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. 

To be more specific, I am referring to a neutral-to-earth fault on the supply side ring circuit that connects each RCD, not on the load side. Such a fault would not cause an MCB to trip, as you explained.

But by definition, the current on the L path through the RCD and the current through the N path can't differ by more than 30mA, or the RCD would have tripped. It doesn't matter what arrangements or faults there are upstream of the RCD. Only if there was a a N-E fault just upstream of the RCD (as in mm or inches) which caused a fault current of many amps to flow (which seems unlikely) could the upstream N get cooked and some of that extra heat get conducted to the N terminal. All seems a bit unlikely.

I am referring to the current in the neutral wire of the ring circuit that supplies power to the RCDs, not the current that goes through the RCDs. Also, the earth rods of different pitches may have different voltages between them because of various factors. So, could this make the current in the neutral  of the ring circuit higher?

I may have misunderstood the setup, but is it not a ring circuit that supplies power to multiple RCDs? And have they not had several RCDs fail on the supply side only? Is it not the current in the neutral wire that goes around from the distribution board through all the neutral terminals on the line side? The current in the neutral wire can still be balanced through each RCD.

What is the rating of the protective device and what is the max demand? If the circuit is on a 40 A breaker, there could be a risk of overloading a 16 A terminal if the load on that terminal exceeds its rating. This could cause overheating and damage to the terminal and its connections.

Hmm if the supply terminals each held 2 wires, a loop in and a loop out as part of that ring, then the terminal may be overloaded. But that probably applies the same to the live path and I thought from the description it was more of a ring with spurs.

Is it ? How many wires in each terminal ?

Mike.

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.

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? 

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. 

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 ?

That sounds good, though how often does an R1/2 test result in a re-tightening of the terminals?  - maybe for some folk, but personally I'm struggling to think of a time. Generally it seems either pass or fail by miles because some vital part  is actually missing - being off  by fraction of an ohm is not a thing that happens to me, unless the cable length/ type is wrong.

Mike

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. 

I agree. If a terminal screw is making light contact, it will pass a loop test. It is when a current of 40 A is drawn that problems arise.

I strongly suspect that there is observer bias going on: over-heated neutral terminals get mentioned more often and once the myth has been established, it perpetuates itself.