Maximum prospective short circuit current
Encyclopedia
The prospective short circuit current (PSCC) is the highest electric current
which can exist in a particular electrical system under short-circuit
conditions. It is determined by the voltage
and impedance
of the supply system. It is of the order of a few thousand ampere
s for a standard domestic mains
electrical installation, but may be as low as a few milliamperes in a SELV (Safety Extra Low Voltage or, occasionally, Separated Extra Low Voltage) system or as high as hundreds of thousands of amps in large industrial power systems.
and circuit breaker
s must be chosen so that they can safely break the current of a short circuit. When a large electric current is interrupted an arc
forms, and if the breaking capacity
of a fuse or circuit breaker is exceeded, it will not extinguish the arc. Current will continue, resulting in damage to equipment, fire or explosion.
In designing domestic power installations the short-circuit current available on the electrical outlets should not be too high or too low. The effect of too high short circuit current is discussed in the previous paragraph. The short circuit current should be around 20 times the rating of the circuit to ensure the branch circuit protection clears a fault quickly. Quick disconnecting is needed, because in live-ground short circuit the grounding pin potential on the power outlet can rise compared to the local earth (concrete floor, water pipe etc.) during short circuit to a dangerous voltage, which needs to be shut down quickly for safety. If the short circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual current device (sometimes called a ground fault interrupter) for extra protection.
The short circuit current available on the electrical outlets is often tested when inspecting new electrical installations to make sure that the short circuit current is within reasonable limits. A high short circuit current on the outlet also shows that the resistance from the electrical panel to the outlet is low, so there won't be unacceptably high voltage drop
on the wires under normal load.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
which can exist in a particular electrical system under short-circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
conditions. It is determined by the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of the supply system. It is of the order of a few thousand ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s for a standard domestic mains
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
electrical installation, but may be as low as a few milliamperes in a SELV (Safety Extra Low Voltage or, occasionally, Separated Extra Low Voltage) system or as high as hundreds of thousands of amps in large industrial power systems.
Importance
When designing an electrical installation fusesFuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
and circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s must be chosen so that they can safely break the current of a short circuit. When a large electric current is interrupted an arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
forms, and if the breaking capacity
Breaking capacity
Breaking capacity or interrupting capacity is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration. The prospective short circuit current which can occur under short circuit...
of a fuse or circuit breaker is exceeded, it will not extinguish the arc. Current will continue, resulting in damage to equipment, fire or explosion.
In designing domestic power installations the short-circuit current available on the electrical outlets should not be too high or too low. The effect of too high short circuit current is discussed in the previous paragraph. The short circuit current should be around 20 times the rating of the circuit to ensure the branch circuit protection clears a fault quickly. Quick disconnecting is needed, because in live-ground short circuit the grounding pin potential on the power outlet can rise compared to the local earth (concrete floor, water pipe etc.) during short circuit to a dangerous voltage, which needs to be shut down quickly for safety. If the short circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual current device (sometimes called a ground fault interrupter) for extra protection.
The short circuit current available on the electrical outlets is often tested when inspecting new electrical installations to make sure that the short circuit current is within reasonable limits. A high short circuit current on the outlet also shows that the resistance from the electrical panel to the outlet is low, so there won't be unacceptably high voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...
on the wires under normal load.