Quick answer
PME earthing failure occurs when the combined neutral and earth conductor (PEN conductor) on a TN-C-S supply loses continuity, causing the earth in your home to rise to a dangerous voltage instead of remaining at zero volts. The most common cause is a broken or corroded neutral connection between the transformer and your property. If you suspect a PME failure, do not touch metalwork in your home and call your Distribution Network Operator immediately. This is a life-threatening emergency, not a job for a homeowner or standard electrician to investigate alone.
PME stands for Protective Multiple Earthing. It is the most common type of earthing system used in UK homes today, and understanding it helps explain why a certain type of failure is so serious.
In a normal PME setup, your electricity supplier combines the neutral wire and the earth wire into a single conductor called the PEN conductor. This runs from the local transformer to your home. At your property, these are separated at the meter, and the earth connection is bonded to your gas pipes, water pipes, and structural metalwork. Under normal conditions this keeps everything at a safe zero volts.
The problem arises if the combined neutral and earth conductor breaks somewhere between the transformer and your home. When this happens, several dangerous things occur at once.
First, the earth in your home is no longer connected to the true earth at the transformer. Instead, it takes on the voltage of the returning neutral current from all the other properties on the same circuit. This can push the voltage on your exposed metalwork, your boiler casing, your kitchen sink, and even your bathroom taps to something close to mains voltage. Anyone touching that metalwork while standing on the ground or touching something else earthed could receive a fatal electric shock.
Second, because the fault is in the network outside your property, your consumer unit, RCDs, and fuses cannot detect it and will not trip.
As a homeowner, there is nothing you can safely do to investigate or fix this yourself. The warning signs to be aware of include flickering lights across the whole house, equipment behaving strangely, a tingling sensation when touching metalwork or taps, or a visible spark or burning smell near the meter or incoming supply.
If you experience any of these signs, especially a tingling from metalwork, follow these steps immediately.
1. Do not touch any exposed metalwork, taps, or pipework.
2. Switch off your main switch at the consumer unit if you can reach it safely without touching metalwork.
3. Leave the property.
4. Call your Distribution Network Operator on their 24-hour emergency line. In England, Scotland, and Wales this depends on your region but the national power cut number is 105.
This is a network emergency and it is the responsibility of your DNO to fix it. Do not re-enter the property until they confirm it is safe.
⚡ North London Electrician
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