70 amperes on the main bonding jumpers?

?s falke

| I recently worked on an electrical system at a fairly large hospital. There | were two 2 (approx) megawatt transformers in parallel supplying power at | 13.8 kv on the primary. Each transformer had the 480 volt side XO solidly | grounded with a copper main bonding jumper. There was approximately 35 | amperes flowing through each bonding jumper as measured using a CTs. | I called the engineer and he said it had been that way for years and to not | worry about it. Has anyone heard of this high of charging current?

I'd be curious about:

1. Is the primary neutral bonded to the secondary neutral?

1. What's the current when clamped over X1,X2,X3 combined?

2. If X1,X2,X3 is zero, then what about H1,H2,H3 (if you are qualified to touch that side)?

1. Does the jumper current vary with changes in utilization?

2. Are there harmonics in the jumper current waveform?

IMHO, it's not right. I don't want to be hooked up to instruments at that hospital until this is figured out.

Exactly what kind of test instrument should he use for this? Have you ever actually SEEN the secondary connections of a 1000 kVA 480V transformer?

I think the operative word here is "parallel." If the transformer secondaries are truly in parallel then the slightest imbalance in transformer parameters will cause a circulating current. It is probably occuring on all the secondary connections, but the load current masks it so it is not readily noticed. In other words, the 35 amps you measure is travelling between the transformers, and is not travelling over the neutral or ground conductors.

That would be my first suspicion.

|> 2. What's the current when clamped over X1,X2,X3 combined? | | Exactly what kind of test instrument should he use for this? Have you ever | actually SEEN the secondary connections of a 1000 kVA 480V transformer?

Yes, I have seen them on an even larger transformer. Yes, you need a BIG clamp. At least you don't have to separate individual conductors for this test so you could do it downstream, too. But unless something is very seriously wrong (which I do not suspect) that current is going to be very small, but at most 70 amps.

And I'd also want to see the waveform if there is any current. It might be just the harmonics doing this (lots of digital stuff with switching power supplies in hospitals these days). And if it is, I'd suggest measuring it a couple times a year to keep track of it (it could be heating the transformer even more than just the utilization current due to the imbalance).

| I think the operative word here is "parallel." If the transformer | secondaries are truly in parallel then the slightest imbalance in | transformer parameters will cause a circulating current. It is probably | occuring on all the secondary connections, but the load current masks it so | it is not readily noticed. In other words, the 35 amps you measure is | travelling between the transformers, and is not travelling over the neutral | or ground conductors.

This could be tested if you can clamp over both just before where they bond together (in the same direction) or the grounding electrode itself.

| That would be my first suspicion.

I had several suspicions come up in parallel. More measurements would be useful.

Fairly common problem on parallel Tx that there is circulating current due to poor parallelling conditions mainly by mismatched impedances or bad jointing techniques

Yes I have seen this and it was mostly from unbalanced loads. My application was an 15kv distribution system using Kerney VacPacs. We could measure up to 80 amps on the grounds with the cables. Ground current when up with the load on the secondary.

Measure your ground. What value do you have? If you pushing 25 ohms it might be time to add some supplemental grounding.

CT scanners, MRI's and most x-ray machines even if connected to 3 phase are single phase loads in my experence. I will bet the electrical system is out of balance a bunch.