“There were multiple mistakes by an "electrician" that resulted in this call-out.
The cable was originally installed at 4mm. When a new shower was installed, it had a far greater Kw rating than the old one, from 6Kw to 9.5Kw. The new shower started to trip the MCB, the installer returned and replaced the MCB with a 45A fuse. Added to this he did not install the isolating switch correctly.
The above conditions meant that to be safe, ALL circuits had to protected by 30mA RCD. The previous "electrician" should have upgraded the protection, not removed it. This meant that the metal light fittings could have given the home owner's family a fatal electric shock.
The result was a cable that now behaved more like a heating element when the shower was on, why this did not lead to a house fire is purely down good old plain luck. The good luck element was that the isolation switch stopped working so we were called to investigate and noticed the dangerous installation. The homeowner did not receive either of the two required certificates.