Protective Multiple Earthing (PME) – TN-C-S – (MEN) and PNB
TN-C-S PME and PNB Earthing Systems AKA Grounding Electrode System
What is Protective Multiple Earth (PME)?
A protection method where the earth continuity conductor (ground wire) in a consumers premises is connected both to the grounding / earthing system as well as to the neutral conductor of the power supply is called Protective Multiple Earth (PME). It is also known as TN-C-S or Multiple Earther Neutral (MEN). This ensures that in the event of a neutral wire breakage, the fault current can still return safely through the earth connection, reducing the risk of electric shock and other hazards.
As shown in the following figure for the PME earthing system, the supply neutral serves both purposes, providing protective earth and serving as the neutral conductor, while the neutral conductor is grounded at multiple points on the supply side. In case of Open-circuit PEN conductor (when a break occurs at the source neutral) is discussed in the following section of this article, covering both protection measures and potential risks of PME.
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What is TN-C-S PME?
TN-C-S PME (Protective Multiple Earthing) is a specific configuration of an electrical distribution system where the external supply source is directly connected to multiple points on earth (T = Terre). On the consumer installation side, the conductive parts of the machinery are directly connected via circuit protective cables (CPC) to the Neutral (N) of the supply source and earth.
The ‘C-S’ designation indicates that both the Neutral (N) wire and the circuit protective cable (CPC) are combined at the supply source side, while they are separated at the consumer installation side.
In the TN-C-S PME system:
- T: Terre (French for “earth” or “ground”)
- N: Neutral
- C: Combined
- S: Separate
In a TN-C-S system:
- “T” signifies that the earth or ground connection is provided separately from the supply conductors.
- “N” represents the neutral conductor.
- “C” indicates that for a portion of the electrical distribution network, the neutral and protective earth conductors are combined.
- “S” denotes that for another portion of the distribution system, the neutral and protective earth conductors are kept separate.
This type of system is designed to ensure safety by providing both a separate earth conductor for grounding purposes and a combined neutral and earth conductor for certain segments of the electrical distribution network.
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What is PNB?
PNB, abbreviated as Protective Neutral Bonding, is a similar method to the PME earthing system, where the Neutral to Earth (TN) connection is made on the consumer side instead of at the power supply or distribution transformer.
A TN-C-S system forms a PNB (Protective Neutral Bonding) where there is only one point at which the PEN or CNE conductors of individual consumers are connected to the source of supply, i.e., the transformer.
The recommended distance between the earth and the consumer insulation is less than 40 meters (≈ 130 ft.). However, to minimize the voltage level in the event of a broken neutral connection, it is advisable to keep the distance as close as possible to the main panel or consumer unit terminals. (earth link bar).
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According to ESQCR (Electricity Safety, Quality and Continuity Regulations), a consumer is not allowed to install the PEN conductor and it is the responsibility of independent distribution network operator (DNO) in case of HV/LV installations.
Why & Where is the PME Earthing System Used?
The benefit of this method is that if the neutral wire breaks ( which is known as Open-circuit PEN conductor), the fault current returns towards the supply source through the parallel neutral available with the earth. This creates a low-resistance path, which melts the fuse or activates and trips the protective device (because a high current passes through the fuse due to low resistance, causing it to melt).
As a result, the risk of electric shock due to broken neutral wire is eliminated (meaning, despite the neutral wire being broken due to earthing from two places, there is no risk of shock when touching it. Remember, if this is not done, the return of current towards the supply source won’t happen if the neutral wire breaks, and touching the neutral wire will result in electric shock).
In Protective Multiple Earth (PME), since the neutral wire of the power supply is also connected to the consumer’s earthing system (or integrated into the consumer’s earthing system), when the neutral wire breaks, the current returns through the earthing system. Therefore, even if the neutral wire is broken, there is no risk of electric shock when touching it.
power supply companies and distributers often use the Protective Multiple Earth (PME) or Protective Compound Earthing method in rural areas where conditions for earthing are not optimal (such as mountainous regions, etc.), and where significant expenses are required to obtain low-resistance paths for each building. It’s important to note that the use of PME requires written permission from the relevant authorities in places where obtaining suitable low earth loop resistance from the supply transformer to the consumer’s terminals is difficult.
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Conductor and Bonding Sizing for PME and PNB Earthing
We have a published a detailed post on sizing of earth conductor, earth lead and earth electrode with a solved example.
While sizing the earth conductor for protective multiple earthing, you must follow 114.1 and 543.1.1 of BS 7671:2018+A2:2022 for its cross-sectional area. The calculation of the cross-sectional area of the earth conductor must be in accordance with Regulation 543.1.3, and the selection of protective conductor sizing must comply with Regulation 543.1.4.
Potential Risk of PME Earthing System
While providing effective earthing using PME is crucial, such systems can also introduce new issues that may pose risks. For instance, it’s essential to ensure that the potential of the neutral conductor does not rise to hazardous levels due to earthing, as all protective metalwork (meaning metal enclosures of machines and equipment grounded for safety) is connected to the neutral of the supply.
This increases the risk of electric shock within a particular distribution network. The concern is particularly heightened if the neutral conductor breaks, which often occurs in rural overhead lines. The potential risks arising from such situations are illustrated in Figure above especially in the following scenario.
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Open Circuit PEN Conductor
Let’s say a user in a distribution system with Protective Multiple Earth (PME) has a load of five kilowatts. (This load is connected to a 240-volt supply, and the load’s resistance is approximately 12 ohms.)
If, for any reason, the neutral conductor of the supply breaks somewhere along the path, there is no direct route for the normal load current to return to the supply. However, in the distribution system, an alternative path for this load current to return to the supply terminals is available through other earthed neutral conductors, as illustrated in the diagram.
(In other words, if the neutral wire of the supply breaks somewhere, the only path for the electric current to return is through the Protective Multiple Earth connection.) Thus, the current flows through these connections in parallel to the earth and returns to the supply terminal via the paths of the electrodes on the other side of the earthed break.
If each electrode’s resistance is also approximately 12 ohms, it means that the supply voltage of 240 volts is evenly distributed across both sides of the load and the break (meaning 80 volts are distributed on one side of the load, and the remaining 160 volts are distributed on one side of the break and 80 volts on the other side of the break). As a result, all the earthed metalwork at every installation supplied by this distribution system is now energized at approximately 80 volts (meaning each earthed metal enclosure and the earth have 80 volts between them).
In such a situation, if someone touches any earthed metal enclosure or metalwork, they would receive an electric shock of 80 volts, which is the most significant danger of this type of earthing.
Adding high resistance to the neutral also has a uniform effect. However, the dangers and risks of electric shock depend on the resistance of the load and the neutral return system. The drawback of this earthing system is that the flow of current towards the load remains constant, and there is no indication of any malfunction or error until someone touches the metalwork. At that point, they receive an electric shock.
Due to these risks, the requirements for Protective Multiple Earth (PME) earthing are quite stringent, and obtaining formal approval from the relevant authority is necessary for implementing this type of earthing in a specific area. Among the basic requirements, the neutral conductor must be earthed at multiple points in the system, and the resistance between each conductor and the earth must not exceed 10 ohms. Additionally, it is recommended to have one earth electrode/ground rod for each installation.
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