Motor protection requirements in ungrounded system

[elecsun]

We ahve delta - delta connected transformer and secoindary is connected to 480volts,3phase,60hz ungrounded Motor control centre(M.C.C).In this type of system what should be protection requirementin the starter compartment for 3phse,60hz.460Volts motors(Motor horsepower rating varies from 3HP to 150HP).MCC Bus bar is having common earth fault indicating lamp circuit at the incoming breaker compartememt.How the motor will be protected in case of phase to earth fault.What are the additional requirement other than normal motor starter components (Circuit braeker-contactor-overload relay)?What are the NEC/IEEE/ IEC requiremts?Expertrs guidence please .......

 

[electricpete]

I’m not an NEC expert. I suspect some of those NEC guru’s may tell you motors shouldn’t be operated from ungrounded systems.

We have ungrounded 480vac system at our power plant. There is no protection associated with ground fault, only indication for ground at the MCC level like you mention (lights).

Prompt action is expected to troubleshoot single grounds (no fault current flowing for single ground). Upon occurrence of a 2nd ground which actually , overload or instantaneous trips would be relied upon for tripping.

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[davidbeach]

electricpete, no NEC prohibition on operating motors on ungrounded systems. I would suggest though that you would find your troubleshooting to be much easier if you convert to a high impedance grounded system so that you get some limited current flow during the single fault condition. With a signal injected onto the ground current, can be as simple as pulsing it back and forth between 5A and 10A, the source can be traced and the fault found without having to turn things off. A high impedance grounded system will also limit transient overvoltages compared to an ungrounded system.