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Author Topic: Another High-Leg Delta Fail  (Read 12916 times)

Mike Sokol

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Another High-Leg Delta Fail
« on: October 10, 2015, 09:49:22 AM »

Just got this post on my No~Shock~Zone blog. It's from an RV owner who plugged the 50-amp/240-volt shore power plug into a newly rewired 14-50 receptacle (stove outlet) at a church that was connected to the Delta-High-Leg bus on a 3-phase panel. Ouch... :o We've discussed how this industrial 3-phase panel works in other threads, so I won't go into the details right now.

As you can see, the OP isn't blaming the electrician for all the damage to his RV's appliances, but I certainly am. Don't these guys use meters? While I can understand that a residential electrician may never have come across a High-Leg-Delta panel, they certainly should know how to measure a 14-50 receptacle for proper voltage, or am I asking too much?

One more reason for each of us to measure receptacles BEFORE plugging in our expensive sound gear. As others have noted on previous threads, lots of modern sound gear uses auto-switching power supplies and wouldn't be damaged with accidentally applying 240-volts to 120-volt circuits. But there's a ton of 120-volt gear out there that won't take a 240-volt hit for even a few seconds. Looks like I've got to write an article on High-Leg-Delta panels for residential electricians and DIY guys.

Mike Sokol

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From http://www.noshockzone.org/accidentally-plugging-into-240-volt-outlet/

I have a situation that’s a little different, we had the panel at our church replaced which is a 3-phase delta system (the third leg is 240v to ground but still only 240v to the remaining two legs. So the 50 amp breaker for the rv was put across the high leg because it’s “240” so any 240 appliance will work fine. What the electrician didn’t realize is that an rv is not a true 240 it’s actually two 120’s. So technically I can’t blame the electrician since it would of worked on any other appliance.
Will I have all the same problems or will it be less or worse? The ac blower and compresser run but does not cool, the display on the microwave doesn’t work but the light comes on when you open the door, the display on the fridge works but not sure if it cools. The slides all work and the tv, all the 12volt lighting works but it could of still been powered by the battery. I didn’t get a chance to check everything since I just packed it back up and came home since I couldn’t stay in it.
« Last Edit: October 10, 2015, 09:55:36 AM by Mike Sokol »
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Tom Bourke

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Re: Another High-Leg Delta Fail
« Reply #1 on: October 10, 2015, 11:14:39 AM »

Sounds like a total fail on the electricians part given that high leg delta is 240 any leg to any leg.  I don't think it is used for new installs any more.  The only reason I could see for that system is a retrofit for a church is to give 3 phase for a new elevator with out having to redo the entire building.
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Mike Sokol

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Re: Another High-Leg Delta Fail
« Reply #2 on: October 10, 2015, 11:58:16 AM »

Sounds like a total fail on the electricians part given that high leg delta is 240 any leg to any leg.  I don't think it is used for new installs any more.  The only reason I could see for that system is a retrofit for a church is to give 3 phase for a new elevator with out having to redo the entire building.

I've been in lots of churches that were placed in revamped industrial parks. High-Leg Delta panels were quite common in commercial buildings that needed a lot of 3-phase power for motors, and limited single-phase power for offices. But yes, I don't think they're currently being installed in modern facilities. Since I do a lot of gigs in churches, I'm going to double-check any 14-50 receptacles for proper 120/240-volt wiring. 

Stephen Swaffer

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Re: Another High-Leg Delta Fail
« Reply #3 on: October 10, 2015, 02:25:08 PM »

I've been in lots of churches that were placed in revamped industrial parks. High-Leg Delta panels were quite common in commercial buildings that needed a lot of 3-phase power for motors, and limited single-phase power for offices. But yes, I don't think they're currently being installed in modern facilities. Since I do a lot of gigs in churches, I'm going to double-check any 14-50 receptacles for proper 120/240-volt wiring.

High-leg delta is a cost savings on the POCO distro side only-either way you still have 3 phase panels.  If the POCO passes the cost on to the customer, then they might opt for the lower cost.  A 14-50 receptacle should never be wired to the high leg.  The reason we use 4 wires is that stoves (and dryers on 14-30s) have 120 volt loads. Wiring to high leg has a 50-50 chance of frying the controls on these appliances-that electrician wasn't thinking straight and he is likely not the only one so checking before plugging in would be a really good idea.
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Jonathan Johnson

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Re: Another High-Leg Delta Fail
« Reply #4 on: October 10, 2015, 06:03:39 PM »

Just got this post on my No~Shock~Zone blog. It's from an RV owner who plugged the 50-amp/240-volt shore power plug into a newly rewired 14-50 receptacle (stove outlet) at a church that was connected to the Delta-High-Leg bus on a 3-phase panel. Ouch... :o We've discussed how this industrial 3-phase panel works in other threads, so I won't go into the details right now.

