Over Load and Over Current

A good way to think is -

Overcurrent is any higher-than-normal current, includes Overload and Short-circuit.

An overload is an overcurrent flowing through the intended path.

A short-circuit is an overcurrent flowing through an unintended path.

Specific to motor protection, overload protection is given by an overload relay, set at rated FLA in IEC countries (maybe different in North America), and Short-circuit protection is provided by fuses or circuit breakers, selected as per Type 1 or 2 coordination.

Over Current is more generic term. It is categorized as Overload Current and Short Circuit Current.

Over Load Current: The term overload is applicable mostly for motor loads. It is generally the current when motor is overloaded by some reason. Overload can be sustained for certain period of time. There should be separate overload protection in this case.

Short Circuit Current: It is due to the fault (line to line, line to ground) in the circuit and applies to all type of circuit. The circuit should be isolated immediately. Motor starter should have short circuit current protection in addition to overload current protection.

The setting of motor Overload and Short Circuit current depends on the standard used. Per NEC article 430.32, the maximum overload setting is generally 1.15% (1.25% for motor of service factor greater than 1.15) of name plate full current and per NEC 430.52, the maximum short circuit setting is generally 800% for instantaneous trip breaker and 250% for inverse time breaker. However, the settings depend on many other factors.

- MS

Overload relays are provided to protect the motor from overload currents .When current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. When the contacts open, the contactor coil de-energizes, which results in an interruption of the main power to the motor.

Overload relays and their heaters belong to one of three classes, depending on the time it takes for them to respond to an overload in the motor. The overload relay itself will have markings to indicate which class it belongs to. These include Class 10, 20, and 30. The class number indicates the response time (in seconds). An unmarked overload relay is always Class 20. Typical NEMA-rated overload relays are Class 20, but you can adjust many of them about 15% above or below their normal trip current. IEC relays are usually Class 10, and you can usually adjust them to 50% above their normal trip current.

The overcurrent protection is provided by circuit breakers.Circuit breakers are usually Rated for ampere, voltage and fault current interruption rating.

Over load is always thermal related. Please remember that a motor can get overloaded even when it is drawing less than its full load rated current, if the cooling is poor. Alternately, even if a motor is drawing higher than its rated full load current, it may still not damage, provided the cooling is better. To say that overload is may or may not be related to over current. So, a overload setting for a motor should be "Thremal Overload Setting" rather than Over current Setting.

Whereas Overcurrent is always current related. It may or may not provide overload protection.

THANK Q Mr. electricalexpert65 for your simple explanation which has cleared my doubt