I just recently installed a 3 phase 700 amp 480 delta circuit for an upcoming project in my lab at work, and of course i check phase to phase and all seems find. 472, 474, 471volts respectly. Then i decided to check phase to ground, and I understand that with a delta connection there is know equal potential to all three phases and ground. but my concern is when I check on A phase I have 45 volts,B phase 445 volts, C phase 494 volts. can you please shed some light on this matter. I have been working in the electrical field since 1977.

James Battle

 

Responding to James Battle's Mar 17, 8:59am query... your systems' capacitance to ground (earth) is unbalanced!

Some unbalance is expected for delta-connected systems. But, the level indicated by your measurements is abnormal for a new system.

You should pursue a course-of-action at once.

Regards, Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])

 

could it be a corner grounded system? someone working in the electrical field since 1977 should know what a corner grunded delta power system is.

 

The grounded corner (say R) ph-gnd measurement will be 0 Volts. The other two (S, T) ph-gnd measurements will be 480 Volts!

Regards, Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])

 

>> with a delta connection there is know equal potential to all three phases and ground.>>

Actually, no.

This is true of wye when the center point is grounded (V_Line/Sqrt(3) ... 480/1.732 = 277V), but only true of delta when phase amp loads are relatively balanced, and the insulation system has the same impedance to ground on all three phases.

Delta "floats", and doesn't have a galvanic (i.e. - hardwired) reference to ground unless added either intentionally, or accidentally, and develops it's voltage to ground across system reactance.

If leakage through the insulation system's capacitive reactances is balanced on all three phases then you'll see 277V to ground on delta. However, if one phase 'gets closer' to ground than the other two then that voltage will drop, and the other two phase voltages to ground will rise. Exactly what those voltages will be isn't at all easy to figure out except in the boundary case.

Aside: We have one delta secondary 3750 KVA PMT in our line-up, and it would
sometimes show voltages similar to the ones you are seeing. Usually we'd find a shorted heater element in our equipment, and fixing it would cure the symptom, but a couple of years after the transformer's installation one of the phases went to ground, but we couldn't find any shorts, and even went to the trouble of powering down and disconnecting all the loads one by one without curing the problem. Over a shutdown our electrical contractor went through the distribution bus between the PMT and feeder switchgear, and found one of the splice bars had been installed improperly, and essentially bolted one of the phases to the bus cover ground! It didn't show up right away because it was connected to a painted section, and I suppose it was necessary for mechanical sway, and expansion/contraction cycles to wear through the paint.

If a phase gets really close to ground (for instance, is bolted to ground) then you'll see 0 volts to ground on that phase, and 480V to ground on the other two. This is called 'corner grounded' delta. There are other delta grounding variations ('red leg', etc.) to consider as well.

Resonance effects can be pronounced with delta systems, and may explain the higher than nominal line voltage (494V) you've observed.

Check out IEEE publication Std 142-1991 "IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems". Section 1.4.2 discusses ungrounded systems.

 

I would beg to differ with you. I have seen corner grounded systems with as much as twenty volts from the grounded conductor to earth depending on where one measures it.

 

james you have what is known as a jumping ground you need to bond the transformer to building steel and the neutral to ground