Wondering if anybody has had the issue. The 8-way fuseboard under the driverside bonnet, first fuse on inboard side of fusebox, 30A. This fuse seems to support everything ! I don't think I've ever used all the consumers at once prior to my run a coup,e of nights ago. However with the fuel pump, heated screen and heater blower running this fuse literally melted (the plastic fuse structure!) due to overload and everything except Hazard lights died. I switched off the non essential loads, blower, heated screen, replaced the fuse and i was away. I need to measure the current with the loads connected so I'll need a 0 to 40 amp ammeter me thinks. I have this wiring loom diagram which i've used before but difficult to interprate into "wiring diagram". Just as an aside the diagram shows a 40A fuse used in one location and checking a similar fuseboard at ECS 30A is max permissable. I think i may need to change the fuseboard due heat damage to the fuse carrier terminals and at the same time separate the circuits via a additional fuses, i.e. install a 10 way board. Has anybody had a similar experiance with this set up.

You may be over thinking this issue which might not be overload, especially if the fuse itself didn't blow. The heat generated which melted the holder is likely to be due to a localised resistance in the fuse holder itself, due in turn to dirty contacts. This would cause a volt drop proportional to the total current draw of the circuit and according to Ohms law. The resulting power loss would he dissipated as heat and a significant amount if the current draw was high. I know you said a replacement fuse got you going again but that might be just remaking the dirty contacts so likely only a temporary cure.

Suggest a thorough clean of contacts if the fuse holder is still serviceable. Check also for water ingress which may have corroded contacts.

Forgot to add that since power is a product of voltage and current any losses from a given resistance will increase by the square of the current! Indeed in electrical engineering such are often referred to as I2R losses.

Lowflyer: You may struggle to find a 40A ammeter. Better to connect your ammeter in series with a fuse and then switch on each load individually, record the result and then add them all up to get the total consumption.
Although your car’s equipment stopped functioning, were you saying that the actual fuse element was still intact? Any tell tail arcing or burn marks on the legs of the fuse suggesting poor contact?

I had this issue on my 2013 +4.

Water ingress into the fuse box had helped the contacts to corrode causing the same problem you have.

The answer to water induced corrosion would be gold plated fuse holders as offered by some marine electrics suppliers. Chances are limited in getting such a fusebox to match OE though.

BTW I had a similar issue to OP on a yacht. The otherwise well respected builder had secured high current 12 volt connections via eye terminals to stainless M10 bolts through a 15mm plywood mounting board. Although the securing nuts were seemingly tight they offered a marginal connection resistance as the wood simply compressed when I tried further tightening. This was only revealed when I burnt fingers on a very hot bolt. I re-engineered with nuts each side of eye terminals so I could really torque up the connections tight. Adding serrated washers helped. Although such a size bolt would have conducted thousands of amps without issue, it was found the eye connection resistance at each end was enough with the currents flowing, to heat the bolt enough to char the wood over time

Thanks for the input gents, I'm sure that there exists some corrison at that particular fuse carrier and i thought I'd got it cleaned up and indeed it had performed ok until as mentioned i switched on all loads, lights, blower, screen, and of course fuel pump.
One of the issues is that Omnitec manufactured "Morgan Dash Control Module", i think its supplied from the same fuse and i think that whilst i was installing an indicator buzzer i accidentally disconnected a wire from it and it also killed everything as i remember....but hey im starting ramble here. That panel i would like a little more info on, its a bit of a mystery and has i believe caused issues for people on here. I did manage to wire my driving lights thro' a spare volt free switch/relay on the panel.
Regarding the suspect fuse csrrier, i think a change out maybe in order which of course means the full 8-way carrier needs to be changed, its a dash drop job. I may choose to just install another carrier and connect the one circuit to it rather than change out all 8 circuits onto a new carrier unless they are spade terminals which would allow for easy changout.
Richard, yes the I^2R heat generated would of course melt the fuse assembly without blowing the fuse, I'd considered this but started to wander of into the hinterlands and vagaries of the Morgan wiring. I guess you've pulled me back on course here. I'd still like to know how much current is pulled via that one fuse.

Once had some electrical issues on a very large distribution warehouse we built, the electrical contractor used a hand held thermal image camera to test all the dist board connections, let's just say it did show some poor workmanship causing hotspots