Safety of clamping two solid core wires in a ring circuit socket

Question

I just bought a house in the UK for the first time and have discovered the joys of ring circuits. One of the double sockets in the house was faulty (switch would always stay on). I took it off the wall making sure not to touch any of the live wires (ha just joking). Inside I found the live wires had badly corroded (but not the neutral - some galvanic effect I assume) and one of them had totally melted.

I'll ignore the obvious safety issue with ring circuits that this may have broken the ring and I had no way of knowing (without tedious manual checking). My real surprise was that apparently the official way to continue to ring is to screw both incoming parts of the ring into the same screw terminal in the socket.

These are solid core wires. This can't possibly be safe can it? In my experience it's extremely difficult to securely screw two solid core wires into the same screw terminal so that they are both securely clamped - far too often the screw is stopped by one of the wires and the other one is loose, possibly leading to a high resistance connection.

Why is this considered safe? Why don't sockets just have two pairs of screw terminals that are connected internally?

Answer 1

This can't possibly be safe can it?

Most people outside the UK consider UK plugs and sockets to be, if anything, over-engineered for safety¹. The use of fuses, shutters, sleeved pins, polarised connectors, angled cable outlets, high insertion forces, finger protection and so on are not universal and ubiquitous outside the UK. Of course, you can find on eBay sellers of non-compliant and even dangerous sockets that are aimed at the UK market but which do not meet UK standards and which would be illegal to sell in the UK.

The specifications for typical UK sockets does not allow for mixing wires of different sizes.

^{Source: MK}

As you can see from the example spec above, the terminals are designed for clamping multiple wires together.

A normal 32A ring main uses 2.5 mm² conductors, you can clamp three per terminal (e.g. in, out, spur)

So long as you check the screw is tightened fully and then check that you cannot pull any wires out, it will be safe.

I believe the intent here is that a gas-tight connection be formed between the conductors and the clamp. Copper is a fairly malleable metal so it should normally be distorted by the clamping mechanism such that the surfaces formed on the wires conform closely to each other and to the clamping surfaces.

As you note, its a loose connection that causes overheating and burning.

It isn't normal to see any significant corrosion other than normal superficial tarnishing of copper from exposure to air.

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Why don't sockets just have two pairs of screw terminals that are connected internally?

You can get sockets with dual terminals, but these are specialist unswitched sockets for use in medical applications - each of the terminals can still be used to clamp multiple wires (four 2.5mm² in this example)

^{Source: M2}

• Dual terminals for live, neutral and earth Each terminal accepts 4 x 2.5mm²

• Screw cap covers minimise dust & dirt traps improve effectiveness of infection control procedures

• Clearly engraved 'MEDICAL EQUIPMENT ONLY'

• Face plate manufactured from a blue urea compound which contains a very effective life-long anti-microbial & anti-fungal additive

• Isolated Power Supply sockets for medical applications

• In line with HTM06-01 and MEIGaN requirements with clean and dirty earth connections

These would be inappropriate for use in a home.

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Notes

¹ If we set aside the issues with split ring-mains and treading on BS1362 plugs.

Answer 2

Arc damage is arc damage and it sounds like you have some.

You shouldn't be putting two wires on a screw unless the screw is listed for that purpose and the labeling/instructions say it is OK to do that. If you can't find an appliance with screws listed for that, use a 3-splice connection to a pigtail, and lay that in the back of the junction box and give it no further concern. The pigtail goes to the socket.

Some sockets do have 2 screw connections per side. You have to shop for that, but it may be regional. Virtually all US dual sockets have 2 screws per side, and have a breakaway tab which allows splitting of the screws so 1 screw powers 1 socket. Even among those, a few allow 2 wires per screw.