NAPIT’s Bill Allan offers more information about the electrical contractor’s responsibility in the event that changes to a property have introduced a risk of shock.
It is common for electrical contractors who have completed work in a domestic premises to be invited to visit the premises again at a later date in order to submit an estimate for further work or to carry out an Electrical Installation Condition Report (EICR). Sometimes the contractor notices that work (which may or may not be electrical) has been carried out in the meantime and has introduced a shock risk that wasn’t present previously.
Under such circumstances, the contractor is obliged to bring this risk to the attention of the home owner.
This article will give a specific scenario by way of example to highlight the responsibility of the contractor.
An example scenario
A contractor has installed electric vehicle (EV) charging equipment on the front drive of a house adjacent to the integral garage. The house has a TN-C-S (PME) supply and the EV charging equipment has a local TT earth electrode.
On visiting the premises some years later to carry out an EICR, the contractor notices that two changes have been made which give rise to concern:
An outside metal water tap has been installed without a plastic insert.
An outside metal porch light has been installed.
Both the metal tap and the metal porch light are within touching distance of the EV charging equipment, so effectively different earthing systems and potentials are within reach.
So what should the contractor advise the customer to do?
Assessing the risk
This scenario underlines the importance for designers and installers of EV charging circuits to have an understanding of the potential dangers involved. It also shows that electricians who are asked to carry out work on such installations at a later date must have the same understanding.
In this instance, it would appear that the electrician who installed the lighting didn’t fully appreciate these dangers. It is also regrettable that the plumber who installed the outside metal tap didn’t have some understanding of the dangers involved. It should be noted that the outside water tap, since it has no plastic insert, presents a shock risk regardless of its proximity to the EV charging point.
Regulation 722.411.4.1 in BS 7671 prohibits the use of a PME earthing facility as the means of earthing for the protective conductor contact of an EV charging point which is located outdoors, or that might reasonably be expected to be used to charge a vehicle which is located outdoors.
Therefore, the use of an earth electrode in such circumstances is common, as in this scenario.
The fault condition of most concern in PME systems is an open circuit in the PEN conductor of the distribution system. This may result in a potential appearing between the metal tap and the general mass of Earth.
An RCD wouldn’t offer any protection because the same fault current would flow in both the line and neutral conductors and there would therefore be no imbalance to cause the RCD to operate.
As this scenario involves conductive parts of separate earthing systems at different potentials which are simultaneously accessible, it may prove helpful to review the distances pertaining to simultaneous accessibility. They’re contained in Regulation Group 417.3 of BS 7671.
Regulation 417.3.1 states that: ‘Simultaneously accessible parts at different potentials shall not be within arm’s reach’. It then explains what is meant by ‘arm’s reach’. The Note to this Regulation states that: ‘Two parts are deemed to be simultaneously accessible if they are not more than 2.50m apart’.
The Note to Regulation 417.3.2 explains that this distance applies to contact directly with bare hands without assistance, i.e. without tools or a ladder.
The voltage between simultaneously accessible metal parts is commonly known as the touch voltage.
The water tap
We’ll assume that the outside tap is connected to the PME terminal of the house via a main bonding conductor.
In normal operation, the main earthing terminal (MET) in PME systems will be at some potential relative to Earth, determined by installation and network conditions. This potential would also be on the exposed-conductive-parts (and the extraneous-conductive-parts via the main bonding conductors) which are connected to the MET.
This means that a person in simultaneous contact with such parts and ‘true’ Earth (i.e. the ground) will be subjected to touch voltages which may cause some discomfort and give rise to a perceived electric shock.
The situation becomes especially dangerous where a person is barefoot or where water is present.
As the metal tap is simultaneously accessible with the EV charging equipment which is connected to a different earthing system (i.e. a TT system), there is the additional risk of a hand-to-hand shock.
Simultaneous contact between different earthing systems must be avoided at all costs.
Plastic insert/plastic water pipe
The problem with regard to the outside water tap is easily rectified by fitting a plastic insert in the metal water pipe supplying the outside tap. However, care should be taken to ensure that simultaneous contact with metal pipework on either side of the insert is not possible if there is likely to be a potential across the insert under a fault condition.
An alternative solution is to supply the metal tap with a plastic water pipe.
The porch light
There are two options to resolve the problem with the porch light:
Move the porch light further away from the charging equipment so that it is no longer simultaneously accessible with the charging point, or;
Change the metal porch light for one which has a non-metallic casing or is a Class 11 fitting.
Where such changes introduce a shock risk, the customer must be made aware of the risk as a matter of urgency. Arrangements must be made for necessary remedial work to be carried out at the earliest opportunity.
Section 722, Electric Vehicle Charging Installations of BS 7671:2018.
IET Code of Practice for Electric Vehicle Charging Equipment Installations.
For more information please visit: https://www.napit.org.uk/