Tech Tip: Ground Current

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Based upon several projects we have had to do a deep dive into expected ground currents in shielded, unshielded, and current loop conditions. While we have not found anything solid in standards, we have found several ‘rules of thumb.’ Note that we are discussing ground and not system neutrals.

Primarily what you should see in the ground circuit is no more than kVA/1000 of the associated ground. For instance, if I have a 480V system that carries a 500 Amp load, the leakage should be no more than ((500 * 480)/1000)/1000 Amps or 0.24 Amps (240 mA).

Causes for having higher current include using unshielded cables with unbalanced voltage, on VFDs or during startup of soft start systems. Other causes would be: leakage from contamination; ground fault; dirty (noisy) ground systems; EMI (Electro-Magnetic Induction); and a number of other conditions. This is one of the reasons you will see a requirement for shielded cabling and spacing between cables in cable trays. Finally, it is also important that you run ground cables away from individual phases in conduit and run all three phases in the same conduit so that they cancel out.

You also need to ensure that all grounds are properly bonded to the motor frame. High resistances can also create these conditions, especially as you need to run a reference ground back to the drive/soft start and one at the motor to ground. In the case of shielded cable, you must ensure that you ground one side of the shielding, as grounding both may cause a ground loop.

Conditions such as ground loops or EMI, in addition to problems with electronics, controls, mechanical and bearing failures (fluting), cause danger to personnel due to the risk of electrocution. A quick check of current on the ground conductors will provide feedback as to problem conditions. Any value of current over 1 Amp should be investigated.

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