In a motor circuit a short circuit condition occurs when the insulation between the phase conductors fails and the load current level exceeds the normal locked rotor current level that the motor would draw during startup. The fault level of the motor feeder supply determines the current level during a short circuit and can be as high as 15 to 50 times the motor’s full load current value.
A short circuit fault is a system level fault. This means that it compromises the stability of the entire network, resulting in total power loss to healthy drives and shutting down an entire process. The MCCB is a maintenance specific item that can appear to be functional even when it is not. This can result in a catastrophic failure on the next fault clearance.
NewElec’s MA motor protection relay is able to protect against thermal overloads, locked rotor and running stall or jam. The MA also protects against short circuit faults and automatically takes care of trip co-ordination so that the main contactor is not used to disrupt high energy faults.
Additional features of the MA include real-time and date stamping recorded by the relay for the last four trips; a recording utility of actual RMS loads; thermal conditioning; mains phase voltage; line voltage; and status of digital inputs and output contacts. The user also has the option of expanding the unit for fieldbus communications via Profibus DP, Modbus RTU and Canbus protocols.
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