The Hazard Of An Open-Circuit Neutral

[bankruatsteve]

If one is using the MEN system and a compliant earthing system that conforms with (for example) AS3000.

If the neutral conductor is broken the potential of the earthing system rises to full line voltage.

The touch voltage to earth by a person in contact with the earthing system will not exceed 50VAC. A mild tingling effect may be experienced.

If the touch voltage to earth does not exceed 50VAC, then how can the potential of the earthing system rise to full line voltage?

Equipotential bonding. The actual touch voltage is difficult to calculate but the main earth and equipotential bonding minimise the voltage rise to less than 50VAC.

The earthing system is at line voltage relative to the neutral at the source of supply, the transformer.

The touch voltage is the voltage between the earthing system and the general mass of earth.

OK, equipotential bonding. So then the main earth and equipment bonding limit the voltage rise (touch voltage) to less than 50VAC by design and presumably AS3000 for the MEN system. Since the neutral is also earthed at the source in the MEN system, seems to me that you can only have a voltage rise of 50VAC maximum relative to the neutral.

Huh? Equipotential bonding and MEN sound like the same thing to me. Where does 50VAC come from?

[InterestedObserver]

Huh? Equipotential bonding and MEN sound like the same thing to me. Where does 50VAC come from?

The touch potential, in this case an arbitrary 50VAC, comes from fault current flowing through the earth resistance (impedance), ground rod, equipment bonding, and conductors of the grounding circuit. During normal electrical system operation this touch potential is very low and not detectably by the human body when touching say a washing machine.

[electau]

If you could measure the voltage between the earthing system and the neutral of the transformer ( hypothetical) it would be at full line voltage when there is a O/C neutral.

The touch voltage is between the earthing system and the general mass of earth where a person is standing. This is limited to less than 50VAC.

If the main earth and earth bonding was to be disconnected the touch voltage would rise to full line voltage and a hazardous situation would be created.

Voltages up to 50VAC are regarded as non hazardous under normal conditions. Voltages over 50VAC are regarded as hazardous and automatic disconnection must occur.(AS3000).

Touch voltages only exist under fault conditions. It is the earthing system that minimises the effects of a open circuit neutral and reversed polarity with the MEN system.

Equipotential bonding is intended to minimise the risks associated with the occurrence of voltage differences between exposed conductive parts of electrical equipment and extraneous conductive parts.(AS3000). It is not quite the same as protective earthing.

Normally the voltage between the earthing system ( eg a washing mahine) and the general mass of earth will be minimal and close to 0 volts AC. Up to 1 to 2 volts AC as a guide.