Introduction to Circuit and Motor Protection - SUMMARY

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.