Do You Know What Overload and Overcurrent Are?

Every electrical device used in power grids or in our daily lives has a rated power. When the device's power exceeds its rated power, it is referred to as overload. Correspondingly, protection against such exceeding of the rated power is called overload protection. Overload conditions in devices can easily lead to short circuits. Protection specifically designed to prevent internal short-circuit faults in devices is termed short-circuit protection.

Both overcurrent and overload are operational states. Overcurrent refers to an operating state where the current in motors or other electrical components exceeds the rated current. Typically, overcurrent does not exceed the short-circuit current and remains within six times the rated current. The overload operating state falls within the category of overcurrent operation, as it involves motor operating currents exceeding the rated current but within 1.5 times the rated value. If electrical equipment operates under overload conditions for an extended period, the temperature rise of its windings may exceed permissible limits, leading to insulation aging or damage.

Overload protection is often implemented using thermal relays, which can operate instantaneously without being affected by short-term overload surge currents or short-circuit currents in motors. However, fuses or low-voltage circuit breakers must also be used in conjunction because when a current exceeding six times the rated current passes through a thermal relay, its operation may be delayed, taking at least five seconds. This delay can cause the heating element of the thermal relay to burn out before it operates.

As mentioned earlier, overcurrent is a state that generally involves currents smaller than short-circuit currents and within six times the rated value. In electrical circuits, short circuits occur far less frequently than overcurrent, especially during frequent motor starts and reversals.

It should be noted that when an overcurrent occurs in a circuit, if the current can return to normal before reaching the maximum temperature rise, the electrical components can still operate normally. However, overcurrent may also cause surge currents to damage the motor, as instantaneous electromagnetic torque can harm mechanical transmission components. Therefore, timely power cutoff is essential.

Overcurrent protection requires the use of overcurrent protection relays, installed in the protected circuit. When the current reaches a preset value, the relay activates, opening its normally closed contacts connected in series with the contactor coil circuit. This de-energizes the contactor coil, disconnecting the main contacts in the main circuit and promptly cutting off power to the motor.

Many are unaware that in power systems, if the insulation of electrical devices or wiring is compromised, issues such as incorrect wiring or load short circuits may occur, leading to short-circuit faults. During a short-circuit fault, the instantaneous current can be tens of times higher than the rated current, not just slightly above it. This can result in powerful electromagnetic forces damaging electrical equipment, generating arcs, and potentially causing severe consequences such as fires.

Thus, measures must be taken to mitigate the hazards of short-circuit faults, which is the purpose of short-circuit protection. This involves quickly cutting off the power supply after a fault occurs, commonly achieved by installing fuses or low-voltage circuit breakers in the circuit.