February 2010. We will start with a story of disaster. I took on a straightforward job to convert a kit built LNER Beyer Garratt. First complication was that, perhaps not unexpectedly it had two motors. As the current draw was modest (about 400mA) I decided on one decoder with the intention of switching off the BEMF if the two motors fought against each other. Tracing the wiring I found that the front chassis unit was live to one rail and the rear chassis unit to the other with some rather thick unsightly wiring between the two units and the motors. I replaced that but could not work out why there were another two wires attempting (unsuccessfully) to pick up off one pair of drivers.

After a bit of head-scratching I found they went to a smoke unit which had not been mentioned. Measuring the current draw this was a little too high for the decoder - but easily overcome by commoning up both functions on the decoder. The smoke unit did look magnificent when working on analogue so I eagerly anticipated seeing it on DCC. Oh dear, before I could even test it there was a slight crackle and a few twitches from the motors and silence - and some smoke from the chimney.
A careful look at the wiring suggested that just possibly one of the smoke unit wires might have been shorting so that was quickly sorted out. On a retry the decoder proved to be dead so I replaced it with another. Loco moved OK but very soon the decoder became an effective smoke generator - but not for long.
To cut a long story short, it was a metal sheathed smoke unit in a metal bodied loco - and one of the wires was connected to the sheath - to my surprise. Hence the track power was connected to the function side of the decoder - hence the double death. I am in the process of ordering a plastic bodied smoke unit which should fix the problem. And I have to go to my supplier with two burnt decoders for replacement. Hooray for the TCS Goof Proof guarantee !!