During periodic inspections of domestic electrical installations, I often find that cables that are concealed in walls at a depth of less than 50 mm have no additional protection by means of an RCD, as is now required for installations complying with the 17th Edition. What Recommendation Code should I give?

Code 4. Although existing installations need to be assessed against the requirements of the 17th Edition, this does not necessarily mean that they require upgrading, unless a departure from the latest requirements constitutes an immediate or potential danger.

Where can I find the limiting values of measured earth fault loop impedance for BS 3871 circuit breakers?

Click here!

What Recommendation Code should be given if it is found that there is no supplementary bonding in a bathroom having extraneous- or exposed-conductive-parts, and the conditions given in Regulation 701.415.2 for its omission are not met?

Recommendation Code 2

Is it necessary to verify voltage drop during a periodic inspection?

Verification of voltage drop is not normally required unless there is considered to be a voltage drop problem.

Where can I find general guidance on the use of Recommendation Codes for domestic periodic inspection reports?

General guidance on the use of Recommendation Codes is available in the Electrical Safety Council’s Best Practice Guide No 4, which may be viewed or downloaded free of charge by clicking here

What should be recommended if, during a periodic inspection, the safety of an installation forming part of a TT system is found to be relying on a voltage-operated earth-leakage circuit-breaker (VOELCB) for fault protection?

If a VOELCB on a TT system fails to operate when tested with an instrument or integral test button, this would normally warrant a Code 1 recommendation (requires urgent attention).

But, subject to the VOELCB being proved to operate correctly, continued reliance on it for fault protection (protection against indirect contact) would normally warrant a Code 4 recommendation.

However, if the VOELCB relies on a water pipe not permitted by Regulation 542.2.4 to be a means of earthing, this should attract a Code 2 recommendation (requires improvement).

What should I recommend in a periodic inspection report if I find that an existing installation still relies for fault protection on a voltage-operated earth-leakage circuit-breaker?

If the device fails to trip when the integral test button is operated, the device should be replaced with RCD protection as a matter of urgency (Recommendation Code 1).

If the device trips on operation of the test button, it may remain in service (Recommendation Code 4).

However, the user should be given clear instructions to test the device at quarterly intervals by means of the integral test button.

The user should also be advised that the voltage-operated device should be replaced with RCD protection at the earliest opportunity to enhance the safety of the installation.

In an existing domestic installation, are 16 mm² single-core pvc/pvc meter tails (having a current-carrying capacity of 87 A clipped direct) adequate if the rating of the distributor’s BS 1361 cut-out fuse is 100 A?

This size of tails may be adequate provided the maximum demand of the installation, taking into account the load characteristics (diversity and load profile), does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met.

As a practical tip, the current-carrying capacity of the meter tails is likely to be adequate if they have been in service for several years and there are no signs of thermal damage at the terminations or to the cable sheath or insulation.

In an existing domestic installation, is a 10 mm² or 6 mm² earthing conductor adequate if the rating of the distributor’s BS 1361 cut-out fuse is 100 A?

This size of earthing conductor will be adequate if the requirements of Regulation 543.1.3 (adiabatic equation) are met.

However, if the supply is PME, the earthing conductor is usually required to be at least 10 mm²
(but see EDQ12 and EDA12)

What is the correct sequence for testing RCDs?

Preferably, RCDs should be tested in the sequence of: x1I_?n, x5I_?n (if required for additional protection), followed by x0.5 I_?n and then finally the test button trip.

However, some automated test instruments test in the sequence of: x0.5 I_?n followed by the x1I_?n test, and then x5I_?n test (if required for additional protection).

In any case, the test button should be operated last in the test sequence.