Neutrals overcurrent protection

All live power conductors, except the earthed neutral, shall employ overcurrent protection selected as low as possible but adequate for the anticipated start-up currents. Control circuits conductors connected directly to supply voltages and circuits feeding control circuit transformers shall be protected. Refer to EN 60204-1 for more information. 

The consumer s mains equipment must provide protection against overcurrent, that is, a current exceeding the rated value (lET Regulation 430.3). Fuses provide overcurrent protection when situated in the live conductors they must not be connected in the neutral conductor. Circuit-breakers may be used in place of fuses, in which case the circuit-breaker may also provide the means of isolation, although a further means of isolation is usually provided so that maintenance can be carried out on the circuit-breakers themselves. 

One purpose of grounding the neutral in a three-wire system is to activate overcurrent protection devices before damage is done when a fault occurs. Should one of the two wires that normally carries current be broken or accidentally grounded, current will flow through the neutral, through the installed ground, and back to the power source. This short circuit will open the protection devices and de-energize the affected portion of the system. 

A new Regulation 473-03-03 provides for the use of RCD rather than overcurrent protection in IT systems in which the neutral is not distributed. 

Circuit protection devices, such as fuses, breakers, or fault-interrupters, may be required in case of excessive current draws as a result of a short circuit, overcurrent, or earth fault. Several options exist with circuit breakers preferred over fuses. In some of the newer devices several functions are combined into one, thereby reducing the total number of components. An example of this is a combined power-switch/circuit-breaker. The switch/breaker is used as a switch and breaker and senses each line and opens all lines, except the grounded line, simultaneously when a fault current is present. Also, the switch/breaker looks and functions as a standard power ON/OFF rocker switch. In some standards simultaneous interruption of the phase and neutral is required, thereby, precluding the use of fuses. 

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