I went on a service call last week, a friend of a friend owns a print shop and was having problems; simple deal to fix. While I was there, I noticed something that looked wierd and I've never ran into it before. Probably a dumb question...
This was a commercial building in an old part of town, with two 240 v services side by side. They're on the outside of the building with surface mounted overhead risers feeding them from the utility.
The utility dropped four wires, three hots (one is a stinger) and a neut. One service was single phase, it got two hots (120 v to ground)and a neut. The other was straight 240 volt 3-phase, 3-wire, no neutral.
My question is, how would the 3-phase service clear a ground fault?
I was in a hurry, but out of curiosity I put a meter between all three legs and ground in the 3-phase service and read 120 on two, 208 on the stinger. I was thinking there had to be a corner ground, which would have been wierd given the transformer feeding these two risers obviously had a grounded neutral. I have very little experience with delta services, all of my work is on new stuff, 120/208 Y.
So, the only thing I could come up with is that the poco is relying on the neutral from the 1-phase service to clear a potential fault in the 3-phase service. Does that seem right to y-all, or should the 3-phase service riser have a fourth wire(neut) connected? Interestingly the 3-phase riser had a fourth wire coming out of the weatherhead, but it was just curled up and not connected. My gut tells me someone should be calling the utility asking them to hook that to the system neut.
[ February 05, 2005, 06:14 PM: Message edited by: sdbob ]
This was a commercial building in an old part of town, with two 240 v services side by side. They're on the outside of the building with surface mounted overhead risers feeding them from the utility.
The utility dropped four wires, three hots (one is a stinger) and a neut. One service was single phase, it got two hots (120 v to ground)and a neut. The other was straight 240 volt 3-phase, 3-wire, no neutral.
My question is, how would the 3-phase service clear a ground fault?
I was in a hurry, but out of curiosity I put a meter between all three legs and ground in the 3-phase service and read 120 on two, 208 on the stinger. I was thinking there had to be a corner ground, which would have been wierd given the transformer feeding these two risers obviously had a grounded neutral. I have very little experience with delta services, all of my work is on new stuff, 120/208 Y.
So, the only thing I could come up with is that the poco is relying on the neutral from the 1-phase service to clear a potential fault in the 3-phase service. Does that seem right to y-all, or should the 3-phase service riser have a fourth wire(neut) connected? Interestingly the 3-phase riser had a fourth wire coming out of the weatherhead, but it was just curled up and not connected. My gut tells me someone should be calling the utility asking them to hook that to the system neut.
[ February 05, 2005, 06:14 PM: Message edited by: sdbob ]
