NICEIC
Approved Contractor
No. 40492

The fault can be attributed to poor installation. The picture shows the Main Switch that was replaced. One of the connections burned out because the fixing screws weren't done up sufficiently tightly. The relatively loose connection would get warm with use, and over the years the warm-cold cycle caused the wire to become looser, which in turn led to higher resistance and higher temperatures. 

Eventually, the whole switch would get so hot when appliances were being used that it would simply cut out.

The customer remarked that it seemed to happen when the washing machine was on the spin cycle, which seemed bizarre. It was the RCD that tripped, and this turned all the sockets off.

It turned out that when the machine was installed, the edge of the machine was left resting on the supply flex. Over the years, the vibration of the machine and the slightly sharp edge of the base of the machine cut through the cable.

Look closely at the coil of cable at the 12 o'clock position. See the brown and blue cables inside the white plastic sheath. The insulation on the blue one was damaged and as a result the conductor could touch the chassis of the machine. (See enlarged part of picture)

This kind of fault is detected when a Periodic Inspection is carried out. For industrial and commercial premises, the recommended period between inspections is usually no more than five years. 

But five years is a long period during which additional equipment may be installed or upgraded. If consideration isn't given to the total electrical load, cables can become overloaded.

Look closely at the red wire on the left - which is wired into the earth connection.

Presumably the installer had run out of regulation green/yellow sleeving, and thought that red would do.

Although this picture is quite shocking, there have been instances where the converse has been the case: wires with green/yellow sleeving used to carry the live 230 volts!

It has a life expectancy of around 20 years, and none has been installed since the 1950s. So any cable encountered in an installation is well past its best-before date.

Look closely at the metal conductors, which are tinned copper. There are deposits on them - deposits of decomposed rubber. 

Over the years, the rubber that insulates the conductors turns powdery. If you move the cable at all, you will discover that the conductors are free to move through this decomposed rubber dust and TOUCH. The cable then explodes with a brilliant white-pink flash.

Look very closely at the wiring inside this 4-way extension lead. Trace where the brown wire goes, and the green/yellow, and the blue. 

When the unit was manufactured, the connections were installed incorrectly, each rotated clockwise from where it should be. So the brown wire (Live) goes to the Neutral outlet, the blue (Neutral) goes to the Earth outlet, and the green/yellow (Earth) goes to the Live outlet.

This extension lead worked OK for years and the problem only came to light when a replacement consumer unit was installed, and the RCD kept tripping. The old-fashioned fuse box couldn't detect this fault, and happily allowed Neutral load current to pass through the earth wire. The practical outcome of this was that whenever a piece of equipment plugged into this extension was switched off, it would still be live.. 

When it was taken down, water gushed out; so much for being waterproof!

Water had built up inside up to the level of the connections to the socket.

The problem here is that a waterproof fitting has been used, and conduit pipe is used to carry the supply cable. The daily warm-cold cycle results in condensation building up inside the conduit and the light fitting. Normal practice is to put a tiny drain hole at the bottom of the fitting so that water can't build up.

In this picture, a polythene terminal strip has melted almost beyond recognition. The insulation on the red and black wires has been melted away.

Imagine what would happen when this assembly was moved, even slightly, when the lightbulb was changed.