If the hum stops, install an isolation transformer on the cable.

Suitably designed isolation transformers block interference caused by ground loops.

Differential noise can magnetically couple from the primary to the secondary of an isolation transformer, and must be filtered out if a problem.

Without the isolation transformer, the lighting circuits on the mast would conduct radio-frequency energy to ground through the power supply.

This can be avoided by using an isolation transformer with the primary and case connected to shore earth, and the secondary "floating".

An alternative is to use an isolation transformer made specifically for this purpose.

Early series street lights were equipped with isolation transformers.

Another solution is to use isolation transformers, opto-isolators, or baluns to avoid a direct electrical connection between the different grounds.

The better isolation transformers have grounded shields between the two sets of windings.

They are often installed in the medical field in conjunction with isolation transformers, which serve to protect the patient from electrical faults.