Saturday, October 29, 2016

What is Multiple Earthed Neutral?

Multiple Earthed Neutral Earthing System Defined


Aus/NZ 3000 (known as Australian/New Zealand Wiring Rules) defined Multiple Earthed Neutral as


A system of earthing in which the parts of an electrical installation required to be earthed in accordance with this Standard are connected together to form an equipotentially bonded network and this network is connected to both the neutral conductor of the supply system and the general mass of earth. (AUS/NZ 3000:2007 Clause 1.4.66)

Source: AUS/NZ 3000:2007

The code explained that under this system the neutral conductor of the distribution system is earthed at the source of supply, at regular intervals throughout the system and at each electrical installation connected to the system. 

Within the electrical installation, the earthing system is separated from the neutral conductor and is arranged for the connection of the exposed conductive parts of equipment. 

Characteristic of MEN system

  • All exposed metal parts in the electrical system is earthed
  • There is a link that connects Neutral & Earth connected in the Main Distribution Panel
  • The neutral of the supply transformer must be earthed.
  • The soil where it is to be installed must be effectively conductive


What is the advantage of MEN System?


The main advantage of this system is it achieves lower earth fault loop impedance that can make circuit breaker operate quickly. This is because the exposed conductive parts and extraneous conductive parts are immediately electrically connected to the general mass of earth. Unlike other earthing system like TN-S where the earth connection can be traced back to the source, there is a risk of high earth fault loop impedance that can delay the operation of circuit protective devices in case of fault.

Risk of MEN earthing system


Equipment is still energized even neutral is disconnected


While it is rare to occur, there is specific problem under this system when the neutral of the supply side will be disconnected. In this case the equipment will still operate due to the continuous path offered by the earth connection while the potential 

The current flowing through the earth impedance will cause the protective earth in the system rise above the “outside” earth potential. In extreme cases this could approach the mains supply voltage. 

Possibility of current flow in bonding conductors


Another thing there is a possibility that large currents will flow through the bonding conductors giving rise to a potential fire risk. The magnitude of the current would depend on where the neutral break is, and the earthing conditions in the system.

Source:

·         AUS/NZ 3000:2007
·         Electrical Wiring Practice by: Keith Pethebridge and Ian Neeson
·         BS 7671

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