practice to not plug items in under load.
Some receptacle/ plug designs may offer better protection then others as well.
Some places you do see a disconnect ahead of the receptacle and the proper procedure is supposed to be to turn the disconnect off before inserting or removing the plug.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Agreed, most plugs aren't rated for "load-break" but are OK to disconnect while energized. Put up some "Turn off equipment before unplugging" signs.
What's the current involved?
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
It is hard to give you a good answer to this question.
I will say this. I was in a plant a few years ago that had many dozens of machines hooked up this way. Each of the receptacles was labeled with the IE. None that I saw even made it into single digits. That suggests to me the risk associated with unplugging under power on these receptacles is quite low. But to reduce that risk even more it would be best to turn off the power to the receptacle first.
I will say this. I was in a plant a few years ago that had many dozens of machines hooked up this way. Each of the receptacles was labeled with the IE. None that I saw even made it into single digits. That suggests to me the risk associated with unplugging under power on these receptacles is quite low. But to reduce that risk even more it would be best to turn off the power to the receptacle first.
kwired
Electron manager
- Location
- NE Nebraska
I have greater concerns over what happens when plugging a cord in that happens to have a fault in the equipment then I am about unplugging one while it is loaded.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A valid concern. In which case you should probably turn off the breaker.
I believe that plugs and receptacles that are listed with an HP value in addition to current are in fact rated for load making and load breaking.
If just amps and "tungsten" rated, you can make and break compliant lighting loads but not motors.
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kwired
Electron manager
- Location
- NE Nebraska
What about the millions of 5-15 and 5-20 receptacles that get motor loads plugged into them? I've never seen one that is HP rated that I can recall.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
They are HP rated. It is part of their listing.
page 467. http://www.ul.com/wp-content/uploads/2014/09/UL-White-Book.pdf
kwired
Electron manager
- Location
- NE Nebraska
I trust you and did not look at your link. I'm guessing they don't need to be marked with HP rating then? This only apply to certain receptacles, in particular 5-15 and 5-20? I never see a HP rating on any other receptacle, until maybe you get into pin and sleeve type connectors.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
It lists basic standard receptacles 15A to 60A like those found in a basic NEMA chart. 5-15 and 5-20 are listed.
For other types it says:
Devices of configurations other than those indicated in the table have horsepower ratings only if such ratings are marked on the device.
kwired
Electron manager
- Location
- NE Nebraska
Now you made me look at the link and I see some common configurations that I see a lot that are not in compliance with the rating. Big one that stands out to me is NEMA 10-50. They have been used for decades on the farms here and is not uncommon at all to see them supplying 10 HP single phase motors. Bad enough that they should probably be using 6-50 before even considering HP rating.
Farmer puts up a new grain bin, he wants one to plug in his portable auger that he uses on several other sites, so what do you do, especially when there has been little or no serious mishaps with those over all those years?
It gets even more complicated when I believe the 2017 NEC will require GFCI protection on such receptacle outlets.
Install something that is NEC compliant and you know they will just make a cheater adapter anyway because they are going to still use it at existing sites.
:blink:
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Here's a much smaller download:
http://productspec.ul.com/document.php?id=RTRT.GuideInfo
Cheers, Wayne
- Location
- Illinois
- Occupation
- retired electrician
The plant safety people around here often require an interlocked disconnect or a Meltric switch rated plug and receptacle for 480 cord and plug connected equipment.
Bigrig
Member
- Location
- Dayton, OH
Combination device?
Combination device?
Depending upon the required current rating you can get twist-lock receptacles with integral disconnects built right in (ex Hubbell SE2730A).
Combination device?
Depending upon the required current rating you can get twist-lock receptacles with integral disconnects built right in (ex Hubbell SE2730A).
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