Earth rods – Protection against electric shock following the loss of the PEN conductor in a TN-C-S earthed electrical system
When an EV charging point is installed, it’s important to make sure that it’s electrically safe. As well as the normal protection against over current and the RCD protection, most charge points need additional protection for what is referred to as “loss of the Protective Earth and Neutral (PEN) conductor”. This fault happens on the electricity distribution system system supplying the property (specifically supplies that have a “protective multiple earth” (PME) or “TN-C-S” supply arrangement) but can lead to a potentially dangerous voltage arising on any exposed conductive surface – such as your electric vehicle when it is plugged into the EV charge point.
The normal way to protect against this fault is to install an additional earth rod in the ground and then connect this an earth terminal in the charge point. (Actually it’s more complicated as the earthing arrangement at the charge point also needs to be changed (so called “TT” earthing) but the point remains that an earth rod is normally required and this involves additional work and disruption).
Myenergi have developed a new way of protecting the zappi from the PEN conductor faults. As this is a new development we’ve applied for a patent, but zappi does not need an earth rod as the supply to zappi will be isolated if there is any indication of a fault current through the vehicle.
Why are Earth rods important?
According to HSE statistics (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.
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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 IET Wiring Regulations (BS7671:2018) tightened up the rules under clause 722.411.4.1 on the installation of EV charge points for domestic installations.
Figure 1: Return current path with broken PEN conductor
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. In 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 require for isolation of the load from the supply as shown by the blue line in Figure 3.
Figure 3: Line to neutral and neutral to Earth voltage due to unbalance on distribution network and broken PEN conductor
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 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 EV charge point that is simple to install and which is packed with features, such as integration with renewable generation, load balancing and circuit overload protection, to create a product which electrical installers and the end customers love.
The detailed technical bit
TN-C-S earthing (otherwise known as PME or “Protective Multiple Earthing”) is widely used on the low voltage electricity supply (230V single phase or 400V three phase) where the Neutral and Protective Earth functions are combined in a single conductor (PEN)i. A local earth connection is then provided by the Distribution Company at the property for final circuit wiring to ensure that any earth fault currents arising in the consumer’s installation are safely returned to the source.
However, under certain supply system fault conditions (PEN conductor of the supply becoming damaged or open circuit external to the installation) a voltage can develop between the conductive parts connected to the PME earth terminal and the general mass of earth. Standard RCD protection will not operate if a current flows from the conductive parts to ground (eg through a person touching exposed metalwork that would normally be considered to be grounded) so the loss of the PEN conductor could lead to a fatal electric shock.
As there are multiple earthing points on the supply network and bonding is provided within the building complying with BS 7671, the risk to a person inside the building is considered to be small.
Extending the earth cable (correctly referred to as the “circuit protective conductor” or CPC) outside of the bonded zone is not generally permittedii due to the potential for electric shock if the PEN conductor becomes open circuit. Indeed, this extended bonding will not protect people outside the building who are simultaneously in contact with conductive-parts connected to the PME earth (part of this bonding) and true Earth.
The IET Wiring Regs (BS7671) define how the EV charge point must be earthed if it is mounted outside the building and it is connected to a PME system. In nearly all circumstances the installation will require a local earth rod. The main earth from the property must not be connected to the charge point and instead it is earthed using the earth rod.
The requirements in BS7671 aim to ensure that the voltage between any exposed metalwork and true Earth is kept below 70V – the voltage where an electric shock could cause injuryiii.
Consider the circuit shown in figure 4. This represents a generic three phase distribution transformer and load with a break in the PEN conductor.
The lumped impedance Z1Z2Z3 represents a combination of three phase and single phase loads connected after the break in the PEN conductor. If the load on the three phases is not balanced the neutral voltage VN will no longer be close to true Earth (0V).
Figure 4: Three phase distribution equivalent circuit with PEN conductor broken
To keep the analysis simple consider the case where Z2 and Z3 are the same magnitude and Z1 is allowed to vary. So Z2 = Z3 = αZ1
Figure 5: Voltage vector diagram for unbalanced loads and displaced PEN conductor
It can be shown that the neutral voltage VN varies with α as
where V1 is the Line-Neutral voltage at the distribution transformer.
In vector form, the voltages can are show in figure 5.
Figure 6 shows the variation in the voltages as the imbalance in the 3-phase load varies, in this case with the distribution transformer operating towards the top of the allowable voltage range. The x axis (imbalance or “α”) extends to the right towards ∞ where the circuit protective conductor voltage (VN) tends toward the Phase voltage.
The figure also shows
the 70V rms limit required by BS7671:2018 722.411.4.1(iii) for the PEN fault protection to operate if the voltage between the circuit protective conductor and earth rises (red dashed line) and
the theoretical Line to Neutral voltage threshold (green dashed line) that would be used to trip a device which does not have an earth reference (ie a local earth rod)
the “safe” operating region where the voltage between the protective conductor and true Earth is below 70V. Outside this region it may appear that everything is working correctly but a dangerous potential is present on the exposed conductive surface of the EV.
Figure 6: Variation in voltages due to an broken PEN conductor and imbalanced loads on the distribution system
What’s different about zappi?
In the zappi we’ve taken a different approach to providing protection against a damaged PEN conductor which means that there is no need for an earth rod. Our approach has been discussed with the IET and with UK Distribution Companies. It’s operation has also been tested in the engineering laboratory at the University of Nottingham.
Zappi already includes an RCD to provide protection against a fault in the wiring or the charger on the car. The additional “PEN Fault Protection” (patent pending) combines the following features.
Protection against over- and under-voltage, based on 230V nominal line voltage ±12% (to avoid nuisance tripping if the supply temporarily goes outside the EU Harmonised Voltage Limits of 230V ±10%). Using these settings will ensure that zappi will trip if the line to neutral voltage collapses (or rises) as the line to neutral to Earth voltage rises.
Measurement of the current flowing to true Earth via the fault path (which for an EV charger includes the chassis and bodywork of the vehicle being charged). This protection operates if it detects even a few mA of fault current, ensuring that the person in contact with the EV or charge point is protected in any circumstances where a current greater than 30mA flows to Earth, even if the neutral to Earth voltage is below 70V.
Isolation of the live, neutral and earth conductors in the case of a fault being detected.
Galvanic isolation between input and output, including all conductors going to the EV (including signalling wires CP & PP). This is achieved through the galvanic isolation provided by the electronic components in the charge point which remain live in order to provide information on the fault and the ability to reset the charger once the fault is cleared.
Manual reset of the protection.
By taking this approach, which builds on the accepted principles of Residual Current protection but then applies these in combination with other key components in a novel application, the “PEN Protection Function” provides more comprehensive protection than a poorly installed earth electrode or simple measurement of the line to neutral voltage
i) Earthing: Your questions answered, Geoff Cronshaw, IEE Wiring Matters Autumn 2005https://www.ee.iitb.ac.in/course/~emlab/assets/earthing.pdf
ii) IET Code of Practice EV charging, page 31, section 5.3.1
iii) Table 2c of IEC 60479-5 Effects of current on human beings and livestock – Part 5: Touch threshold values for physiological effects