[Case notes: right connections]
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: The electrical installation
comprised lighting and power in the loft conversion of a period cottage, and subsequently an upgrade of the electrical mains, including a ‘high integrity- 17th Edition’ consumer unit providing provision for non RCD protected fi nal circuits, and two independent 30mA RCD protected MCB banks. The type of system earthing was TT.
What went wrong:
We got involved after the householders had complained of several instances of receiving unpleasant ‘tingling’ sensations associated with mild electric shock when touching the kitchen sink.
During an inspection of the property it was discovered 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.
that a downstairs lighting circuit, including lighting in the conservatory, had been disconnected, and the owner said that an independent electrician had disconnected this from a 6Amp ‘B’ type MCB connected to one of the non RCD protected ways in the consumer unit. The electrician had been called in by the householder after a routine maintenance contract visit to check highlighted problems. On further investigation, by carrying out an insulation resistance tests, it was discovered that there was a neutral to earth fault on the lighting circuit, which included the conservatory. This gave credence to the householder’s observation that the ‘electric shocks’ only occurred when the conservatory lights were switched on – the ‘shocks’ being due to the voltage difference between the sink and the TT earth. This was subsequently proven by us under strictly controlled conditions where the lighting circuit was reconnected to the non RCD protected 6Amp ‘B’ type MCB, and the ‘leakage current’ measured (with a clip-on ammeter) rose to 1.5Amp when the conservatory lights were switched on.
64 ECA Today Winter 2010
The outcome: Fortunately, no one was injured in this instance. However, had the situation been allowed to persist, and had a fault subsequently occurred between live to earth on the lighting circuit in question, then this could have caused serious injury or even proved fatal. The earthing system was TT, and we measured the
external earth loop impedance value (Ze) giving a result of 22ohm. The maximum permitted total earth fault loop impedance (Zs) permitted to give disconnection within 0.2 seconds required to comply with BS7671:2008 is 7.67ohm for a 6A ‘B’ type MCB. Based upon a supply voltage (Uo) of 220volt and a total earth fault loop impedance of not less than 22ohm, the 6Amp ‘B’ type MCB would disconnect the circuit in not less than 600 seconds (10 minutes). Therefore, fault protection is not afforded by the 6A ‘B’ type MCB, as the requirement to disconnect in 0.2 seconds is not achieved and exposed conductive parts could rise to a voltage of 220volt (Uo) for up to 10 minutes.
What should have been done? It seems obvious to assume that the lighting circuit was originally connected to an MCB on one of the 30mA RCD protected banks and, because of the neutral to earth fault, the RCD could not be switched on (persistent tripping). It is therefore logical to conclude that, for this reason, the electrician connecting up the consumer unit subsequently decided to connect this on the non RCD side. It is a requirement of BS7671:2008 that all PVC
insulated and sheathed cables not afforded suitable mechanical protection and installed up to 50mm from the surface where buried in the building fabric of a wall, be afforded ‘additional protection’ by use of an RCD having a residual tripping current rating of not more than 30mA. This requirement was not satisfi ed on this installation for the downstairs lighting circuit supplying the conservatory.
It is assumed that the electrician also failed to realise that, as a consequence of his actions, he had failed to comply with the requirements of ‘fault protection’ for this circuit, which could have had a greater consequence for the reasons previously described.
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