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2C139 Shaft Overcurrent Protection Relay
If the bearing insulation on rotating machines breaks down, the voltage induced between the shaft ends, due to machine dissymmetries, creates a shaft current that can damage the bearings. The level of damage depends on the magnitude & duration of the shaft current.
It is therefore desirable to provide sensitive protection that can detect shaft currents of 1A or less. The shaft current measurement principle enables more sensitive protection than shaft voltage measurements or vibration monitoring. Vibration monitors only operate after bearing damage has occurred. By applying the shaft overcurrent protection relay type 2C139 together with a special current transformer, the machine can be tripped & the cause of the bearing insulation breakdown determined before the bearing is damaged.
Rejection of the 50Hz component is important where the shaft current transformer is exposed to large fundamental leakage flux from the machine. The 2C139 relay may be employed to overcome this problem by monitoring the 150Hz component of the shaft overcurrent & is available with a minimum setting sensitivity of 0.1% of In.
Where the 50Hz fundamental frequency is to be monitored the 2C138 model should be specified.