3.4 Code of Practice requirements
3.4.1 The Multiple Earthed Neutral (MEN) system
3.4.2 Earthing of LV distribution centres
3.4.3 Exposed conductive parts within 2.4 metres above ground
3.4.4 The Common Multiple Earthed Neutral (CMEN) system
3.4.5 Earthing at a distribution centre (CMEN)
3.4.6 Exposed conductive parts within 2.4 metres above ground associated with high voltage in a CMEN system
3.4.7 The Single Wire Earth Return (SWER) system
3.4.8 Exposed conductive parts within 2.4 metres above ground associated with high voltage in a SWER system
3.4.9 Earthing of high voltage distribution centres
3.4.10 Exposed conductive parts within 2.4 metres above ground associated with high voltage transmission systems
3.4.11 Exposed conductive parts within 2.4 metres above ground associated with low voltage only
3.4.12 Induced voltage on the sheath of underground electric lines
3.4.13 Exposed conductive parts higher than 2.4 metres above ground
3.4.1 The Multiple Earthed Neutral (MEN) system
Where an MEN earthing system is used, the following conditions should be met:
- The neutral conductor should be earthed at or near each LV distribution centre; and
- Where necessary, the neutral should be earthed at other points along the distribution system.
- The earthing should be arranged so as to ensure that resistance of neutral to earth at any location does not exceed 10 ohms.
In the MEN system the high voltage and low voltage earthing systems should be kept separate.
The high voltage earthing system provides an earth return path for the high voltage system. The earthing system protects plant and equipment capable of being energised by the high voltage system, eg surge arrester or transformer tank. This earth should always be insulated from the low voltage earth.
Where the LV distribution centre is in a remote location, permanently electrically isolated from other distribution centres and is rated at not more than 63 kVA, the limit of 10 ohms may be increased provided that:
- Where there are bare high voltage conductors above the low voltage conductors of an overhead electric line, the low voltage conductors must be insulated with 0.6/1 kV grade insulation; and
- the high voltage protection will clear any high voltage to low voltage fault in less than 0.2 seconds.
3.4.2 Earthing of LV distribution centres
If an MEN system is used, the low voltage earthing system should be separate and distinct from the high voltage earthing system for the distribution centre. The earthing systems should be designed and installed to prevent any significant portion of the high voltage system voltage gradient being superimposed on the low voltage earthing system and as a result, transferred to the customer’s electrical installation through the MEN system.
- When the two earthing systems are located where a person may make contact between the earthed exposed conductive parts or conductors connected to the earthing systems, adequate separation or insulation should be provided between the exposed conductive parts or conductors or both.
The following should be connected to the high voltage earthing system:
- transformer tank or tanks;
- HV surge protection devices;
- HV cable sheaths/screens/guards;
- exposed conductive parts of all other HV equipment.
The design and impedance to remote earth of the high voltage earthing system should be arranged so that:
- the HV protection system will operate in the event of a high voltage to exposed conductive parts fault; and
- the prospective touch voltage on uninsulated conductive parts up to 2.4 metres above ground level and prospective step voltages meet requirements set down in the prospective touch and step voltages section.
The low voltage earthing system should be bonded to:
- the neutral terminal of the distribution transformer, and
- any earth leads of low voltage surge protection devices at the distribution centre.
The impedance to remote earth of the low voltage earthing system should not exceed the value for the high voltage earthing system.
3.4.3 Exposed conductive parts within 2.4 metres above ground associated with high voltage in an MEN system
Exposed conductive parts that may become energised from the electricity supply system if there is an insulation failure or contact with a conductor, should be:
- insulated; or
- earthed to a separate and distinct earthing system from the high voltage earthing system. The impedance to remote earth should allow the HV protection system to operate in the event of a high voltage to exposed conductive part fault; and
- be designed so that the prospective touch and step voltages meet the requirements set down in the prospective touch and step voltages section.
3.4.4 The Common Multiple Earthed Neutral (CMEN) system
If an electricity entity uses a Common Multiple Earthed Neutral system, the low voltage neutral conductor, and the low voltage earthing system, should be connected to the high voltage earthing system. This requirement includes the earthing system of transformer stations, zone substations and at poles carrying exposed conductive parts associated with high voltages.
The CMEN system should only to be used for distribution voltages up to and including 33 kV and where the design limits prospective touch voltages – including within any part of the associated LV installations – to within curve A1 of Figure 2.
The resistance to ground of the LV neutral at any location should be no greater than 1.0 ohm.
