Overcurrent and Overload in Electricity Metering
I. What is Overcurrent?
Overcurrent, simply put, is a situation where a circuit experiences excessive current due to overload (especially a short circuit). Once a short circuit occurs, a massive current surges into the circuit, causing the load voltage to drop to almost zero, easily leading to insulation damage, fire, equipment failure, and even explosions!
For example: When a 125A circuit breaker (magnetic trip rated trip point 200%) is connected to a 100A load circuit, the circuit breaker will trip when the load current reaches the 125A limit; if the current increases to 200A (125A × 200%), the circuit breaker will immediately trip, quickly protecting the circuit from overcurrent damage caused by the short circuit.
II. Overcurrent Protection
This refers to protective measures to prevent equipment damage from overcurrent (such as short circuits). Common devices include fuses, circuit breakers, overcurrent relays, current limiters, temperature sensors, and solid-state power switches. Thermal-magnetic trip circuit breakers provide both overcurrent and overload protection.
III. What is Overload?
Overload occurs when the current in a circuit exceeds its normal rated current; essentially, it's a form of "prolonged overcurrent."
For example:
1. A power line has a rated safe current carrying capacity of 20A. According to regulations, a circuit breaker with a rating of ≤25A (20A × 125%) should be used for protection. If a 30-35A circuit breaker is mistakenly used, it will allow an overrated current to flow through the wire, causing it to overheat and catch fire, damaging the circuit or equipment, while the circuit breaker will not trip (incorrect rating).
2. A 1kW generator/inverter/transformer connected to a 1.5kW load, with the current exceeding the rated value by 1.5 times, is also considered an overload.
IV. Key Difference Between Overload and Overcurrent
Overcurrent: Typically exceeds twice the rated current;
Overload: Typically exceeds approximately 1.5 times the rated current and persists for a certain period.
V. Overload Protection
The core of overload protection is to prevent the circuit from overheating due to prolonged overcurrent. Commonly used devices include:
Slow-blow fuses, overload relays; thermal-magnetic circuit breakers ("magnetic element" protects against overcurrent, "thermal element" protects against overload; the larger the current, the faster the trip).
Generally, the overload protection circuit will activate when the initial current flowing in the circuit is 120% to 160% greater than the rated current of the power supply.
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