If I understand correctly, it would have been 240V leg-to-leg, 120V from the leg on the split-phase hot to neutral, and 208V from high leg to neutral.

So if the RV has a 120/240V service, some 120V appliances would have received 120V and been just fine; others would have received 208V and likely been damaged. The 240V appliances would have received 240V and been fine.

But if the RV has only 120V service, as most do, and was connected via an adapter, you've got a 50/50 chance of getting either 120V or 208V.

I think maybe the electrician did not understand that high leg to neutral is 208V. I wonder how many electricians understand the vector math that results in 208V between phases?
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Jeff Bankston

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Re: Another High-Leg Delta Fail
« Reply #5 on: October 10, 2015, 06:32:29 PM »

240 High Leg 3 pole with neutral Delta is all over Los Angeles. Services are also mixed in some areas. I rewired a friends 4 space building that had 2 seperate services. A single phase 120/240V 2 pole with neutral service and right next to it a 240V 3 phase 3 pole no neutral for equipment use. Both services looked the same and were very old and did not have labels. Very bad if you didn't meter them. He had 2 sets of 3 wires coming from one power pole to insulators on the side of his old building. There was a 120/240v transformer several poles away. This is very common out here. A BMW dealership down the street added a new showroom and had the same setup. The 120/240V 2 pole and 240V 3 pole no neutral services were in seperate buildings. You defintely have to meter stuff. 
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Ray Aberle

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Re: Another High-Leg Delta Fail
« Reply #6 on: October 10, 2015, 07:21:14 PM »

Very bad if you didn't meter them.

You defintely have to meter stuff.

And these are the key points, and hopefully as more RV owners blow up thousands of dollars of VERY EXPENSIVE shit, maybe they'll start getting the picture. A Harbor Fright DMM is less then $7, and sometimes "free" with a purchase-- that's a small price to pay (well, know how to use it, of course) to hopefully head some of these problems off at the pass...

-Ray
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Mike Sokol

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Re: Another High-Leg Delta Fail
« Reply #7 on: October 10, 2015, 08:47:34 PM »

And these are the key points, and hopefully as more RV owners blow up thousands of dollars of VERY EXPENSIVE shit, maybe they'll start getting the picture. A Harbor Fright DMM is less then $7, and sometimes "free" with a purchase-- that's a small price to pay (well, know how to use it, of course) to hopefully head some of these problems off at the pass...

-Ray

Since campgrounds are now allowed to use 2 legs of 3-phase 208 to power 50-amp "single-phase" 14-50 "stove plugs", I suspect we'll see more of this type of over-voltage failure and the resultant expensive repairs.

I do wonder just how much 3-phase training the average journeyman electrician gets. I had a ton of industrial 3-phase experience very early in my electrical training, so I've always had a good understanding of how this all works and measured everything. The first time I encountered a High-Leg Delta service was at the Old Mill Inn, a club my band played back in the mid 70's. I was putting in tails for my lighting distro and measured 208 volts from neutral to one of the legs. Of course, I stayed away from that leg for the gig, and studied it at work the next week. High-Leg Delta made sense, but seemed really dangerous if you didn't know what you were doing. A few weeks later another area band tied their lighting distro into the same panel, and ended up feeding 208 volts to a bunch of PAR cans, blowing up dozens of 500 and 1,000 watt bulbs in seconds. After that I REALLY paid attention to what I was connecting into.

Steve M Smith

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Re: Another High-Leg Delta Fail
« Reply #8 on: October 11, 2015, 08:41:35 AM »

What is the reason for the high leg?  We manage with three phases all of the same voltage.

EDIT: I think I get it.  It's so you can have a three phase supply for motors and a 120/240 supply all from the same three phases plus neutral.  The 208v is just a consequence of having neutral half way between the other two phases.

Is there much equipment which uses 208v as its supply?


Steve.
« Last Edit: October 11, 2015, 08:45:59 AM by Steve M Smith »
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Mike Sokol

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Re: Another High-Leg Delta Fail
« Reply #9 on: October 11, 2015, 08:52:01 AM »

What is the reason for the high leg?  We manage with three phases all of the same voltage.

EDIT: I think I get it.  It's so you can have a three phase supply for motors and a 120/240 supply all from the same three phases plus neutral.  The 208v is just a consequence of having neutral half way between the other two phases.

Exactly right. It's a cheap way to provide mostly 3-phase 240-volt for motors with some single-phase 120/240-volts for the offices.

Quote
Is there much equipment which uses 208v as its supply?

Not really. But if the gear needs s full 240-volts we can add a buck-boost transformer to "boost" the 208 up to 240 volts. However, that 208 connection from a High-Leg Delta transformer isn't really "full power" so we don't like to use to for large loads.
« Last Edit: October 11, 2015, 09:49:49 AM by Mike Sokol »
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ProSoundWeb Community

Re: Another High-Leg Delta Fail
« Reply #9 on: October 11, 2015, 08:52:01 AM »


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