Terminal Burn Out.

I have read a lot of American stuff about AFDDs and the assumptions in the justification for AFDDs, fire statistics etc is that all electrical fires are caused by arcing. 

Is this the case? If there is a poor connection with oxidation generating heat will there be an arc? This is a genuine question and I am hoping someone on the forum knows. I have seen a few badly overheated terminals and I am not sure that there was necessarily signs of arcing. I would expect to see bits of molten metal or holes in either the conductor or the connector from loss of metal to the arc and I have not seen these in most cases. 

Over the years I have found a few but not many loose connections on incoming tails. A neighbour recently suffered a loss of power. It turned out to be a loose neutral connection in the recently fitted smart meter, he got a £30 credit from his supplier for the inconvenience! Not enough to cover my Sunday callout charge if I had charged him. 

Looking at the thread on the fire after a EV charger had been installed. How many of us can guarantee that we have always  checked all CU connections when working on a CU?

You are right to be wary - copper copper and copper to brass casual contacts do not generally strike and maintain an arc, if you have an arc welder and time to waste, try and draw an arc with an off-cut of copper wire in the grip instead of a steel rod. In general it is almost impossible, even with a machine whose voltages and currents have been optimized for striking arcs, mostly it will either blow clear like fusewire or melt and stick solid. However arcs with carbon rods, steel rods, and between bits of aluminium are relatively easy.

It is telling that the certification test for an AFDD requires PVC covered copper wire to be pre-charred 'conditioned' by a few kV- a mechanically damaged wire alone will not do, and may well not be detected.  Overheated insulation is more likely in the vicinity of a loose contact,  or in cables that are not fused down correctly, so perhaps some char is realistic, but HV pre-conditioning to make an AFDD work seems an odd thing to require.

In the UK we do not see much aluminum wire on low current circuits, and copper clad steel only turns up in the  worst 'fake mains cable' scams, while our layered approach to over-current protection  means that overloaded cables to the point of charring should not occur, at least not in the fixed wiring, perhaps extension leads. I can see a far stronger case for AFDDs when local wiring standards mean these are more likely.

There is no substitute for getting the cable to fit well into the terminals and doing them up properly. And then giving it a wiggle and re-checking. And maybe if that cannot be ensured, then a smoke detector.

Mike.

Read this kfh, it might give the description you are looking for!

engx.theiet.org/f/wiring-and-regulations/28497/implications-of-the-term-recommended-in-7671

Mike has left a bit out too, a steel-steel arc without a coated electrode is very difficult, this is actually a major part of electrode coatings, that they can easily form the ions to start an arc. It is also why wet rods are not much good!

I am finding with modern switches and larger cables for cooker or shower circuits, that I can not always double over the ends of the cables when terminating to ensure good connections that will not come loose when fitting the accessory to its back box, sometimes with much struggling with the larger sized cables. I was happier with the old two screws per terminal for cooker connections and shower connections, but alas these have disappeared.

Yesterday I was very impressed by the solid good quality terminals on an Aga Rangemaster. It has a gas hob and 11.5kW of electric ovens. Far better that the plastic flimsy excuse for a cooker termianl box on the back of most cookers.

So now I terminate, tighten the screws, move the accessory to near its final fixed position, test the screw tightness again, and even a third time after moving the cable.

Z.