Blowing fuses

[willnotwish]

In my house I have a really old 60s style "fuseboard". This is not a modern consumer unit but a set of rewirable fuses.

When I switch the bathroom fan on, there is a flash at the switch and the 5A lighting circuit fuse blows. If I replace it with fuse wire and try again, the same thing happens.

If I isolate the bathroom fan (it has a dedicated switched fused spur), I can operate the switch OK, but -- of course -- the fan doesn't work! This proves that the wiring to the spur switch is OK.

[willnotwish]

I have had trouble with the fan before. It is a capacitor start-and-run single-phase induction motor, which ought to be very reliable.

It originally had an internal fuse on one of the windings (I can't remember which). This blew ages ago (same problem possibly), so I moved the fuse to the terminal box where I could monitor it without having to dismantle the motor itself.

I removed the motor and checked the 2A fuse. It is fine.

So, the question is: why is the main 5A fuse blowing when I turn the fan on, but the 2A fuse is not?

[willnotwish]

I previously replaced the original capacitor with two identical class X 0.47uF capacitors in parallel. They are rated at 250V AC.

[willnotwish]

I have the motor on the bench. I am going to run some tests to investigate the inrush current when starting.

The fan is largely unloaded while on the bench. I would need to connect some ducting to load it to simulate a more typical use-case (the bathroom). High starting current is to be expected if the ducting were blocked in the bathroom. It isn't.

I will go slowly using a current limited supply (dim bulb tester) via a variac to start with.

I am also interested in the fuse characteristics of plain 5A fuse wire when compared to a 3A cartridge fuse or (maybe) an MCB.

[willnotwish]

I will also investigate the characteristics of different motor capacitors.

[willnotwish]

It's still a mystery as to why the main 5A fuse blows, but the local 3A does not.

[willnotwish]

The new fan seemed OK for about a week. However, when operating the switch to turn the fan on one evening, there was a pop, a flash, and the 5A fuse in the fuse board blew again, taking out the lighting circuit. This is the same behaviour as with the old fan.

[willnotwish]

When diagnosing the issue again, I discounted a faulty fan (it's new) and turned my attention to the 3A - 5A fuse issue.

It occurred to me that the 5A fuse blowing, together with the loud pop and flash from the switch, pointed towards the switch itself - not the fan.

I unscrewed the switch from the wall and examined it carefully.

I could see some black coloured burn marks on the switch body and in the earthed metal back box next to the mounting screw hole.

Careful examination of the switched live to the fan revealed a pinhead sized area lacking any black plastic insulation. There was also a similar sized shiny mark on the metal back box.

I think the switched live from the fan wiring was pinched between the switch body and the back box.

What was happening was that, when operating the switch to turn the fan on, sometimes (but not always), the bare cable touched the (earthed) back box and caused a direct short across the ring lighting circuit. This caused the main 5A fuse to blow, bit did not affect the motor protect 3A fuse.

The problem is made worse because the switch is actually a 3-gang type, with three grey T+E cables coming into the back box, which is itself reasonably shallow and not necessarily designed to accommodate six wires.

[willnotwish]

I rewired the switch and positioned the wires more carefully. Let's see if it's fixed!

[willnotwish]