Single-device motor protection relays

In all these conditions protection against 2, 3, 4 and 5 is generally applicable to large LT motors, say 100 h.p. and above, and all HT motors. This protection is normally provided by a single-device motor protection relay, discussed in Section 12.5. 

Purpose Unfavourable operating conditions Fault conditions Protection Single-device motor protection relays Summary of total motor protection Motor protection by thermistors Monitoring of a motor s actual operating conditions Switchgears for motor protection Selection of main components Fuse-free system... 

Overload devices in current use are typically thermal overload relays to BS 4941, motor starters for voltages up to and including 1000 V A.C. and 1200 V D.C. or BS 142 electrical protection relays. Relays to BS 4941 generally provide overload and single-phasing protection. Those complying with BS 142 are also frequently fitted with instantaneous earth fault and over-current trips. 

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