N2 terminal on 50A Mk dipole switch overheats

[kez1]

Have 10.8Kw shower attached to 50A Mk Switch.
Outward apperance of switch is fine but switch used to get warm during use.
Noticed neon light flickering so pulled switch, peeled wires back to new and replaced with another MK 50 A dipole switch.

Photos show N2 terminal was overheating and had partially melted blue sheathing around it.
Ideas what has caused this?

 

[Jackrae]

Loose screw, faulty switch (unlikely as heat seems to be from termination rather than switch contact) or more likely blue insulation partially trapped under screw so decreasing current carrying capacity of termination. Can I suggest careful removal of blue wire end to see if there was trapped insulation.

 

[kez1]

Loose screw, faulty switch (unlikely as heat seems to be from termination rather than switch contact) or more likely blue insulation partially trapped under screw so decreasing current carrying capacity of termination. Can I suggest careful removal of blue wire end to see if there was trapped insulation.

Terminal screw was tight when I first removed wires / switch from wall.

Blue N2 wire is stripped back 2 - 3 mm beyond where the grub screw bites when fully pushed in as far as it will go, so seems no blue insulator from the wire was under the grub screw.

Looking carefully at the wire it seems very good contact was made by grub screw with one strand and a bit of contact with one other strand but not apparently with the other strands.
Grub screw certianly seemed tight when removed. Had to apply a bit of pressure to all 4 grub screws to get them out.

 

[Jackrae]

The 'earth' terminal looks pretty green suggesting ingress of water; but even that wouldn't result in gross overheating of the neutral terminal.
You can see the plier marks on the cable end, caused by the sparky twisting the strands together. Clamping screws can only bear on one or two strands but if the wires are twisted, which yours were, then the strands effectively form a 'solid' conductor. I looks like a not-fully-tight termination. Once the termination has been suitably 'cooked' the screw would probably be tight due to products of heat (oxides)

 

[AndyPRK]

maybe best not to twist the wires, for better contact ?