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As for any electrical installation, a mobile or transportable unit must be inspected and tested before being put into service to verify, so far as is reasonably practicable, that the requirements of the Regulations have been met (Regulation 641.1 refers).
This article looks at the information that should be provided as part of the initial verification of a mobile or transportable unit that is intended to be connected to a low voltage (230 V) supply derived from a fixed electrical installation (Regulation 717.313 (ii) refers).
Requirements of Section 717 of BS 7671
A mobile or transportable unit is classed as a special installation and so in addition to the general requirements of BS 7671 such installations need to comply with the relevant requirements of Section 717 of BS 7671. Regulation 717.1 refers to such an installation as a ‘unit’ and defines it as: ‘…a mobile or transportable structure in which all or part of an electrical installation is contained.’
Examples of such units include vehicles for the entertainment industry, medical or health screening services and welfare units.
Identification of the unit
The details recorded on the Electrical Installation Certificate (EIC) should provide a unique means of identification irrespective of location or particular application served by the unit. Otherwise it might be impossible to confirm, at a later date, which particular unit the EIC covers.
Therefore, in addition to the client’s details, full details of the particular unit must also be recorded. Including, (as shown in Fig 2): the model name and description, any unique identifier, and for a vehicular unit the 16 digit vehicle identification number (VIN) should be inserted.
Requirements for connection of the unit
As a consequence of being moved from one location to another there is an increased risk that a unit may be inadvertently connected to an inappropriate supply arrangement. In view of this, it should be confirmed that any limitations imposed by the designer regarding the connection of the unit to an electrical supply are clearly recorded in the user documentation (Regulation 717.132 refers).
A permanent notice should be displayed at each supply inlet connector (or a similar prominent position) to provide information throughout the lifetime of the unit. The notice displayed should be durable and include the specific information listed in Regulation 717.514; including, the types of supply suitable for connection to the unit and its maximum power rating.
Electrical testing
The range of electrical tests appropriate to the initial verification of such a unit is limited to those performed with the supply to the unit de-energised, commonly referred to as the ‘dead tests’. Live tests, such as polarity, earth fault loop impedance and RCD testing are reliable only for the particular supply connected at the time of testing and as such are not applicable to the initial certification.
In view of this, it is essential that the full details of the work covered by the certificate, including the limitations on the electrical testing, are accurately recorded on the Electrical Installation Certificate (EIC). The results of continuity and insulation resistance, together with the results of the inspection, provide evidence that the circuits have been installed in accordance with the relevant requirements of BS 7671.
The completed EIC should be issued to the person ordering the unit, once the unit is delivered to site the dead test results should be reconfirmed to verify that no damage or deterioration has occurred in transit.
The low voltage supply connection
Typically, the connection of the unit once delivered to site is normally not under the control of the person performing the initial verification, so the person responsible for the connection of the unit will need to check the relevant documentation provided and confirm that the relevant requirements of BS 7671 are satisfied before the unit is energised.
Where the protective measure of ADS is used, Regulation 717.411.1 requires the supply to the unit to be protected by an RCD having a rated residual operating current not exceeding 30 mA. Once the unit is connected to the supply the live tests should be carried out and an EIC completed and issued for the installation of the unit and the supply.
It should be noted that the connection of a unit to a PME earthing arrangement is prohibited, unless the unit is under the continual supervision of skilled or instructed person(s), competent in such work and the adequacy of the earthing arrangement is confirmed before each connection of the supply (Regulation 717.411.4 refers).
Periodic inspection and testing
As with all electrical installations, the time interval between periodic inspection and testing of a mobile or a transportable unit is a matter of engineering judgement and will depend on a variety of factors including amongst other things, the level of supervision, the frequency of use, quantity of road mileage and the severity of the external influences to which it is exposed. However, as a minimum, such units should be inspected and tested by a skilled person(s), competent in such work, at least annually.
In addition, a visual inspection should be carried out on the connecting cable and all plugs and socket-outlets before each and every use of the unit, and the results of the visual inspection should be entered in a log-book as a permanent record of the condition of the electrical equipment.
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