Wild Leg
Back in my mis-spent youth I built a 48 KW lighting system using (48) 1,000 watt PAR cans on 24, 2KW dimmers for my rock band. Since I had my Master Electrician license we would tie it into our own cam-locks I installed in the clubs service panel after measuring everything very closely. And I found that by turning on all the PAR's on one leg (24KW of load) we could get the neutral leg to drag over to the load side (as per Langston's great video) by up to 10 volts. I determined that the service panel had an undersized neutral wire, most likely because it was a converted flower mill which used 3-phase power for the heavy motor loads, and 3-phase motors don't produce any unbalanced neutral currents at all. So I could watch the heavy loaded leg of our lighting system drop to 110 volts while the light loaded leg rose to 130 volts. This is just the baby version of what happens when you open the neutral entirely.
The other part of this story is that the panel was wired as 3-phase Delta High-Leg, instead of 3-phase WYE. See High-leg delta - Wikipedia, the free encyclopedia for an overview. This was a wiring shortcut often done in the 50's and 60's for industrial buildings that needed mostly 240V 3-phase for motors, and some 120/240 volt single phase power for the office. Notice that if you meter from the Neutral center-tap to two of the legs you'll measure 120 volts, but from the neutral to the "Wild Leg" you'll measure 208 volts. They often call this the "Wild", "Red", or "High" leg for obvious reasons, and it's supposed to have orange tape around the feed wire as a warning. Now I was lucky enough to be working for Corning Glass at the time as my day job, so I knew exactly what a High-Leg 3-phase circuit was all about and I carefully metered the panel before installing cam locks. But one of the other bands who played there decided to take a pair of battery jumper cables (like you start your car with) and grab 120-volt power for their own lighting system because they were tired of tripping breakers. Unfortunately for them, they knew nothing about 3-phase power or Wild Leg transformers, so they tied their lighting system into the 208 volt "Wild Leg" and blew out a few dozen PARs in a second or two. I later asked their road guy why he didn't meter the panel first (let alone what he was doing inside of a live box), but found he didn't own a voltmeter of any kind. Of course this was the 70's, so some flames and sparks only added to the show (just kidding). :lol:
I had this same High-Leg Delta scenario happen at a campground in Texas last year. The camp "electrician" wanted to run some new 30-amp 120-volt power pedestals for additional campsites, and found one of the legs on the campground service panel was empty of circuit breakers. Thinking it was his lucky day, he plugged in a dozen 30 amp breakers to the empty (Wild Leg) buss and sent 208 volts to the 120 volt outlets at the campsites. A few new campers had pulled in with their RV's and were blowing up their RV appliances, but the campground electrician attributed it to a "lightning strike". It wasn't until a few days (and a few more trashed RV's) that somebody put a meter on an outlet and found 208 volts where there should be 120 volts. Yikes!!!!
The lesson is meter everything you can, every which way you can, and be suspicious of everything. I know I'm paranoid, but am I paranoid enough? :roll:
