The Microshield O/C relay is an advanced microprocessor based unit that was designed to provide high value three phase and ground overcurrent protection and optional circuit breaker auto-reclosing. Applications include distribution feeders, transformers, line protection back-up, motor protection, and others. The Microshield O/C provides monitoring, metering, and fault recording as well as protection.
The Microshield O/C can be applied with voltage transformers (VTs) connected for operation at 69 or 120 volts AC phase-to-ground (Wye), 120 volts or 208 volts AC phase-tophase (Open-Delta). (Other voltages such as 110v or 100v nominal are also suitable and easily accomodated.)
The unit is packaged in a steel case suitable for conventional flush mounting on panel. It can also be semi-flush mounted by the use of available case spacers when unit depth is a concern. Available bezels make retrofit of older electromechanical and solid state relays possible. The full draw-out design allows for quick removal without the need for removal of hardware. The microprocessor based logic along with the power supply, VT’s and CT’s can be totally withdrawn from the case and interchanged with other cases without the need for re-calibration. CT shorting bars are an integral part of the unit case. All connections to the Microshield O/C are made at terminals on the rear of the unit.
Protective Functions
The Microshield O/C contains the protective elements necessary for medium voltage circuit overcurrent protection. Its applications include distribution feeder protection, line backup protection, motor overcurrent protection, and transformer overcurrent protection. The dual phase-timeovercurrent
elements allow the user to design a composite curve; for example for motor protection applications to provide locked-rotor and overload protection. A summary of the elements follows.
- Three-phase time-overcurrent elements (51P, 51LT)
- Ground time-overcurrent protection (51N)
- Phase instantaneous overcurrent (50P)
- Gound instantaneous overcurrent protection (50N)
- ANSI Trip and reset times conform to IEEE C37.112