Why Pod Point homechargers do not require an earth rod

The system we pioneered disconnects the charging point, including earth from the car in the event of a failure developing on earth or neutral. We worked closely with regulators during the testing and launch of this system, and we are pleased to note that one of the key amendments in Feb 2020 of 18th Edition of the Wiring Regulations specifically details the technology we have brought to market.

Read on for more details...

Protective Multiple Earth (PME) Systems

Some homes in the UK are wired using a Protective Multiple Earth (PME) system. In a PME system, the home’s earth wiring all runs back to the main supply point where it connects to the neutral conductor coming into the home. This allows the mains supply to share a conductor for both earth and neutral entering the house. The EV chargepoint is also connected to this same point, using the same earth. The neutral conductor is then earthed multiple times along its route back to the upstream sub-station, this earth must also be bonded in accordance with the wiring regulations (e.g. to water and gas pipes).

PME systems ensure very low neutral to earth impedance in the home and allow the RCDs fitted in your consumer unit to reliably detect earth faults.

Risk in Case of Neutral Failure

However, PME systems have a rare but possible fault situation should the neutral connection to a home somehow be disconnected upstream, e.g. should the combined neutral and earth connection points along the cable corrode and degrade over time. With no neutral all the appliances in the home would appear dead, but if any filament light bulbs or heaters are turned ON then the neutral will rise to the live potential, as these devices are connected from live to neutral (but the neutral is now not connected to anything). As the house earth is connected to neutral, the house earth will also rise towards the live potential. NB: any RCDs fitted will not protect for this failure.

Inside the home this is not as dangerous as it might seem, because if all conductors are at the same potential there is no potential difference between them to do harm. However, because the car is located outside the home and is connected to the earth inside the home, there is a chance of a difference in voltage between the earth (chassis) of the car and the actual ground potential outside the home, which would then be apparent to a person stood on the ground and touching the metalwork of the car.

Upstream neutral faults are rare but they do happen.

Protecting Against Neutral Failure

In order to protect against neutral failures, 18th Edition states that the earth impedance between the vehicle charging earth and the true ground (or earth), for chargers fitted outside the home, must be under 200 ohms to minimise any voltage developed between the two and ensure correct operation of an RCD.

It can be possible to achieve this 200 ohms earth impedance by sinking one or more dedicated earth rods for the chargepoint. Pod Point installation guidance recommends 100 ohms as electrodes and ground conditions are subject to wide variations due to both locality and season.

Earth Rods

An earth rod is a long copper spike that is driven into the ground and connected to the chargepoint’s earth.

Whilst this system can work, it is often difficult to install, and even when fitted it is often difficult to ensure a sufficiently low impedance connection to true earth consistently (this depends on e.g. soil type, moisture degradation/corrosion of the earth rod over time). It is also important to ensure that any rod installed is not close to any other earth rod or underground feature, such as a water or gas pipe, that may be connected to the PME earth in the locality - this is very difficult to determine as a low earth loop impedance might be due to either a “good earth” or a false “connection” to the PME.

In addition, other metallic objects (existing outside taps, lighting, metal cladding etc) may use the same PME earth (i.e. they are in the same equipotential zone). In this case installing an earth rod for the chargepoint can make this equipotential zone (essentially the area surrounding the parked car) more dangerous.

In summary, earth rod installation is far from a fool-proof solution to neutral failures, the guidance for domestic earth rod installation in BS7430 can be difficult to apply in practice making this a risky grey area for electricians.

Exceptions and Alternatives to Earth Rods

According to the 18th Edition and as expanded by Amendment 1:2020, the fitting of a dedicated earth rod to the charging point is not required if one of the exceptions mentioned in regulation 722.411.4.1 is met.

Pod Point hardware meets the requirements of exception (iv) that requires the service voltage between live and neutral to be monitored and for charging to be stopped and the car isolated from the supply (including earth) in the event of a fault condition appearing.

Development of Pod Point Neutral Failure Detection Circuit

Since 2013 earth rods have been required on PME systems and the flexibility that some installers used to not fit an electrode was removed from the 18th Edition in January 2019, but the difficulty of successfully fitting an earth rod predates both regulations.

Safety has always been the first priority when designing hardware at Pod Point. Our original hardware (manufactured 2011 to mid 2017) was designed and tested to the requirements of BS EN 61851-1:2011. In 2017 Pod Point made some design changes to the internal circuitry of the Pod Point hardware to include the additional electrical safety features included in the IEC’s update to that standard (IEC 61851-1:2017). Pod Point hardware is designed to meet, and is independently tested against, the requirements of these standards. In addition to meeting those requirements in full Pod Point’s design also complies with and is tested against the requirements of the Low Voltage Directive.

None of these standards define any requirements for connecting a charging point to a PME supply or what should happen in the event of a failure of the neutral in that supply. When doing our design work we also considered the requirements of the then 17th Edition of the wiring regulations. Guidance for connection to PME supplies in the 17th Edition was less clear and installers were permitted to connect a charging point to a PME supply if it was “not practicable” to install an earth rod.

In light of, and considering together, the following hazards across all installation cases:

• Neutral failure
• The risk of an installed earth rod picking up a false/unknown connection to a PME earth
• The possibility that any earth connection (rod or PME) might degrade over time
• The risk that other PME connected metalwork might be added in the vicinity of the car in future

We chose to add features to the charging point to provide additional protection for the user in the event of any earth problems. The features added at that time match the requirement that is now formally defined by exception (iv) in Amendment 1:2020.

The Pod Point System

The additional protections are afforded by inclusion of a circuit that detects a neutral / earth failure (or even partial failure) and immediately puts the charger into a fault mode while at the same time disconnecting live, neutral and earth from the car via a single earth relay and double in series live and neutral relays.

So in the event of a partial or full neutral fault, Pod Point chargers completely galvanically isolate the car to prevent current flow and isolate the car from earth, thus protecting the user without relying on an earth rod, transformer or difficult to ensure low ground impedance. The chargepoint enters a fault state that it will not leave until the chargepoint is powered down at source and then powered up. If the neutral failure remains in effect when power is restored, the chargepoint will return to the fault state.

Reporting of Failures and Future Improvements

The Pod Point can send diagnostic data on this and other safety related issues - e.g. disconnected earths, reverse polarity supplies, elevated neutral voltages, under/over voltages and charging cable faults - via its WiFi connection. In case of comms failure, the safety features remain fully operational.

Pod Point has detected several anomalous events on customers’ supplies and in each case has instructed them to contact their DNO immediately as the anomalies have been consistent with an upstream neutral failure. We have also had visibility of brown out events which we have subsequently verified with the DNO.

As with all modern devices the Pod Point has on board software which is occasionally updated to improve performance and add features, including additional safety features so it is sensible to advise customers to maintain the Wi-Fi connection to the Pod-Point to take advantage of these updates.

Engagement with Professional Bodies regarding the Pod Point System

This system was developed as a significant improvement over and above standard industry practice. Because of this Pod Point regularly briefed professional bodies, including the NICEIC, the IET and the pan European EV Ready Mark Committee during the development process, to ensure that it was both understood and caused no concerns.

What about TN-C-S Systems?

Although TN-C-S and PME systems have many similarities there is no prohibition on connecting a charging point to a TN-C-S system. In the event that a neutral failure were to occur on a TN-C-S system the Pod Point would detect the failure and enter into a fault state as described above.

Note if you are moving an old Pod Point

Any Pod Point hardware manufactured prior to mid 2017 may not have the inbuilt safety features and should therefore include an electrode should the installation change significantly (moving it’s location, connecting to a PME supply etc).