Earthing an outbuilding: earth or its own MEN

A shed, garage or sleep-out with its own switchboard needs its metal parts earthed. AS/NZS 3000 gives two ways to do it. This page explains which buildings may use which, and what each needs.

Individual and combined outbuildings#

• An individual outbuilding (Cl 1.4.88) is a structure with a switchboard, standing on its own ground apart from the building whose switchboard supplies it. A detached shed is the usual case.
• A combined outbuilding (Cl 1.4.89) is two or more structures on one foundation, or sharing a metal roof or a metal frame, with more than one supply, standing apart from the supplying building. Two sheds on one slab are an example.

The two arrangements#

Cl 5.5.3.1 sets which arrangement each may use.

• (a) An individual outbuilding may take either:
  • (i) a protective earthing conductor run with the submain, connected at the main switchboard; or
  • (ii) a separate MEN installation, under item (c).
• (b) A combined outbuilding takes a protective earthing conductor with the submain only. It may not be a separate MEN installation.

A separate MEN#

In a separate MEN installation the submain has no earth. Its neutral carries the fault current as well as the load current, so it is a combined protective earth and neutral, a PEN conductor. Item (c) then sets these conditions, in short:

1. One MEN connection at most in the outbuilding.
2. The outbuilding board is treated as a main switchboard for the MEN link.
3. The lead from that board to its electrode is treated as a main earthing conductor.
4. The submain runs straight from the main switchboard, or through other outbuilding boards to one board only in this outbuilding.
5. Where it runs through other outbuilding boards, their terminals are not relied on for the PEN.
6. The PEN should not be put in parallel with metal pipes or structural metal by earth or bonding conductors.

Item 6 matters most on a farm or a section with a metal water pipe between the buildings. The pipe can carry part of the neutral current. An earth with the submain avoids that path.

The conductor sizes#

• The protective earthing conductor with the submain comes from Table 5.1, by the active size. A 16 mm² copper submain takes 6 mm².
• The PEN conductor of a separate MEN must meet the neutral size rules and be no smaller than the earthing conductor (Cl 3.5.2(c)). On a single-phase 16 mm² submain that is 16 mm².
• The main earthing conductor to the outbuilding's electrode also comes from Table 5.1.

Open a 16 mm² submain as a separate MEN.

The electrode#

Table 5.2 lists the electrodes, with their least dimensions and coating. A driven rod is the common choice: copper-clad or copper-plated steel at 12 mm diameter with 250 µm of copper, stainless at 12 mm with 500 µm, or galvanized steel at 16 mm. A strip, rod, pipe or bare cable laid in a trench is the other kind.

Cl 5.3.6.3 sets how deep it goes:

Work out an outbuilding with the outbuilding earthing tool. For the main earth and bonds of the house, use the earthing calculator. A worked shed is in Shed earthing on a 16 mm² submain.