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In this regular section, we look at cases of poor-quality workmanship, mistakes in installation and shoddy practices. John Rothwell from ELECSA provides an insight on what can go wrong – and how best to avoid it

outbuilding. Also, the sub-main could only be isolated by removing the REC main fuse, which is not permitted and would involve removing the main seal, an operation only allowed to be undertaken by persons employed by the REC. Further investigation, and the results of an earth loop impedance test at the incoming terminals of the consumer unit, revealed that the disconnection time required by BS7671 for the 100Amp BS1361 fuse could not be satisfi ed, because of an excessively high earth fault loop impedance.

The outcome: The issue of the loss of the house supply because of a fault on the submain to the outbuilding is generally regarded as an inconvenience rather than a safety issue. However, the failure of the REC’s main fuse to operate within the required disconnection time was potentially dangerous and it could be considered fortunate that this did not result in an incident involving electric shock.

The installation: This was a lighting and power installation in an outbuilding, adjoining a domestic property, being utilised for breeding animals. A sub-main comprising a split concentric cable had been installed to supply a consumer unit, installed within the outbuilding adjacent to the front entrance door. The supply to this sub-main cable was taken from Henley blocks connected from the load side of the electricity meter, and the cable was cleated at high level along the front elevation of the house. The lighting and power comprised PVC insulated and sheathed cables installed in plastic conduit run surface on walls. The installation had been made live but had not been fully completed.

About the author

John Rothwell John Rothwell IEngMIET is a senior inspector in ELECSA’s inspections team and has been working in the electrical industry for 42 years.

What was wrong: This is a common method of supplying an outbuilding, but the problem was the protection afforded to the sub- main cable against fault current and overcurrent, and the means of isolation of this cable. The sub-main cable was afforded protection only by the REC’s 100Amp BS1361 cartridge fuse in the supply cut out, and this fuse was the only means of isolating the sub-main. Operation of the REC main-fuse, because of overcurrent or fault current in the sub-main cable, would result in the supply being disrupted to the house and the

60 ECA Today May 2011

What should have been done: BS7671:2008 requires: 1) Every installation to be divided into circuits as necessary to avoid hazards and minimise inconvenience in the event of a fault (Regulation 314.1).

2) Due account shall be taken of the consequence of the operation of a single protective device (Regulation 314.2).

3) In a TN system, a disconnection time not exceeding fi ve seconds is permitted for a distribution circuit (Regulation 411.3.2.3).

4) The distributor (REC) agrees that their main fuse provides overload and fault protection between the origin of the installation and the main distribution point where further protection is provided, and thus a consumers’ main protective device(s) is/are not required (Regulations 433.3.1/434.3).

5) Each installation shall have provision for disconnection from the supply (Regulation 537.1.3).

6) A main linked switch or linked circuit breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation (Regulation 537.1.4).

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