Q & A of the Day - Does this sub-main distribution panel need an earth bar?

Question: We have a sub-main distribution panel that has a connection facility for the incoming earth cable, which is then linked to the metal frame. However, it does not have an internal earth bar, so the outgoing cables rely on the metal casing of the panel as an earth path.

Is this acceptable, or should it have an internal earth bar?

Answer: Regulation 542.4.1 of BS 7671:2008 requires that a main earthing terminal (MET) be provided for every installation. The MET must connect the following to the earthing conductor of the installation (Regulation 542.4.1 refers):

(i) The circuit protective conductors.

(ii) The protective bonding conductors.

(iii) Functional earthing conductors (if required).

(iv) Lightning protection system bonding conductor, if any (see Regulation 411.3.1.2).

For practical reasons, the MET is normally situated at, or near, the origin of the installation (that is, the position where the electrical energy is delivered).

BS 7671:2008 does not stipulate a particular form for an MET, but leaves this to be dictated by the needs of the particular installation, subject to certain regulatory requirements being met. Hence, METs take a variety of forms, ranging from simple to complex, either within or separate from the main switchgear.

The MET must be selected to take account of the following requirements:

• Regulation 9 of the Electricity at Work Regulations requires that 'if a circuit conductor is connected to earth or to any other reference point, nothing which might reasonably be expected to give rise to danger by breaking the electrical continuity or introducing high impedance shall be placed in that conductor unless suitable precautions are taken to prevent that danger', and...
• ...sufficient terminals are to be provided to accommodate separately the earthing conductor and all other conductors referred to in Regulation 542.4.1, not forgetting future additions that may reasonably be expected, and...
• ...sufficient terminals are also to be provided for the separate connection of any duplicated protective conductors required by Regulation Group 543.7 for circuits supplying equipment having high protective conductor currents, and...
• ...the terminals are to be suitable for the conductor material, number and shape of wires, cross-sectional area, number of conductors to be connected together (for example ring final circuits), and the effects of temperature and vibration, as required by Regulation 526.2, and...
• ...the terminals are to be suitably arranged or marked so that the conductors connected to them can be identified for inspection, testing, repair or alteration of the installation as required by Regulation 514.1.2 (unless the conductors are identifiable by other means, such as markers attached to them), and...
• ...a means for disconnecting the earthing conductor for testing is to be provided as required by Regulation 542.4.2, and...
• ...where necessary, the MET and its connections are to be protected (eg, by an enclosure) against vandalism, unauthorized interference or other external influences such as impact, damp or dust, as required by Section 522, and...
• ...where the MET is separate from the main switchgear, a permanent label, with the words 'SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE', is to be fixed in a visible position at or near the MET, as required by Regulation 514.13.1.
  

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