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. 

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.

not sure I have understood you, I may have the wrong image of your idea or it sounds a bit unlikely to me. Currents in the CPC will not affect the RCDs - it does not even go through them, and the only neutral - earth bond will be at the site transformer or main incomer - on the load side the only time earth ever meets either neutral or live is during a fault, and the RCD fires some tens of milliseconds after.

However, burnt neutrals really are a thing and really seem to be more common than burnt phases even on single phase parts - cooker switches, shower switches you name it, only this week I was greeted  by a colleague holding some burn out floortrack (the kind that is a bus system to boxes with 13A sockets) with guess what, a burnt neutral, and we shook our heads and both agreed we saw more cooked  N than cooked L. Perhaps makers just cheap out on the neutral terminals or the live side is more likely to self clean and sizzle and then spot weld back to a low resistance state for the same degree of corrosion or whatever ??

If you do drill the rivets and open it onto a table, post pix if you can - post mortem are always interesting.
Mike.

I appreciate your expertise, but I have a question regarding a possible scenario. What if there is an existing neutral-to-earth fault on the supply ring main, which may not require RCD protection? Could the potential difference between the earth rods then increase the current in the neutral incoming terminal?

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.