Myenergi’s Chris Horne looks at the requirements for protection against electric shock following the loss of the PEN conductor in a TN-C-S earthed electrical system.
According to HSE statistics (see Table 1), each year there are around 400 reported incidents of broken Protective Earth and Neutral (PEN) conductors on TN-C-S (PME) earthed electrical installations, with around 10% of these causing an electric shock. When the PEN conductor is broken the neutral voltage can rise with respect to true earth and the normal protective earth forms the return path for any current that could flow.
For an EV charger this means that the body of the car forms the return path, presenting a real risk that anyone touching the car will get an electric shock. This is why the 18th Edition of the wiring regulations (BS7671:2018) tightened up the rules under clause 722.411.4.1 on the installation of EV charge points for domestic installations.
The new rules mean that in nearly all circumstances the installer will need to install a dedicated earth rod for the charge point and connect the charge point as a TT earthed installation.
Providing earth rods and making sure they have a low enough earth impedance is time-consuming, can be very difficult, particularly in stony ground, and presents new dangers if there any buried services where the earth rod is being installed. All this pushes up the cost of the installation.
Clause 722.411.4.1 does allow for protection to be built into the EV charge point, but this protection has often been referred to as the “Unicorn device” – some people claim to have seen one but it’s generally thought to be a myth!
Simply measuring the voltage between the live and neutral conductors does not provide full protection. n an unbalanced three phase system, the line-neutral voltage one phase could remain within statutory limits whilst the neutral voltage rises above the 70V threshold that the IET requires for isolation of the load from the supply, as shown by the blue line in the graph below:
Is there any equipment that meets the new requirements?
Myenergi have developed a solution that does provide full protection against a potentially dangerous electric shock when the PEN conductor is broken. The patent-pending protection device, which is built into the new Zappi EV charger, isolates the output if the voltage is outside of the statutory limits. Further protection is provided by tripping the output from the charge point if there is any indication of a fault current – extending the concept of RCD protection to this new application.
The result is a smart, OLEV approved EV charge point that is simple to install and is packed with features, such as integration with renewable generation, load balancing and circuit overload protection. Zappi is available in tethered and untethered versions, and for single phase (7kW) and three phase (22kW) installations.
Myenergi has published more extensive details about PEN protection and the requirements for installers at: https://myenergi.com/pen-protection