V6 blowing Main Fuse/100A?

[Mrs Feynmans Bongos]

Just back from a fab BH expedition into Wales in our V6 AFT.

Except for a Bongo wobble on 2nd campsite, where we'd parked all day while we walked.
On return it started fine, I moved it to a suitable pitch, raised the AFT while running, then turned Ignition OFF.
With no ac hookup available, I then turned ON my Coolbox, also pumped some water for cooking. (We couldn't afford a LB when we bought it, so the Conversion wired both those into the Standard Battery: submersible Pump hard-wired; 50W ac/dc Coolbox via an Accessory socket he retrofitted.)

While I visited "site facilities", MrsF tried to close the cab windows. She turned Key to "I:Acc" but nothing, nor when we tried to start engine, not even clicks, only headlights worked. Next day despite some kind offers of jump-starts, I found the 100A MAIN FUSE BLOWN. Fortunately I was carrying a spare

Questions, all towards the obvious "why on earth did my main fuse blow?"
- as a matter of good practice, should any retrofitted accessories or sockets be fused? (I don't think mine are )
- if they are not, could any startup transients due to windows/accessories somehow get magnified given other devices on-circuit, giving a big spike >100A?
- similar pumps to mine are rated 10-20W, ie new/pristine. Could one such but 4 years old stall at much higher than 2A ?due to limescale/bubbles/mould?

Advice appreciated.
Meanwhile I have ordered 5 more spares fuses; we will only pump or cool with Ignition OFF (ie stop to close windows/blinds); am saving for a LB fit

[g8dhe]

Any cable connected to a power source needs to be protected by a fuse - the fuse protects the cable not the device on the end so a cable rated at say 20Amps should have a fuse no more than 20Amps protecting it. Its normal to pickup from existing cabling for low current devices anything less than 10 Amps would normally be supplied from the Ancillaries circuit, so that you can power it up without turning on the engine itself. If you have run cables directly back to the battery then you MUST fit a fuse panel close the battery itself, but be warned its NOT a good idea to use the starter battery for these circuits its only too easy to leave them drawing power and leaving you with a flat starter battery by morning.

As to what has blown the 100Amp fuse, normally its fairly obvious as it takes a fair old wack to blow one! The most common cause is a damaged alternator or its wiring, but a careful inspection would normally identify the cause, if you do have added wiring which isn't protected by fuses then it needs to be sorted ASAP as its a significant danger, as there is a lot of crumple zone in a Bongo as the engine isn't under the bonnet to absorb any impact so batteries and wiring get damaged very easily and can then cause fires!