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
The installation:
This was a total refurbishment of a cottage, including a full re- wire of the electrical installation. At the time of the inspection, fi rst fi xing was in progress, and provision had been made for the use of 230V power tools by the installation of temporary BS1361socket outlets. The type of supply system was via an overhead line, and necessitated the use of a consumer’s earth electrode to provide a return path for fault
current so that protection could be effectively provided. This is the earthing system that the BS7671:2008 wiring regulations defi ne as ‘TT earthing’.
Several trades were involved in the refurbishment
work, each requiring power for electrical power tools, such as drills, saws and so on. As working conditions can be onerous in this type of environment, and therefore the measures used to protect the users of electrical equipment under these conditions are particularly stringent, it is always the focus of the inspection to ensure that the protective measures afforded and that those used to prevent electric shock are adequate.
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 went wrong: It was established that the 230V socket outlet provision was not afforded 30mA RCD protection. It was also ascertained by testing that the earth loop impedance was infi nitely high and, as such, the protective devices affording overcurrent protection would not provide protection against fault current in these circumstances. On further investigation, it was discovered that the earthing conductor originated at an earth electrode comprising of a metallic tube driven into ground at the front elevation of the property. The earthing conductor was disconnected at the earth electrode, which accounted for the infi nitely high earth loop impedance value obtained in the test previously described.
64 ECA Today March 2011
The outcome: Fortunately, the failure to provide adequate safety measures to prevent electric shock did not result in injury or fatality. It is likely that the contractor would have had to have defended themeselves against a charge of negligence had serious injury or death occurred.
What should have been done? Construction sites are particularly diffi cult work environments, where hazards are sometimes compounded, for example, by the presence of water and use of heavy plant and equipment. Section 704 of BS7671:2008, entitled ‘Construction and Demolition Sites’, outlines specifi c safety measures in these environments. The requirements for supplies for socket outlets and other circuits supplying hand- held equipment up to and including 32A rating are that they should be protected by one of the following measures: (i) Reduced low voltage; (ii) Automatic disconnection of supply with ‘additional protection’ provided by RCD(s) rated at not more than 30mA and tripping within a time not greater than 40mS when tested at fi ve times the rated residual current;
(iii) Electrical separation, with the isolating transformer supplying only one item of electrical equipment; or
(iv) Separated extra-low voltage (SELV) or protective extra-low voltage (PELV).
While measures (i) and (ii) are those commonly
used, it should be pointed out that Section 704 strongly recommends that portable hand lamps for general use and portable hand tools and local lighting up to 2kW should be supplied by reduced low voltage.
This could have been avoided by: Verifying that the earthing of the incoming supply was effective prior to starting the work, and also ensuring that 30mA RCD protection was afforded at the main intake. In this case, the work necessitated the installation of a proprietary earth electrode (as, in our opinion, that in situ was not suitable) within an inspection pit complying with
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