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This article discusses the protective measure Automatic disconnection of supply (ADS), typically used in most electrical installations as a means of providing protection from electric shock. Also included is an overview how ADS may be verified in an installation.
Introduction
Regulation 411.1 of BS 7671 states that ADS is a protective measure which consists of provisions for basic and fault protection as described below:
Basic protection
Basic protection is necessary in preventing contact with live parts and is usually verified during an inspection. The risk of contact with live parts may occur as a result of damage to cable insulation or enclosures or because of missing barriers.
For example, it is not uncommon to find a metal-clad consumer units with all the ‘knock-outs’ removed or the absence of a suitable barrier for an unused spare way, as shown in Fig 1.
It is a requirement of Regulation 411.2 that all electrical equipment should comply with at least one of the provisions for basic protection described in Section 416 and where appropriate 417.
The basic requirements for protection against the ingress of solid objects are that:
– barriers or enclosures should provide a degree of ingress 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 IPx4 or IPXXD, Regulation 416.2.2
IP2X denotes that the enclosure is protected against access to hazardous parts when tested with a British Standard test nger having a diameter of 12 mm and length of 80 mm. Such a test confirms that any object 12.5 mm or greater in diameter is not liable to insertion.
IPXXB describes a level of protection whereby a standard test finger 80mm long and 12 mm in diameter can enter an enclosure, but there will be adequate clearance from live parts.
IP4X denotes that at no point on the surface of an enclosure must the insertion of a wire or object greater than 1 mm in diameter be possible.
IPXXD indicates that where the insertion of a wire or object 100 mm long and 1 mm in diameter is possible, adequate clearance from live parts is provided.
For areas where there is an increase in risk of electric shock, any particular requirements pertinent to that installation or location must be satisfied. For example, in a room containing a bath and/or shower, the particular requirements of Section 701 supplement the requirements of Part 1-6 of BS7671.
For the protective measure of ADS, fault protection should be provided in accordance with Regulation Group 411.3, in which the requirements for protective earthing, protective equipotential bonding and automatic disconnection of supply under fault conditions should be satisfied.
For protection against electric shock, the magnitude of the fault current needs to be sufficient to cause automatic disconnection of the protective device for the circuit within the maximum permitted time specified in BS 7671.
Verification of fault protection
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 643.7.3 refers.
Where it is safe to do so, earth fault loop impedance testing should be carried out at any relevant point within the installation, including:
-The origin.
-The furthest point of every distribution circuit.
-The furthest point of every final circuit.
Carrying out a test at the origin of the installation determines the integrity of the external earth fault loop impedance Ze which forms part of the earth fault loop path for every circuit within the installation, as shown in Fig 2.
Carrying out an earth fault loop impedance test will confirm that the intended means of earthing is present and its measured value of impedance is appropriate for the type of supply. However, before undertaking such a 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 other bonding and circuit protective conductors likely to form parallel Earth paths.
NOTE, before the installation is re- energised the earthing conductor must be reconnected.
The maximum external earth loop impedance values measured or otherwise determined in accordance with Regulation 313.1, may differ from the values published by the Distribution Network Operator (DNO) for each type of supply, for example:
– 0.8 Ω for TN-S,
– 0.35 Ω for TN-C-S.
Where the measured values of Ze significantly exceeds those stated, the person ordering the work must inform the relevant DNO.
Earth fault loop impedance testing remains the most common method used to verify circuit disconnection under earth fault conditions.
Earth loop impedance of distribution and final circuits
The test should be carried out at the furthest point of each distribution and final circuit to confirm that the value of line-earth loop impedance Zs is suitably low to achieve ADS for the circuit within the relevant maximum time specified in Regulation Group 411.3.2.
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 device in addition to maximum permitted disconnection times.
Where the protective device is a non-time delayed RCD the maximum values of Zs can be found from Table 41.5. The values in Table 41.5 are intended for a TT system however, these may also be applied to a TN system.
Verification of test results
Before comparing measured values of Zs with the maximum permitted values given in BS 7671, the measured values obtained should be adjusted to allow for an increase in conductor temperature, which is the difference between the temperature of conductors during the test and under fault conditions.
The requirements for disconnection are satisfied when the measured value does not exceed 0.8 times the relevant tabulated value as described in Appendix 3 of BS 7671.
To conform to Regulation 14 of the Electricity at Work Regulations 1989, unnecessary work carried out on or near live conductors should be avoided where ever possible.
As an alternative to taking a direct measurement and to minimise the risk of contact with live parts a calculated value of Zs may be acquired for a circuit by adding the (R2 + R1) test results obtained during continuity testing to the external earth fault loop impedance Ze test result.
In addition, it is recognised that where an RCD is installed Regulation 643.7.1 permits ADS to be verified by confirming the effectiveness of the RCD using suitable test equipment. This method of verification excludes the need to obtain further values of Zs. However, the related circuit should also incorporate an overcurrent protective device, Regulation 411.4.5 refers.
Summary
Automatic disconnection of supply (ADS), is typically used in most electrical installations as a means of providing protection from electric shock and is made up of two parts; basic protection and fault protection. Basic protection is a preventive measure against coming into contact with live parts. Fault protection requires the protective devices to operate within a specified time under fault conditions.
Carrying out an earth fault loop impedance test is the most common method used for verifying circuit disconnection, although this may increase the risk of unnecessarily coming into contact with live parts.
Where an RCD is provided upstream to the protective device, verification of ADS maybe achieved by confirming the effectiveness of the RCD.
* Guidance on an appropriate safe isolation procedure is outlined in the Electrical Safety First publication Best Practice Guide 2
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