NewElec's NJ and NK series motor protection relays offer a cost-effective solution providing protection against motor overloading for both sustained and cyclic overloading conditions. They protect against phase current imbalance and phase loss protection, as well as earth leakage protection with a sensitivity of 250 mA and a trip delay of 100 ms.
The relays' design includes full thermal memory with user-selectable thermal curves in the range 2,5 to 32,5 s. Running stall protection with a factory preset trip threshold of three times motor full load setting is activated as soon as the motor starting sequence is completed. This feature is ideal when protecting mechanical equipment against the effects of impact torque conditions.
As the relays are immune to harmonics, when they are used in conjunction with larger kW motors, nuisance earth leakage trips are eliminated, especially when starting these motors on D.O.L.
Their built-in motor load transducer, a 4 to 20 mA output loop, may be used as an analog input into a controlling PLC. Providing reliable and effective protection for low voltage motors, the relays obviate any further requirements for a separate earth leakage protection relay as well as another transducer. These built-in features are both cost reducing and space saving factors for MCC manufacturers.
The design of the protection relays allows adequate provision for preventing the main contactor from opening on an earth leakage fault current whose magnitude could rise to 'infinity' within a very short time span with possible disastrous results to the main contactor. This is achieved on the NJ model by intelligent assessment of the actual fault current magnitude prior to energising the main trip contact. If the relay assesses that the fault current is exceeding eight times motor full load setting, the trip is blocked to allow for the backup protection to disrupt the supply to the motor. In the case of the NK model a totally separate trip contact is energised. This is designed for connection to the shunt trip mechanism of a backup circuit breaker.
An additional set of four N.O. contacts provide a hard wire link to a remote control room as to the motor status and fault type. Localised, remote and voltage actuated relay reset instructions are included in the package. The relays are designed for door and chassis mounting and can be used over a broad spectrum of current ranges.
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