Motor protection requirements in ungrounded system
Motor protection requirements in ungrounded system
(OP)
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 .......






RE: Motor protection requirements in ungrounded system
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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RE: Motor protection requirements in ungrounded system