hello h20dr
I'm still waiting for a description of the system.
A.C. corrosion. I have seen rapid corrosion of sheet piling subjected to A.C. welding currents.
Insulation resistance vs ground currents. There is a relation but not a direct correlation.
The best way to measure ground current is to identify the ground path and measure the current in the ground path with a meter.
Where is the corrosion taking place. Is your water conductive? Are you using cooling towers or deep wells or another method of rejecting the heat of condensation. Is your piping conductive and does it have any non-conducting connections or couplings?
I am assuming that your 4160 motors, compressors, and condensors are mounted on a common skid. If not, please advise.
There would be a metalic connection between the motor, the condensor, the compressor and the skid. Any leakage would pass through the metal and return on one of the grounds you mentioned. However, if a pump or fan motor at the cooling tower or a well pump motor was poorly or improperly grounded and was starting to fail this could cause current flow in the water.
PLEASE NOTE: For electrical currents to cause corrosion, the current must be flowing in the water, not near the water.
Is the 4160 volt system delta or wye? Is it solidly grounded, high resistance grounded, low resistance grounded, Zig-zag grounded, directly grounded etc.?
More importantly, what is your pump and fan voltage and what type of system and how is the system grounded. This will be a more likely cause of corrosive ground currents.
What is the condition of your zincs? If your zincs have eroded or are fouled you can expect rapid corrosion.
yours