3.4.5 Earthing at a distribution centre (CMEN)
If a CMEN system of earthing is used at a distribution centre the following should be connected to it:
- the transformer tank and any high voltage surge protection devices;
- the low voltage neutrals and earth leads of low voltage surge protection devices;
- any exposed conductive parts that may reasonably be expected to become energised from the electricity supply system if there is an insulation failure or contact with a conductor;
- a separate earthing system with a resistance to earth that will ensure the HV protection system operates in the event of a high voltage to exposed conductive parts fault and where the low voltage neutral may be disconnected.
3.4.6 Exposed conductive parts within 2.4 metres above ground associated with high voltage in a CMEN system
Exposed conductive parts that may reasonably be expected to become energised from the electricity supply system if there is an insulation failure or contact with a conductor, should be:
- insulated; or
- bonded to the CMEN system low voltage neutral.
3.4.7 The Single Wire Earth Return (SWER) system
If an electricity entity uses the single wire earth return earthing system, a separate high voltage earthing system and low voltage earthing system should be used. Thus, for each installation, there will be only a low voltage earth electrode system at the transformer and an earth electrode at the customer’s premises earthing the low voltage neutral.
The high voltage and low voltage earthing systems should be separated to avoid any significant portion of the SWER earthing system voltage gradient being superimposed on the low voltage earthing system and as a result, transferred to the customer’s electrical installation through the MEN system.
Where the two earthing systems are located so that personal contact may be made between the earthed exposed conductive parts or conductors connected to the earthing systems, adequate separation or insulation should be provided between the exposed conductive parts or conductors or both. This does not apply to spark gap devices fitted to protect the transformer against lightning damage.
The earthing of SWER distribution centres should be designed for continuous passage of electric current, in addition to protective earthing.
For a SWER distribution centre, two separate and distinct earthing systems should be provided:
- a SWER earthing system; and
- a low voltage earthing system.
The following exposed conductive parts should be connected to the SWER Earthing System:
- transformer tank or tanks;
- HV earth bushing;
- HV surge protection devices; and
- exposed metalwork of all other HV equipment.
The SWER earthing system should be designed so that:
- at least two earthing conductors are installed with maximum separation. The conductors should:
- be connected to an inter connected earthing system that consists of at least three earthing electrodes not less than three metres apart; and
- arranged so that in the event of one or two earthing conductors between two electrodes being severed, at least one earth path will remain.
- any joint in the SWER earthing conductors between the transformer terminals and the earth electrodes should not be disconnectable;
- at a SWER LV distribution centre, earthing conductors within 2.4m of the ground should be insulated with 0.6/1.1 kV grade insulation and must be mechanically protected; and
- the maximum voltage on the earth lead with respect to remote earth under operating conditions resulting in maximum continuous earth current, should not exceed 20 Volts.
The low voltage earthing system should be bonded to:
- the low voltage neutral terminal of the transformer, and
- any earth leads of low voltage surge protection devices at the distribution centre.
The design and impedance to remote earth of the low voltage earthing system should be arranged so that the MEN system complies as documented in the MEN section.
3.4.8 Exposed conductive parts within 2.4 metres above ground associated with high voltage in a SWER system
Exposed conductive parts that may reasonably be expected to become energised from the electricity supply system if there is an insulation failure or contact with a conductor, should be insulated.
3.4.9 Earthing of high voltage distribution centres
Earthing of exposed conductive parts of HV distribution centres including external metal fences should comply with the ESAA Substation Earthing Design Guide and the Institute of Electrical and Electronics Engineers Guide for Safety in AC Substation Grounding – IME Std. No. 80. Earthing of Works other than distribution centres.
3.4.10 Exposed conductive parts within 2.4 metres above ground associated with high voltage transmission systems
Exposed conductive parts that may reasonably be expected to become energised from the electricity transmission system in the event of failure of insulation or contact with a conductor, should be designed so that the prospective touch and step voltages do not exceed the allowable prospective touch and step voltages.
3.4.11 Exposed conductive parts within 2.4 metres above ground associated with low voltage only
Exposed conductive parts that may reasonably be expected to become energised from the electricity supply system in the event of failure of insulation or contact with a conductor, should be either insulated or bonded to the low voltage neutral of the system.
3.4.12 Induced voltage on the sheath of underground electric lines
Cable sheaths or screens should be earthed to ensure that the prospective touch voltages that may appear on any accessible exposed conductive parts of the underground electric line for both load and fault current conditions, meet the requirements set down in the prospective touch and step voltages section.
3.4.13 Exposed conductive parts higher than 2.4 metres above ground
Non-current carrying exposed conductive parts located 2.4m or higher above ground and not exposed to personal contact need not be earthed. If these parts are associated with high voltage or low voltage works or both and are required by the electricity entity to be earthed, these parts should be earthed in accordance with sections on high voltage in an MEN system, CMEN system or low voltage only systems.
Last updated July 7, 2005
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