480 3 phase
So, we were working on a facility that is new to us, it is an older factory. While working on one of the pieces of equipment in the older section, we discovered a situation with the electrical. We observed the following voltages
A to B 480
A to C 480
B to C 480
All normal, everything looks fine.
Then we checked each leg to ground, a habit I picked up working around 240 volt 3 phase with a wild leg.
A to ground 480
B to ground 480
C to ground 0
I have since discovered that this is called 480 with a dead mans leg, I have a few questions
1st, why would this system ever be allowed to operate here in the USA, land of work place safety rules and regulations.
2nd, and most important to me, is the modern equipment able to run on this power with no damage to the motor windings?
3rd, what other safety concerns should I have that I have not thought of yet due to this power source
This corner grounded system or as you call it dead man's system Is an older style system. I am not sure why they used to engineer systems that way. The phase to phase voltages are the same so in that sense it won't short out motor windings, however phase y I ground voltages can be higher so if the motor isn't rating for the extra voltage it may lead to a ground fault. Check equipment manufacturers ratings.
As for safety, people working on this system might benefit from being advised before working on it that it is a corner grounded. Qualified electricians practicing safe work habits should have no problem identifying and safely working on this system. Whenever work is performed energized at least two journeyman must perform the work per Osha regulations