Introduction to Circuit and Motor Protection – SUMMARY


  • Starters use overload relays to provide protection to a motor.
  • An overcurrent is the result of an overload current, a short-circuit, or a ground-fault current.
  • Fuses are used to protect a circuit from short-circuit faults.
  • A single-element fuse is also known as a non time-delay fuse.
  • Dual-element time-delay fuses can also protect conductors and circuits from short-circuit and ground faults.
  • When used as a disconnect device, a circuit breaker can open and close a circuit nonautomatically.
  • Circuit breakers use two types of tripping elements—bimetal and magnetic.
  • The activation of a magnetic circuit breaker is instantaneous.
  • Overload relays are used to protect a motor.
  • The eutectic melting alloy overload uses a heater element surrounding a solder-like mechanism that holds the pin of a ratchet wheel element.
  • A solid-state overload relay is unaffected by ambient temperature.
  • Heater elements are rated by NEMA according to the amount of time it will take to melt the alloy when the motor is drawing six times its full load current.
  • Heaters are grouped into three categories—class 10, 20, and 30.
  • A compensating bimetal strip should be used in the relay when the ambient temperatures of the motor and the overload are different.


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