EV Charging and the PEN Fault
18 March 2022
In February 2020 the current Electrical Regulations, also known as BS7671, were amended to introduce some important changes relating to electrical installations for electric vehicle charging. In particular this amendment introduced additional measures for protection against electric shock.
The PEN Fault
While rare, the main concerns addressed by this amendment related to a grid fault that can result in the simultaneous loss of both the earth and neutral from the electrical supply. This in turn affects the electrical supply in your property. However as this is a grid fault it is the responsibility of the electricity supply company, your DNO, to repair.
Should this fault occur, since there is now no 'protective' earth and no neutral (PEN), only the live conductor, which will still carry 240 volts, will remain connected. This will look like a power cut. Your lights and other appliances will no longer work as they require the neutral so that electrical current can flow and the protective earth to provide protection from electric shock from faulty appliances.
PEN Fault and EV Charging
With a PEN fault present it is feasible that an electric vehicle standing outside, potentially in the rain, would be connected only to live should it be plugged into charging equipment. Should the vehicle also be electrically faulty, perhaps due to age and wear, the metal work could then become live as well.
Whilst this fault is very rare, if somebody was unfortunate enough to then touch the vehicle they would receive a 240 volt electric shock. This could potentially be fatal and as such the electrical regulations have taken measures to prevent this condition ever arising.
Two Protective Measures
While the PEN fault cannot occur with some properties, those that have their earth derived directly from an earth electrode driven into the ground (this is not too common), and does not present the same risk if the vehicle is charged inside, for example in a garage, the electrical regulations have two techniques available to protect against electric shock from a PEN fault.
The use of a earth electrode also know as an earth rod. This is installed specifically for the EV charging equipment and provides a permanent protective earth for this equipment and hence also for any vehicle plugged into the EV charging equipment.
A device for the electric vehicle charging equipment that is able to detect a PEN fault and which will fully disconnect the live, neutral and earth supplying that equipment. This also fully disconnect any connected electric vehicle and hence eliminates the possibility of electric shock.
An earth rod provides full protection against electric shock provided it provides a good connection to the earth. Sandy, dry or very stony ground can prevent this happening which also means that rain or water can also affect the protection offered. However a well installed earth rod provides a good protection against electric shock.
Many properties are however on ground that doesn't provide for a good connection to ground or they are constructed in a way that makes the use of an earth rod problematic. Concrete, tarmac or other materials around a property can also make placement of a ground rod difficult as can underground services such as drains, sewers, water, gas and electric supplies that can also make the installation of a ground rod hazardous for the electrical contractor.
PEN Fault Protection Device
The recent electrical regulations have allowed for a PEN fault protection device, with this device disconnecting live, neutral and earth from the EV charging equipment. However, it was not until the amendment in February 2020 that a practical specification for a device was approved and published by the IET. This specification is based on voltage measurement rather than direct PEN fault detection, which has been very difficult to do.
The UK grid supply is regulated. The nominal voltage is set at 230 volts but this can fluctuate upwards due to the use of renewable supplies, like wind and solar, or downwards due to heavy use of electrical power. There are regulated voltage limits. The DNO's and electrical generation companies are compelled to keep the voltage between these limits which are 207 volts at the low end and 253 volts at the high end.
When a PEN fault occurs there is a very high likelihood that the voltage on the remaining intact live cables will present a voltage outside of the regulated grid supply voltage. As such the IET approved the use of devices that measure grid voltages that are higher than 253 volts or lower than 207 volts for a period of time of up to 5 seconds. Under these conditions the devices will then disconnect live, neutral and earth connections removing power to the EV charging equipment and hence providing PEN fault protection. The removal of power to the EV charging equipment by the protection device will be alerted to the user. The manner of the alert will depend on the manufacturer of the protection device which may also be built into some charging equipment.
The removal of power to the EV charging equipment when the PEN fault protection device 'trips' due to a grid voltage fault condition is a safety feature and does not indicate a fault with the charging equipment installation. Instead the device has responded to a grid fault condition. While most PEN protection devices will automatically reset once the grid voltage is again within the regulated low and high limits, it will not be possible to charge an EV while the grid voltage fault exists.
Since the grid voltage is always fluctuating, often due to the use of wind and solar, and not all local supplies are nominally at 230 volts (the UK used to be 240 volts) it is possible and not uncommon for a PEN fault protection device to disconnect the EV charging equipment from the supply even when no PEN fault exists.
This type of disconnection, sometimes called nuisance tripping, will stop a connected EV from charging. It is easy for the vehicle owner to believe this failure to charge is due to fault in the charging equipment or a fault in the vehicle. However, if a PEN fault is indicated, neither of these is true.
In fact the charging equipment is working as designed. The PEN fault detection devices is working as designed and as a safety measure and as required by the regulations it has disconnected the supply. For the EV owner this is a real nuisance, but while there is no fault with their charging equipment or with their vehicle there is a however a real fault present.
The fault causing nuisance tripping is a grid fault. The regulated grid voltage has exceeded the mandated limits and it is the responsibility of your local DNO to correct and prevent this.
High or Low Grid Voltages
Only your local DNO can correct and prevent high or low grid voltages and they are obliged to do this.
If you notice nuisance tripping you will need to contact them. You are their customer so you will almost certainly have to do this yourself due to data protection regulations that make it very difficult for your EV charging equipment or vehicle supplier to do on your behalf.
Insist that they install voltage monitoring equipment. This will be installed for a week. Ask them for the results and if any voltages are detected as being outside of the regulated range your DNO is obliged to correct this.
What If My DNO Will Not Do Anything
This is a troubling question.
It may be possible to ask your EV charging equipment supplier to change the installation to incorporate an earth rod. Do not expect them to do this without cost. They have installed working and safe equipment and it is the DNO that is at fault. Your EV charging equipment supplier will expect to be paid for the labour and the parts needed to change the installation and you should expect to pay this.
High grid voltages, which are more common due to the use of renewable technologies than low grid voltages are not only potentially dangerous, possibly bursting light bulbs and potentially starting fires, they are for sure also costing you money. Your electricity bill is based on your use of power. Higher grid voltages mean you will be using more power and as such, without any real benefit. A grid voltage that is 5% higher that it should be will be increasing your electricity bill by 5%. With the new energy prices in April 2022 this could mean you are paying £150.00 per year more than you need to for the average homeowner.
Since grid voltages will always vary there are now voltage conditioners available that can be used to ensure the voltage at your property always remains at 230 volts. These do cost money to install but they not only eliminate nuisance tripping, they can also reduce your energy bills so they are worth consideration should your DNO not meet your expectations.