How I troubleshooted a tripping MCB

A tripping MCB is nothing uncommon and I have troubleshooted the faulting device many times before, but this time was interesting and challenging; hence the post.

We had this nagging issue of our master MCB tripping every time we turn on the ceiling fan in one of our bedrooms. To complicate the problem further, the fan is a designer fan and has 3 light bulbs and a built-in speed control (via hanging strings). So the faulting electrical component could be one of the bulbs, OR the speed-regulator inside the fan, OR one of the fan's electrical components (capacitor, coil etc.,), OR the speed-regulator on the switch board OR something that I didn't think of then.

The regulator on the switch board was the least of my suspects, as my first suspect was the bulbs (hunch!) or something in the fan. Removed the bulbs and tried, no use. Opened the fan's casing and looked for any obvious burns or something -- no sign of any electrical fault. This doesn't rule out a problem with the fan, but I wanted to cover all easy approaches before getting into disassembling a fan.

Just decided to checkout the switchboard. As I opened the switchboard, it was evident! As you see below, I could see a burnt regulator casing with some internal leakages.

It appeared like, this was the issue. Since our fan has a builtin regulator and I didn't have a spare one handy, I decided to short the regulator off the circuit and use the fan's strings to control the speed. Changed the wirings quickly and tried, and Viola! the fan is running normally without tripping the MCB. Even the fan's built-in regulator worked.

(Pic: had just taken the regulator off the circuit) 

With sense of satisfaction, I switched off the fan and closed the switchboard completely and screwed it tight. All done, so wanted to try for one last time that all is well. I turn on the fan, and the MCB trips! Wow, that's something :)

I had to stop for a while, as I couldn't come up with a theory for this. As you might know, the typical household switches control only one polarity (typically phase/line), the neutral goes to the appliance (fan) directly. The possibility of a short-circuit inside a switch board is pretty less (except if the switch board has a high amperage plug or something, that controls both polarities). And in this case, the MCB trips only when I turn on the fan. So it should be something related to the switch, but the switch has only one polarity! I was pretty sure, I'm missing something, because I couldn't explain the behavior.

The hint I got was the fact that, it all worked until I closed the switchboard. I just realized that our MCB also has Earth Leakage Protection. So it's probably not a short, but earth-leakage. On a closer look at the wiring that goes off the switch, towards the fan, I could find this:

In one of the joints, the insulation had burnt a bit, exposing few strands of the wire. When I closed the board tight, these strands came in contact with the metal casing inside the switch board, which is grounded. This triggers a leakage on the earth line and the MCB trips. I just had to redo the insulation with new tape, and that was it. The fan is running fine since then, without tripping the MCB.