Hi DMB, probably loose connection but just a thought. Are the pitch sockets connected by actual ring final circuit? Maybe this is a bit out there, but due to the earth rods etc of the pitches there could be a potential difference between these rods/plates buried in the ground. This could make a circulating current in the ring circuit that could also make the neutral connection of the RCDs too hot. The circulating current is a current that goes around in a circle inside a circuit without going through the RCD. 

Hi DMB, probably loose connection but just a thought. Are the pitch sockets connected by actual ring final circuit? Maybe this is a bit out there, but due to the earth rods etc of the pitches there could be a potential difference between these rods/plates buried in the ground. This could make a circulating current in the ring circuit that could also make the neutral connection of the RCDs too hot. The circulating current is a current that goes around in a circle inside a circuit without going through the RCD. 

A few things make that difficult...

Firstly each electrode will have a significant resistance around it (due to soil being a less than ideal conductor) - even in relatively conductive damp soil you'd likely be looking a several tens of Ohms for a normal 4' rod. So even if you had a very significant voltage difference - 20V or 30V say, you'd likely have less than an amp flowing - unlikely to be significant for overheating if the joint is otherwise sound.

Then there's how to get that current to flow along the N conductor(s) in the ring - getting the current to circulate around the entire ring would be tricky - you'd likely need some very odd combination of faults to get the c.p.c/armour and ring N to act as some kind of 1-turn transformer, especially if you wanted to get any significant current transferred (normally, being a ring, things would tend to cancel out). A N-c.p.c. short is maybe more likely - in which case the extra current would likely divide between the two legs of the ring (although not necessarily equally) - but as it was a small current to start and it's only going to get smaller.

Then how does that extra current flowing in the ring N get to overheat the RCBO terminal - clearly if it's on a spur it's not going to happen, so next most likely is that the ring N is looped into the RCBO's incoming N terminal - but in that case - as it's a single hole terminal - most of the current will flow direct from one wire to the other (the terminal just physically holding the two wire ends together - in the manner of a screwit) - only a small proportion will have to flow around the terminal itself - so again starting an extra amp or so the effect on the terminal is going to be very small indeed.

There may be other possibilities - e.g. supply N being at an unusually high voltage (e.g. due to a network fault (broken PEN) or an uncleared L-earth fault in another TT installation), which together with a N-PE fault could cause currents to flow in N conductors where they shouldn't (in PME systems which have bonding to extraneous-conductive-parts that themselves have a low impedance back to the source, such currents can be substantial) - but again in this situation we have the limiting effect of the electrode resistance. In such cases the likely complaint is much more likely to be tingles/shocks from electrical metalwork than burned out terminals.

   - Andy.

“If the circuit is protected by a 40 A breaker, there is a risk of overloading a 16 A terminal if the load on that terminal exceeds its rating” The location of the overload in the ring main depends on the position of the RCD on the circuit and the length of leg A and leg B of the ring. The current in each leg varies depending on these factors. When the RCD is closer to the consumer unit, the current in both legs is higher. When it is farther from the consumer unit, one leg has lower current and the other has higher current.

Imagine a scenario where there are three sockets on a 50m2 ring main. Each socket has a load of 16 Amp. Socket 1 is positioned 5 metres from the consumer unit, socket 2 is 15 metres and socket 3 is 25 metres away. Leg A at socket 1 is 14.4 Amps, leg B is 1.6 amps. Leg A at socket 2 is 10.5 amps, leg B is 5.5 amps. Leg A at socket 3 is 8 amps, leg B is 8 amps. Therefore, the total current on leg A is 32.9 amps and on leg B is 15.1 amps. It seems that the terminals of a 16 amp device could be overloaded in this case.

Yeah, but that doesn't explain why only the N has burnt out. Any such overload would be very similar on both the L and N legs of the ring.

The neutral pole is many times the first suspect also on a shower pull switch if your shower draws more current than the switch can handle. What are your thoughts? 

AMK said:

It seems that the terminals of a 16 amp device could be overloaded in this case.

No, because the terminal is not drawing more than 16 A. The CSA of the cables in the terminal is double the cables in the ring.

We have have discussed this in here previously. If the switch is 2-pole and the terminals are the same size (why would they not be) how can one run hotter than the other?

Let’s wait and see what DMB concludes. 

Remember that effective communication is a two-way street, and it’s important to listen to others’ perspectives as well as expressing your own. Instead of saying “no,” you could try saying “I disagree” or “I see things differently.” This can help keep the conversation constructive and productive. I hope this helps.

AMK said:

I hope this helps.

No.

AMK said:

I hope this helps.

No.