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. 

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.

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? 

There have been several discussions on that very subject (and not just on overloads). Several theories abound, but my favourite is that shower pull switches (especially the types with terminals on the "faceplate") are very easy to create loose connections (as large stiff conductors, typically held by a small single screw attempt to twist as they're pushed back into the backbox) - that should happen equally with L and N of course, but then a loop test (or R1+R2) is done - so loose connections on L are much more likely to be spotted and therefore corrected; while on the other hand none of the standard tests check N continuity, so loose connections there are much more likely to go into service.

   - Andy.

AJJewsbury said:

shower pull switch

I am asumming that it is located in the room of the shower and thus is exposed to high humidty and ingress when the shower is in operation.  What IP rating does the shower pull switch have other than a very basic IP2x ?

AJJewsbury said:

shower pull switch

I am asumming that it is located in the room of the shower and thus is exposed to high humidty and ingress when the shower is in operation.  What IP rating does the shower pull switch have other than a very basic IP2x ?

Moisture ingress wouldn't account for why (empirically) N terminals suffer more than L ones.

  - Andy.

Good point.  I would expect to see both terminals with the same issue.  Could that question be put to the manufacture maybe.  So someone like MK add their thoughts on why the N burns out more often than the L in a shower pull cord.  I am summing that the L and N terminals are built the same.