Before crossing the "Golden Bridge" (on the outer sides) to the campus this morning. I liked the clouds. So mild and windy, compared with the damp chill and fog of the previous two mornings.

Extra, with a bit of a horror story. Well, it was horrifying to me. 

Faulty bits from someone's previous repair, hence the solder and teeny bit of other cable on the left, after removing the sleeving over the joint. I very much suspect the repair was for the same fault, but they failed to clean the heating element's contact which the crimp (thing on the right) slides on to, and didn't fit the crimp tightly enough... which was why I couldn't find at first. It had fallen off! Other side of element - no problems.

Just a small amount of resistance from a loose connection or dirty contact can eventually lead to this happening. It may take years, as the copper slowly tarnishes with the heating and cooling from the extra heat generated from the high current and that little resistance. 

Eventually the tarnishing works its way into the contact(s), and the resistance increases very slightly more. The heat then increases. When it becomes sufficiently high, that PVC insulation (not a good choice for connecting to a heating element!) starts to melt, and gives off various acidic substances (probably hydrochloric acid) which attacks the copper, hence the green corrosion. This reduces the conducting cross-sectional area, which in turn increases the resistance, and the process accelerates rapidly. The cause of many house fires from the likes of washing machines, tumble dryers, and immersion heater circuits, especially if someone fitted the likes of a plug, and missed just a tiny single strand of the copper wire. May work okay for years, but eventually...

The piece of equipment this was from, was a small autoclave - hence some water about, and the need for a heating element. It is very much based on the same well known company's pressure cookers. Have repaired a couple of identical types at work before.

For the repair I used thicker cable with heat-resistant sleeving - a bit left over from an oven repair - a new crimp, some solder and a double layer of heat-shrink sleeving over the joint to the cable. Cleaned up the area from fumes etc, and the element's contact, and ensured a tight fit for the crimp. The owner asked how much it cost, "Just a few pennies, don't worry about it, or a packet of Aldi's plain chocolate digestives if you really want to make my day." Gave them full details of what I'd found and done, along with a photo like the above.

They gave it a good testing, and it's working fine. That was last week. It was this week in conversation that they revealed they'd previously sent it back to the manufacturer for repair. It's a circa £1,000 piece of equipment, and the company would have (hopefully) done a full safety test on everything before sending it back, but other than that, the material repair costs would have been very much the same as I worked out, as there were no signs of any other works being done - so a few pennies. How much did they charge? The horrifying bit... £400.