eli5 how is a neutral and earth able to use the same wire (PME) in electricity distribution? (In the UK)
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The earth wire needs to be connected to earth at some point. This can be with a earth rod at the house – a long metal rod buried in the ground. However, these tend to give a bad connection and may be unreliable if not installed correctly, or if the ground dries out. Therefore a lot of care is needed if an earth rod is used, and there is still a risk that it won’t work properly.
It is therefore nice if the distribution network operator can supply the earth wire. Historically, electrical cables were built with an outer jacket made of lead. This is a conductive metal and is buried in the earth, so gives a really good earth connection – it also connects directly to the heavy duty earth connections at the electrical substations. This gave a super reliable and very good quality earth connection to any houses which connected to it.
After lead cables went out of fashion and were replaced by plastic, this type of earthing is not longer available in most places. However, if the distribution network operator can provide a good quality earth connection then that would be better than individual rods.
If you combine the neutral and earth wire together as a combined neutral and earth (CNE) conductor, then this does actually work as a good earth connection. However, there is a potential danger if that wire gets broken. If there is a fault, then the downstream end of the CNE wire is connected to appliances, and then to the live wire. This can cause dangerous voltages to appear on the downstream end of the wire, which includes the earth wire and everything connected to it (like metal appliances).
For this reason, it is strongly recommended to keep the neutral and earth separate. If the neutral breaks, the earth wire remains safe to touch.
The trick with a PME earthing arrangement is for the CNE connection managed by the distribution network operator, to be connected to numerous separate earth connections, in a highly reliable and interconnected way. This way, if the wire breaks, then there is still guaranteed to be an earth connection on both ends of the cable.
For example, a street has 2 substations, one at the north end and one at the south end. A mains cable runs from North substation to South substation, and it loops in to each house as it goes past. It is connected to earth at both substations. Even if the wire gets cut, each piece is still connected to earth, and so dangerous voltages can’t build up.
Because house wiring isn’t built in such a reliable way with multiple earth connections, the safest option is to separate out the neutral and earth into separate wires, until they connect to a highly reliable connection.
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