Hi,

I am working on a project where I need 'electrician's practical advice', rather than detailed code analysis.

As I've stated in the past, I am doing research on electric motor systems, and have been studying code in order to better know how these motor systems will be applied once they reach production. We are now working on a project where we will be mounting a diesel gen-set on a vehicle on a temporary basis, and using the output of the generator to run an experimental electric motor/drive system.

The system will be operating at 440V three phase with approximately 80KW peak loading. Because the electronic drives are using a simple (dumb) six step rectifier without power factor correction, we are planning on using a gen-set with at least 130KVA capability. These are the only loads on the generator, so we don't care about harmonic voltage distortion on the generator output; we simply care about damage to the generator itself because of excessive harmonic currents. We will be operating two motors through two electronic drives, and may split the gen-set to two smaller systems.

NEC does not apply to this application, although OSHA rules almost certainly do apply. Customer requirements for protection of their vehicle, and personal requirements for the protection of my own skin set the standards for this installation.

All cabling between equipment (generator to electronic drives, drives to motors) will be SO cable. The SO will be sleeved in copper braid to provide EMI shielding, and then further overwrapped with sleeving to protect the copper from abrasion. In addition to any ground bonding done using conductors inside the SO, the copper braid will be bonded at both ends.

Each electronic control will include a circuit breaker on the input, at their rated current level, as well as a lock-out compatible on-off switch. The electronic control will provide overload protection for the motors. The motors will have direct embedded temperature monitoring. The generator will have its standard overcurrent protection.

First question: is there anything in the above (code or not) which makes you jump up and say 'you are asking for trouble'?

What I am looking to do is add ground fault detection to this mix, shutting down the generator if there is insulation failure anywhere between the generator and the motor. I'd like the ground fault pickup to be adjustable between 1A and 10A. Extremely sensitive ground detection will result in nuisance tripping, since there will be some amount of capacitive coupling between the motor winding and frame, especially at the electronic control switching frequency.

The Cummins generators that we have been looking at have Ground Fault Detection available as an option, however the minimum pickup level is 100A.

Second Question: Where should I be looking to find a cost effective and reasonable way to add ground fault detection to this system? I am currently searching for 'ground fault relay', and going down the list of suppliers to call, but I'm hoping that someone here has already installed sensitive ground fault detection on a generator, and can point me in the right direction.

Best Regards,
Jonathan Edelson