Customer has a 240V, 3 phase, 3wire service to his shop. Phase A(black)= 208V to GRD, Phase B(red)= 0V to GRD, Phase C(blue)= 240V to GRD. When MAIN DISCO. is turned OFF(no load), Phase A(black) jumps up to 240V to GRD, like it should be. Turn MAIN back on(load applied), 208V to GRD again. Phase B & C stay same. Phase A has a crimped Butt-Splice in the meter can, and "sizzles" for a few seconds, when MAIN is turned back on. I had POCO disconnect power to shop until I could replace splice in morning. Anyone ever come across this before? Why would voltage drop under load? Also, not much experience with 240V, 3 wire, 3 phase. Phase "B" was grounded. Does that act as a neutral? POCO double checked their XFR, and verified their voltage was correct, 240V phase to phase. It is a 5 year old, underground service. No one has been trenching near service, but still not 100% sure the splice is the problem.
Last edited by Up2code; 12/17/0910:12 PM. Reason: missing info
Well the bad splice is creating a resistance in series with the load. Remember basic ohms law. V=I*R when the load is disconnected there is no current flowing through the resistor (bad splice) so there is no voltage drop accross it. When the load is applied, current is drawn through the splice (masquerading as a resistor) and it causes the 32 volt, or so, drop that you are seeing. As for phase B, yes it acts as neutral or, more technically, the grounded conductor. It is what is known as a corner grounded delta service. I have never dealt with one before but, I have heard of them. I didn't think that they were that common any more but, you say it is a fairly new service (5yrs). So, well there it is.
Re: Bad 240V butt-splice
#191102 12/18/0906:00 AM12/18/0906:00 AM
240V 3phase, 3 wire is for all his cabinet making machinery. Yes,there is a transformer inside for his 120/240V loads. Crimped butt-splice is not lava, but is blackened, with some type of "goo" leaking out. Short to ground on Phase A was initially what I thought, until kicked main off, and it jumped back to 240V. That was when I got stumped. Never thought of butt-splice acting as a type of resistor, but makes sense. I also thought with winter here, if there was a cut in insulation in conduit somewhere, water in conduit turning into ice(expanding) could be causing a "voltage leak", short to ground, even though customer claims pipe is 36" deep. Oh well, going back today to change out splice and see what kind of results we get.
I also agree with IanR: The odd Voltage Readings on Phase "A" are result of a bad Termination in series with a connected Load.
The System described sounds like a normal 3 Phase 3 Wire Corner Grounded Delta, with Phase "B" as the System's Grounded Conductor.
Phase B is a Grounded Conductor, quite similar to other System Grounded Conductors; Bond it at one point only (such as the Service) to a Grounding Electrode System, plus a Main Bonding Jumper. Then bond all Equipment Grounding Conductors to the Grounded Conductor at the same spot where the the GES and MBJ are connected. Identify the Phase B Grounded Conductor with White (or Gray) Coloring.
The differences between a Grounded "Neutral" Conductor and this "Grounded Phase B" are:
Grounded "Neutral" Conductors are used on "Dual Voltage" Systems, across Multiwire Circuits.
Grounded Conductors are part of a 2 Wire L-N, or 3 Phase 3 Wire L-L-L Circuit.
The Phase B Grounded Conductor of a 3 Wire Grounded Delta is One of the Three Phases involved with a normal L-L-L 3 Phase Circuit, and this Circuit acts no different than any other 3 Phase 3 Wire Circuit.
Additional notes include:
Do not install Fuses in series with Phase B. If a device opens all 3 lines at the same time, Phase B may be run through Circuit Breakers and Disconnect Switches.
There is an allowance for final running protection of Motors to use single Overload Devices in series with the Phase B Grounded Conductor. This does not include the Disconnecting means at equipment.
Check out the Technical Reference section for Schematics of Delta Systems.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
Found culprit. Bad crimped splice. When I cut into butt-splice(500kcm copper), water literally poured out. One end of underground service goes 25' up POCO pole where it connects to there XFR. Apparently rain water has been going down open end of copper wire at XFR, for 5 years and was traveling all way to encapsulated splice in meterbase. Well every time it would freeze, the water would turn to ice and push the crimped connection further & further apart. When I cut into bad splice, POCO side of splice was barely in crimped connection, as if both ends of wire had been slowly pushing apart. We had freezing temps here for a week or so now, and apparently had pushed crimp to breaking point, to where it was barely making connection. I suppose IanR's explanation on splice acting as resistor is why I was reading 208V on that phase underload, 240V without a load. I put new splice in, with water able to drain out of splice. Not quiet sure what else I could have done. POCO said water in there conductors was not uncommon, and over time would go away. Not sure how. Powered up and all readings were back to normal, with & without load, 240V(A to B), 240V(A to C), 240V(C to B). Thanx to all.
Last edited by Up2code; 12/18/0907:11 PM. Reason: missing info