The protective measure Automatic Disconnection of Supply (ADS) is used in most electrical installations to provide protection against electric shock. This article provides an overview of how ADS is achieved in a typical domestic premises.
The general requirements for ADS consist of provisions for basic and fault protection, which are considered as follows (Regulation 411.1 of BS 7671 refers):
Basic protection is required to prevent contact with live parts and is generally verified by inspection. In domestic premises, contact with live parts may occur as a result of damage to cable insulation or enclosures or barriers are missing. For example, the possibility of contact with live parts may exist where a suitable barrier (as shown in Fig 1) has not been fitted in the spare ways in a consumer unit or unused entry holes in the enclosure are not suitably closed. For these reasons, all electrical equipment should comply with at least one of the provisions for basic protection described in Section 416 (Regulation 411.2 refers).
For a domestic installation, the basic requirements for protection against the ingress of solid objects are that:
- barriers or enclosures should provide a degree of protection against solid objects of at least IP2X or IPXXB (Regulation 416.2.1).
- the horizontal top surface of a barrier or enclosure which is readily accessible must provide a degree of protection against solid objects of at least IP4X or IPXXD (Regulation 416.2.2).
To satisfy compliance an understanding of IP codes is required.
IP2X means that the enclosure is protected against access to hazardous parts with a British Standard test finger having a diameter of 12 mm and 80 mm long, and will not permit the insertion of any object 12.5 mm or more in diameter.
IP4X means that at no point on the surface must the insertion of a wire or object greater than 1 mm thick be possible.
In areas that present an increased risk of electric shock, such as rooms containing a bath or shower, the particular requirements of BS 7671 (Section 701), must also be satisfied.
For the protective measure ADS, fault protection should be provided in accordance with Regulation Group 411.3. The requirements for protective earthing, protective equipotential bonding and automatic disconnection in case of fault should be satisfied. For domestic premises, earth fault loop impedance testing is the most common method used to verify circuit disconnection.
For protection against electric shock, the magnitude of the fault current needs to be sufficient to cause the protective device to automatically disconnect the circuit within the relevant maximum time specified in BS 7671. In order to verify compliance with BS 7671, knowledge of the earth fault loop impedance is required for each circuit that relies on ADS as the protective measure against electric shock (Regulation 612.9 refers).
Where it is safe to do so, earth fault loop impedance tests should be carried out at the following points within an installation:
- The origin
- The furthest point of every distribution circuit
- The furthest point of every final circuit
A test is carried out at the origin of the installation to determine the external earth fault loop impedance Z,e which, forms part of the earth fault loop impedance of every circuit connected to the installation (Fig 2 refers).
The purpose of the test is to confirm that the intended means of earthing is present and its measured impedance value is appropriate for the type of supply. Prior to carrying out the test, the installation should be isolated from the supply and the earthing conductor should be disconnected from the main earthing terminal or otherwise separated from all parallel earth paths such as bonding and circuit protective conductors.
Before the installation is re-energised, the earthing conductor must be reconnected
Distribution Network Operators (DNOs) publish typical maximum impedance values for each supply type, for example, 0.8 Ω for TN-S and 0.35 Ω for TN-C-S supplies. In circumstances where the measured value of Ze, is significantly higher than the typical maximum value, the person ordering the work should be advised to inform the relevant DNO.
For each distribution and final circuit the test should be carried out at the furthest point. Tables 41.2, 41.3 and 41.4 of BS 7671 give maximum permitted values of Zs for different types and ratings of overcurrent protective devices in common use and different maximum permitted disconnection times. Table 41.5 gives maximum permitted values of Zs for RCDs installed to provide earth fault protection.
Verification of test results
However, before comparing measured values of Zs with the maximum permitted values given in BS 7671, the test values should be adjusted to compensate for conductor temperature (that is the difference between the temperature of conductors during testing and fault conditions). Where the measured value does not exceed 0.8 times the relevant tabulated value given in BS 7671, the requirements for disconnection are satisfied (Appendix 14 of BS 7671 refers).
Alternatively, a calculated value of Zs may be obtained for a circuit by adding the (R1 + R2) test result obtained during continuity testing to the external earth fault loop impedance test result.
Where an RCD is installed in a circuit to provide fault protection, it must be tested to verify satisfactory operation. The circuit should also incorporate an overcurrent device (Regulation 411.4.4 refers).
The NICEIC and ELECSA publication; Inspection, Testing and Certification (7TH Edition) provides practical guidance on inspection and testing.