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Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "current limiting" is also used to define a type of overcurrent protective device.
Circuit-breakers can be placed at portions of a circuit in series to the path of current it will affect. If more current than expected goes through the circuit-breaker, the circuit breaker "opens" the circuit and stops all current. A fuse is a common type of circuit breaker that involves direct effect of Joule-overheating.
Lack of overcurrent capability (low short circuit ratio) of a weak grid creates a multitude of problems, including: [2] transients during the large load changes will cause large variations of the grid voltage, causing problems with the loads (e.g., some motors might not be able to start in the undervoltage condition);
A circuit breaker or protection relay may fail to operate. In important systems, a failure of primary protection will usually result in the operation of back-up protection. Remote back-up protection will generally remove both the affected and unaffected items of plant to clear the fault.
Two thermal switches (Thermal Cut) Schematic symbol for a thermal overload switch A thermal switch (sometimes thermal reset or thermal cutout (TCO)) is a device which normally opens at a high temperature (often with a faint "plink" sound) and re-closes when the temperature drops.
When the circuit is closed, the thermistor's resistance limits the initial current. After some time, current flow heats the thermistor, and its resistance changes to a lower value, allowing current to flow uninterrupted. It is inherently impossible for 100% of supply voltage to appear on the protected circuit, as the thermistor must continue ...
Fuses and circuit breakers are two commonly used mechanisms for overload protection. [4] A fuse contains a short piece of wire which melts if too much current flows. This effectively disconnects the power supply from its load, and the equipment stops working until the problem that caused the overload is identified and the fuse is replaced.
The selection of over-current-protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.