Effectively Connected Extraneous Metalwork

NAPIT’s Bill Allan offers advice on how to check that extraneous metalwork in a bathroom or shower room is effectively connected to the main earthing terminal.

Anyone who teaches the IET Wiring Regulations, BS 7671 will tell you that Regulation 701.415.2 on supplementary equipotential bonding in bathrooms or shower rooms can generate much discussion among students.

The first half of this long regulation tells us how supplementary equipotential bonding – we’ll refer to this as ‘supplementary’ bonding from now on – is to be carried out and this includes examples in items i, ii and iii. But the second half gives three conditions – iv, v and vi – which, if complied with, render supplementary bonding unnecessary.

One of these conditions – item vi – has long been a topic of discussion among conscientious electricians when deciding whether or not supplementary bonding can be omitted in such a location.

Item vi states that: “All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2.”

Regulation 411.3.1.2 requires that extraneous-conductive-parts are connected to the main earthing terminal (MET) by main protective bonding conductors. This regulation applies only to electrical installations which use Automatic Disconnection of Supply (ADS) as the protective measure against electric shock. ADS is the most commonly used protective measure and applies to most electrical installations.

This article will consider how to determine that the extraneous-conductive- parts are effectively connected to the MET in a bathroom or shower room.

It is a means of providing additional protection and involves connecting together the simultaneously accessible metalwork of an electrical installation, that is, the exposed-conductive-parts and the extraneous-conductive-parts (see Regulation 415.2).

By this means, the touch voltages between such items of metalwork in an earth fault condition are reduced.

This article is concerned only with the bonding – main bonding and supplementary bonding – of extraneous- conductive-parts.

ADS installation
The first sentence of the final paragraph in Regulation 701.415.2 reads: “Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2, supplementary equipotential bonding may be omitted where all of the following conditions are met.” The following three conditions are then given:

(iv) Compliance with Regulation 411.3.2
All the final circuits of the location must comply with the requirements of Regulation 411.3.2 for automatic disconnection within the required time in case of a fault. This is a requirement of the ADS protective measure.

(v) Compliance with Regulation 701.411.3.3
All the final circuits of the location must have additional protection by means of an RCD in accordance with Regulation 701.411.3.3 (see Regulation 701.411.3.3 for the exact wording). This is a requirement for rooms containing a bath or shower.

(vi) Compliance with Regulation 411.3.1.2
This refers back to the main bonding requirement of Regulation 411.3.1.2 and requires that a check be made to ascertain that all extraneous-conductive-parts of the location are “effectively connected” to the MET.

This check can be done visually or by means of a test instrument.

As all three conditions are requirements for bathrooms and shower rooms supplied by ADS systems, supplementary bonding will not normally be required.

It is possible that an extraneous-conductive-part, in the form of a metal water pipe, could enter the premises via the bathroom or shower room. In that case, the pipe will need to be connected by main protective bonding to the MET.

Where doubt exists about an extraneous conductive part 

There may be doubt if an item of metalwork within an electrical installation is an extraneous-conductive-part. For example, it may be suspected that plastic pipework has been installed somewhere along the run of metal pipework. Where there is such doubt, a test instrument can be used in conjunction with the following formula:

Rx ≥ U0
          Ilimit – Rb
Where,
Rx = the resistance between the metallic part in question and the MET;
U0 = the nominal voltage line to Earth;
Rb = the resistance of the human body;
Ilimit = the value of current through the
body which must not be exceeded.

The following values are given in British Standard Published Document PD 6519, IEC 60479, Guide to effects of current on human beings and livestock:

U0 = 230V;
Rb = 1,000 ohms (for hand-to-hand
contact in dry conditions);
Ilimit = 0.01A (the let-go threshold).

These values give the following maximum
value for Rx:

Rx ≥ 230
         0.01 – 1,000
      ≥ 23,000 – 1,000 ohms
      ≥ 22,000 ohms

A reading below 22,000 ohms will mean the metallic item is an extraneous-conductive-part which does require bonding.

Where doubt exists about a bonding connection
Where, after careful investigation, it isn’t possible to see a main bonding or a supplementary bonding connection, a low-resistance ohmmeter may be used to confirm that a bonding connection has been made.

A reading that’s ideally around 0.05 ohms – measured between any two metallic parts – is usually considered adequate to confirm that bonding is in place, but be aware that not all test instruments are capable of such low readings.

The 0.05 ohms figure is, in effect, a continuity test and should not be confused with the 22,000 ohms figure which is used to determine whether or not an item of metalwork is an extraneous-conductive-part.

Parallel paths
The existence of parallel earth return paths will tend to reduce continuity readings below their true values. In practice, these parallel paths can take various forms and it can be impractical to disconnect some or all of them, but it is something the contractor should be aware of.

Review of Regulation 701.415.2
It shouldn’t normally be necessary to install supplementary bonding in a room containing a bath or shower.

Where supplementary bonding is installed outside the room, it should preferably be installed close to the point of entry of extraneous-conductive-parts into the location. This may be done in an airing cupboard.

Requirements for the minimum cross-sectional areas of supplementary bonding conductors are found in Section 544.2 of BS 7671.

Further reading:
– IET Guidance Note 5, Protection Against Electric Shock.
– IET Guidance Note 7, Special Locations.
– IET Guidance Note 8, Earthing and Bonding.

All are available from the IET website: www.wiringregulations.net/IET/GuidanceNotes

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