Leakage current on shore power cord question

[jackrabbit59]

I have a 50 amp 240V shore power cord. The boat has a Charles 12KVA Isolation Transformer. The shore power cord input is directly connected to the isolation transformer, Red Black and ground wires only. If I place a clamp meter over the shore power cord it reads anywhere from 4 to 7 amps!. I have tested other boat power cords and they read around 0.04 amps. I bought a new 50 amp cord, $600 and I get the same 4 to 7 amps. I have tried plugging into other dock posts with the same results.
On the output side of the transformer to the breaker box the clamp meter reads 0.02 amps to 0.03 amps.
I have tried disconnecting the output side of the transformer completely and the input side still reads 4 to 7 amps.
I had the DC leakage tested with a line from the prop shaft to the other electrode in the water. It tests fine. I do not any unusual zinc deterioration.
Anyone have an idea what could be wrong with the isolation transformer? Should I replace it?

[Dsanduril]

4A@240V=1000W. If the measurements are good then something should be getting very warm very fast.

[Sailmonkey]

Use a multimeter or preferably a megger to measure between the transformer input winding and the ground connection. This will let you know if the problem is in the transformer.

Also.

Is the current measurement AC amps or DC amps?

[jackrabbit59]

It is in AC amps. The reading floats around from 4 to 7 and back continuously. They charge for power here so it is definitely not really using 4 to 7 amps or my bill would be much higher. An electrician and I spent several hours going over everything. We pulled all the wires and checked the input sockets for corrosion. Could not find a thing. I will have the electrician back this week and check the input winding to ground. I assume there should be no connection?
As to the other comment, the transformer hums and is always a little warm. But not hot just a little warm as one would expect.
Thanks

[jackrabbit59]

I checked the clamp meter on the line for DC amps and it reads 0.2 A. It was floating between 3.8 and 4.7 A in AC mode.

[Dsanduril]

Are you measuring the cable as a whole or each individual wire? How do the measurements vary between black, red, and green?

[jackrabbit59]

Doing all measurements as a whole. Clamp goes around the entire cable. If remembering correctly, I get nothing on the individual wires if I have the breaker off on the output side of the transformer. I have to pull the connection socket from the boat to measure the individual wires so I only did that one time with the electrician. We were sure it had to be corrosion in the dock cable so I bought a new one. But nothing changed at all with the new power cable.

[Sailmonkey]

If you remove the ground connection from the isolation transformer does the current measurement disappear?

[Dsanduril]

Whole cable measurements don’t tell you a lot, the 4-7A represents on imbalance when measuring the cable. If I had to guess (and I am guessing) you have very low/zero current on black and red (so essentially no power being used), and the imbalance is a low voltage current on the ground wire. But without measuring each wire impossible to tell.

[rwidman]

Unless something has changed since I was working in the field, I don't believe you can measure current in a cable without separating the wires and measuring each one individually.

I would say you are misusing the meter and actually have no problem.

[Sailmonkey]

Originally Posted by rwidman

Unless something has changed since I was working in the field, I don't believe you can measure current in a cable without separating the wires and measuring each one individually.

I would say you are misusing the meter and actually have no problem.

A whole cable measurement is used to measure leakage. If everything is perfect the current will be zero. If there is an imbalance/leakage (ac or dc) this will show up by clamping the whole cable. Leakage current will be in addition to the two hot wires, with no countering current the clamp meter will detect it.

[Stewie12]

I too agree with measuring each cable seperately.

[canyonbat]

Originally Posted by Sailmonkey

If you remove the ground connection from the isolation transformer does the current measurement disappear?

Be VERY careful doing this. If you do have a “leak” that is coming back through the ground wire and you remove the ground wire, you could become the return path. Buy me a beer some day and I will tell you how I know!

[captstu]

This is an easily confused issue. When you measure the cable as a whole, you are measuring the current in black-red+white to ground. Hard to explain, but on a split phase circuit, there is a black to white and a red to white circuit - each providing 115 volts. The black to red circuit is 230 volts (or 220/240, but the result is the same.

If the red to white current is identical to the black to white current, by coincidence, than no power is flowing thru the neutral wire. If the red to white OR black go white circuit is open (zero current) the current on the white wire is identical to that on the red or black, whichever one is not zero.

Your case is different: The sum of red-black +white is not zero - thus there is leakage in the circuit, most likely an internal short to ground in the isolation transformer.

Measure with a wrap around ("amprobe " or similar) the current on the Green wire leading form the isolation transformer to earth ground. It should always be near zero - or if possible, measure the voltage by isolating the isolation transformer from ground and reading the voltage between the green wire on you disconnected from the transformer and the connection the green wire was attached to.

That voltage mst be less tan 3 volts - no exception - any more creates a lethal shock hazard for anyone swimming near the boat plus other less serious issues.

This is the primary voltage a ground fault interrupter measures to trip the safety circuit.

As said earlier, 4 amps at 240 volts would generate significant heat - but, most likely, you have an internal short that is producing only a few volts, but since the circuit is of such low resistance, it is carrying several amps.

Sorry this is so complicated - but in essence:

If your isolation transformer measures 3 volts or more two earth ground (green) when otherwise disconnected from ground, it is internally shorted (there are other even more complicated reasons) and is unsafe to operate.

[smac999]

either something is wired wrong (ie ground cable connected from input to output of transformer) or the transformer itself has failed somehow. IE the wire isolatioin has failed and the coil wires are touching the ground shield.

there is no way to have current mismatch / leakage on a properly wired and working isotransformer. as the ground does not go anywhere. and the current on the 2 hots through the primary coil has to be the same in and out.

unplug the shore cable from the boat.

open the isotransformer.

measure ohms from input ground to output ground
from input line 1 to input ground,
from input line 2 to input ground

all should be infinate