Immersion Heaters and Potential Fire Risks

Immersion heater terminal box. © Eugene Brennan

So I was doing some gardening for the sister today and she mentioned a smell that was coming from her immersion, which immediately rang alarm bells in my engineering head, knowing what was likely. So I investigated.

I turned it on and there was a puff-like sound and small flash from under the cover of the terminal box. There was also charring around the cable where it entered the cover. I told her she was very lucky she had told me what had happened and that no clothes were on the bottom shelf of the hot press, close to the element. I told her she needed to get an electrician. Luckily, someone came out in the afternoon, and I explained the symptoms to him, and we agreed on what likely caused it—a loose connection, possibly due to insufficiently tightened terminals. It could also have been caused by another phenomenon known as conductor creep. (See below. )The electrician took the cover off the terminal box, and we discovered that cable conductors and terminals had undergone a complete meltdown. So a new element will be required. The electrician mentioned that this can happen more often around the neutral terminal. I suggested that both neutral and live carry the same current. He suggested that neutral can carry a bit, more, which isn't true. Anyway the moral of the story is to monitor your high power appliances regularly, especially where they connect to the power source. This could be an isolating switch such as a "sink/bath" switch of an immersion element or the plug in a socket outlet, or the socket outlet itself. Plugs and switches can get warm, especially the fuse in a plug, but shouldn't get hot. Terminals in socket outlets can get loose and/or worn from repeated insertion and removal of plugs. Also, the spot welds or rivets that hold the spring contacts onto the busbars inside sockets can also get loose from repeated movement.
Loose connections can potentially cause fires.
I never leave high power (and consequentially high current) appliances unattended, and I never use the time switch for the immersion.

Conductor Creep and Stress Relaxation

Another phenomenon, which could have caused it, and which I did some research on when I got home, is conductor creep. When a screw is initially tightened in a terminal, copper wire resists the pressure and deforms elastically, like bending a piece of springy steel. So it pushes back on the screw, However, over decades, the metal permanently deforms plastically and flows, relieving pressure on the screw. So the screw in turn effectively doesn't put as much pressure on the conductor. If you've ever opened a plug and tightened the screws and they seemed loose, but you knew were diligent about tightening, this is what could have happened. Contrary to popular belief, screws don't actually loosen themselves and unscrew, although this can be caused by vibration, once pressure is taken off them. This loosening can cause arcing. Another similar mechanism that causes reduced pressure in brass terminals is stress relaxation. Both conductor creep and stress relaxation can be accelerated by thermal cycling as metals expand and contract due to heavy currents and hotspots caused by increased resistance at connections.
Some info here on stress relaxation and conductor creep.

 

Further Reading