Burning smell

Hi guys

as u know I'm learning at the moment so please don't berate me if I'm wrong. My friend called me to say that he was home when his whole flat went out at once, when he went to his cu (old board with ceramic fuses maybe?) he said the knob on the far right had turned to off. ( i dnt know what type of switch this is?) He tried to turn it back on and it wouldn't turn until after several attempts. When he finally turned this knob on all power resumed as normal, however when he went upstairs he noticed his immersion heater was not on and he could smell burning from the airing cupboard he thinks.

Now I said to him that it sounds like a loose connection, possibly the burning smell was insulation? Could be caused by wear and tear, could be something that has bitten into it?

Am I right in saying there sounds like a fault in the immersion circuit? This is on it's own dedicated circuit as shown in the cu by a blue 15a fuse. Has the fault blown this fuse and then tripped the main switch (which i presume is the knob?) due to circuit overload?

Thank you for your help

A fault causing something to get hot to the point that insulation breaks down can cause the fuse to pop or an RCD to trip. My guess is what he is calling the main switch is probably an RCD. Main switches don't operate on overload, they are a switch only.
RCDs on the other hand operate on earth leakage.
That aside it is a good idea to leave the fuse for the immersion heater out until it has been checked out by a competent person.

Thanks for that so you think it's an earth leakage as upose to an overload? Oh I forgot to mention the main switch is actually an earth leakage circuit breaker. If I was the spark goin in would I isolate and conduct an insulation test on the immersion circuit? And then find the fault visually?

rotating knob is most likely an RCD. IIRC old Dorman Long and some old MEM were like that

it looks like it rotates but actually only goes about 1/4 of a turn.