At the worst possible time, just one day before my technical exam, I started the engine and saw a nasty cloud of black smoke emerge from the engine bay. I killed the power and ran over with the fire extinguisher to see what was burning. When I opened the hood, there was nothing. No smoke, no fire, no scorch marks. Zip.

The smoke had appeared to come from around the fuse box, so I checked all the fuses and wiring, but still nothing. With no better ideas, I sat a GoPro camera on the radiator and pointed it at the fuse box, then started the engine again. No smoke, this time, but a very rough start.

It took a while to find any useful information, but thanks to the Transit Forum, I found the culprit.
Underneath the main fuse box is a red wire that looked like it had decayed over the centuries. It had actually burned out when one of my glow plugs had given up (possibly short circuit, but probably just age), causing the power to the rest of the plugs to be cut off. That explained the rough start; the cylinders were cold. But this is an odd way to fuse a circuit. A 100A plate fuse would seem like a better option.

The slightly melted insulation of the fusible link, as found underneath the main fuse box.

If you have a blown link, it might be a bit of a pain to replace, if it's connected underneath the main fuse box, as mine is. To get to it, you need to lift the main fuse box out, which is easier said than done, owing to its mass of tightly-interwoven cables, as you can see. There's very little wiggle room and the clips that hold the main box on are tricky, too. Be careful, because they're also pretty sharp. Wear safety gloves and press the clips inward or outward (depending on your van) with a thin screw driver. Do it gently, otherwise they will snap. Once all clips are undone, the box will probably pop up a little bit under the pressure of the cables. If so, you'll need to put a bit of pressure on it to get it back together after replacing the link.

Once you get the box up, you then have to get in and extract the cable. If you're lucky, it'll be connected somewhere close and you should be able to unplug it and replace it relatively easily. If you're not so lucky, you might have to cut it and use a strong cable junction to connect a new cable, rated at 100A. That's what I had to do. Not impossible but definitely fiddly.

With the new cable fitted, you need to find out which glow plug(s) have given up, otherwise you're going to blow the fuse again when you try to start the engine. Unless you have a new problem, the plug(s) that died probably did so because they are old, in which case, you should get them all replaced. That doesn't mean you shouldn't check for other possible causes. You know it never rains, but it always pours.

If you want to tackle this one by yourself (and you might struggle to find anyone else who will accept the liability), have a look at this article to get an idea what's in store for you.

Hidden inside a plastic cable runner, the blown link is just visible as charred wires.

The main fuse box is in the engine bay. It protects high voltage circuitry and houses solenoids and other components that control lights, fans, and more. The secondary fuse box is usually inside the dashboard. That protects smaller circuits like the dashboard and interior lights, heater, indicators, and the 12v lighter socket. The secondary fuse box is also where you will diagnose issues with the brake light switch, horn, tail lift and more. Always check your manual to find out which fuse is for which circuit, or you'll waste an awful lot of time and sanity pulling fuses.