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This could have been avoided by: Ensuring that the location of the neutral to earth fault had been traced and rectifi ed so that the circuit could have been afforded 30mA RCD protection. To this end, in this sort of situation the following tips are suggested: ■ It is probable that the location of the fault is at terminations – so visually check joints and terminations for cable ‘pinching’ or pressure.

■ Test the circuit by splitting into sections – half, then quarter, until the leg of the cable at fault is located.

■ It may be cheaper to replace a faulty leg of cable where the fault is concealed rather than to trace to the exact location and rectify.

Had the electrician done this, his competence would not be called into question. However, had an accident occurred as a consequence of what he did, we believe that his competence as defi ned by BS7671:2008 would have been found wanting under scrutiny.

The outcome: Fortunately, while potentially dangerous and an electric shock hazard, this did not result in an accident. But under different circumstances, serious injury or a fatality could have resulted.

What should have been done? It was common practice in situations like this (where the external earth fault loop impedance is excessively high and therefore fault protection – formerly protection against direct contact – cannot be achieved by the use of MCBs) to install a 100mA time delayed RCD in place of the incoming mainswitch. An acceptable degree of discrimination is achieved by this confi guration of RCDs in the instance of earth faults.

This could have been avoided by: Ensuring the adequacy of the type of system earthing and main bonding is provided, and the design and installation of circuit protection refl ects this. In carrying out this, reference to and/or the following checks need to be made:

The installation: The electrical installation comprised of a re-wire of a domestic terraced property, installed when BS7671:1991 including amendments 1 and 2 were in force. The consumer unit installed was a split load type consisting of an up front mainswitch supplying an ‘unprotected’ MCB bank and a 30mA RCD protected MCB bank. The connections to the unprotected MCBs included lighting fi nal circuits, and the connections to the 30mA RCD protected MCB bank included ring fi nal circuits. The system earthing was TT.

What went wrong: We carried out an inspection of the installation after an apparently long-running dispute between the client and the electrical contractor had resulted in the latter refusing to return to complete the work. The installation had, however, been made live and the house was occupied, as it had been during the rewire. It was discovered that the external earth fault loop impedance (Ze) by measurement was 99 ohms and therefore, in theory, the maximum fault current that could fl ow under fault to earth conditions by the application of Ohm’s Law, assuming a nominal voltage (Uo) of 220Volts, would be 2.22Amps. This fault current is less than the nominal setting (In) of a 6Amp ‘B’ type MCB, the smallest MCB that was installed on the ‘unprotected’ side of the consumer unit. Therefore, should a phase earth fault have occurred on any circuit supplied by the ‘unprotected’ ways, this would have persisted for an infi nite period of time, thus raising all exposed conductive parts associated with these circuits to the dangerous level of 220Volts. Disconnection should occur in not more than fi ve seconds, and for this to be achieved the maximum permitted earth fault loop impedance (Zs) for a 6Amp ‘B’ type MCB is 8 ohms.

■ The accessibility of the termination of the earthing conductor to the means of earthing, suitably protected against corrosion and labelled ‘safety electrical connection do not remove’. (In this case, the connection to the earthing conductor was inaccessibly buried in concrete when, in fact, it should have been installed in a proprietary inspection pit.)

■ Check the maximum

permitted earth fault loop impedance values for the MCBs (to be used for

overcurrent protection) against the maximum permitted disconnection times.

If the requirements of the second point above cannot be satisfi ed and/or there are other requirements to install RCDs for supplementary (additional) protection, then two or more suitable RCDs, bearing in mind discrimination, will have to be installed in addition to the MCBs affording overcurrent protection. Alternatively, individual RCBOs should be considered. We believe that, had the contractor needed to

prove competence, the argument that the omission of RCD protection was justifi ed because the work was incomplete could not be upheld. Similar cases brought before the legal system, where accidents resulting in serious or fatal injury have occurred, bolster our view. Temporary, incomplete and electrical installations on construction sites must comply with the requirements of the regulations current at the time, and arguments for relaxation of those regulations due to the temporary nature of the work would not bear scrutiny.

Winter 2010 ECA Today 65

More info

ELECSA offers an inspection service providing expert witness reports for clients, including local authorities, trading standards, solicitors, electrical contractors and private clients. It is listed in the UK Register of Expert Witnesses and accepts commissions from clients related to incidents of electrocution, fi res and disputes regarding compliance with British Standards, the diversity of which includes lighting and power, emergency lighting and fi re detection and alarm systems. See or email:

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