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Author Topic: Today's cautionary tale  (Read 5248 times)

Don T. Williams

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Re: Today's cautionary tale
« Reply #20 on: March 03, 2018, 12:49:27 pm »

Yep, I ran into a similar situation again just now.  Two 120v circuits/receptacles meter just fine, but a third metered at 213v.  Always check the power before plugging in!

Jeff, it sounds like you have what was explained to me as "wild leg" three phase.  I'm not making that up.  In my home town, the third leg usually measures a full 240 volts.  This is in our "older" downtown area with most of the major buildings built in the 1950's.  3 phase motors run reasonably well (at least without problems-yet), but it can kill audio gear!  Be afraid. Be very afraid!

As has been stated by others, 120/208 three phase confuses a lot of generator suppliers.  I've had generator suppliers come in early in the mornings of multi-day events, and not only refuel the gen set, but "correct" my 3 phase settings by changing it to single phase, and once even to 460V.  I check BEFORE I fire it up and before I turn on the main breaker! 
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brian maddox

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Re: Today's cautionary tale
« Reply #21 on: March 03, 2018, 01:18:21 pm »

Yep, I ran into a similar situation again just now.  Two 120v circuits/receptacles meter just fine, but a third metered at 213v.  Always check the power before plugging in!

Yeah, i got this exact thing last genie i worked with.

Gotta meter ALL the things...
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Mike Sokol

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Re: Today's cautionary tale
« Reply #22 on: March 03, 2018, 03:50:54 pm »

Yeah, i got this exact thing last genie i worked with.

Gotta meter ALL the things...
We've covered this before, but it's always good to revisit a topic that's potentially so dangerous. High-Leg Delta service was often brought into industrial buildings that needed mostly 3-phase motor power that didn't even require a neutral. But you could get 120/240-volts off of the lower pair of legs on the center-tapped winding. What really grinds me is electricians hooking into the High (or Wild) Leg, and never use a meter to confirm the correct voltage.
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Jeff Lelko

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Re: Today's cautionary tale
« Reply #23 on: March 03, 2018, 04:18:30 pm »

We've covered this before, but it's always good to revisit a topic that's potentially so dangerous. High-Leg Delta service was often brought into industrial buildings that needed mostly 3-phase motor power that didn't even require a neutral. But you could get 120/240-volts off of the lower pair of legs on the center-tapped winding. What really grinds me is electricians hooking into the High (or Wild) Leg, and never use a meter to confirm the correct voltage.

This is exactly what I suspect happened.  This is a newly installed power box (the kind you find at venues for bus/RV/trailer hookup).  The two 5-20Rs are 120v as advertised, the 14-50R meters 240/120 as expected, but the TT-30R came in at 213v when it should have been 120v.  I'm very involved with this particular project and know the electrician had to install this equipment prior to the utility service being connected (hence he couldn't test this himself).  Good thing I did!  I ensured the whole box was de-energized and locked out/tagged out until the electrician could return to correct the problem.  You just can't be too careful with these things!
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Stephen Swaffer

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Re: Today's cautionary tale
« Reply #24 on: March 03, 2018, 07:29:54 pm »

The NEC requires "high legs" to be marked with orange (orange is also used in 480/277 panels).  If you ever see orange in a 208/240 panel it should raise a red flag.

The other place "high" or wild leg services are found are in places that are mainly 120 volt loads, but need or want 3 phase for AC/air handling.  The POCO can do a wild leg with 2 transformers instead of 3, saving them money (sometimes cost that is passed on to the customer at the time of install-so its not always the POCO being cheap).

One of the odd/annoying things is that POCO's don't play by the NEC-they use the NESC.  In my area, the POCO uses blue/yellow/red to mark 480-electricians use brown/orange/yellow.  The NEC requires high leg to be "B" phase, I think the POCO usually runs their equipment with it on the "C" phase.  Obviously an easy place for a mistake/miscommunication to happen.
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Steve Swaffer

ProSoundWeb Community

Re: Today's cautionary tale
« Reply #24 on: March 03, 2018, 07:29:54 pm »


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