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SNZ TR 6120

2014 Edition, July 4, 2014

Complete Document

Low voltage supply earthing systems

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Product Details:

  • Revision: 2014 Edition, July 4, 2014
  • Published Date: July 4, 2014
  • Status: Active, Most Current
  • Document Language: English
  • Published By: Standards New Zealand (SNZ)
  • Page Count: 56
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:


The supply of electricity to electrical installations at voltages that are above extra low voltage requires the adequate management of risks of serious harm to persons, mainly due to shocks, and significant damage to property, mainly through ignition of combustible material. Consequently, there is an onus on regulators, the electrical industry, and the electricity supply industry (ESI) to ensure that the safety-related risks are managed sufficiently well that a reasonable person will consider the residual risks to be acceptable.

Leaving aside the supply of electricity at high voltage, which requires very effective management of the associated safety related risks, the supply of electricity to electrical installations at low voltage (LV) (defined in the ESRs as ‘standard low voltage') still requires the exercise of care to ensure that the supply is as safe as may reasonably be expected. This implies that, if any fault develops that may increase the risk of serious harm to persons or significant damage to property, some protective action, such as the blowing of a fuse or the opening of a circuit breaker (CB) or residual current device (RCD), will take place to remove the supply or mitigate the effects of the fault to an acceptable level.

Central to the requirement for protection is the need for the supply system to be provided, or not provided, with a reference to earth. Various internationally recognised earthing systems for LV supplies are described in this report. In New Zealand, there is a mandatory requirement that electrical installations operating at standard low voltage must be connected to a multiple-earthed neutral (MEN) system, defined and described in detail in this report. However, the MEN system does not deliver ‘perfect' safety in all situations and locations and the use of other earthing systems should be considered as alternatives to the MEN system where their characteristics are such as to better manage the safety related risks in certain fault situations or locations.

This report includes the deliberations, decisions, and recommendations of a Low Voltage Supply Earthing Systems Committee established by Standards New Zealand specifically to examine the MEN system and any alternatives that the regulator might permit to be used if convinced that their use will better manage the risks associated with LV supply. The committee was charged with considering the likely performance of the MEN system and its alternatives for various foreseeable fault situations and supply locations and making any recommendations that it wishes the regulator to consider.

For the avoidance of doubt, the report does not advocate any widespread conversion of supplies to electrical installations from the MEN system to an alternative system but rather the selective use of the alternative system where there are clear benefits in the reduction of risks of serious harm or significant damage. In addition, other than pointing out the need to amend the ESRs and AS/NZS 3000 should government decide to permit the use of an alternative system in addition to the MEN system, the report does not include proposals for amending wording since these will be considered and recommended by other parties.