The protection fuse is intended to be the weakest point in the circuit, for safety reasons of course.
But in actual fact, you rarely see the fuse itself actually fail !.
I consider that the two thin clamping points that that retain the 13 Amp fuse inside the plug, to be the weakest point.
The three main contact posts of the plug are pretty substantial, but the fuse carrier clamping points themselves are pretty thin and therefore vulnerable.
The load passes through the live post, up through the fuse ( held by the thin clamping points ) then leaves via passing through the fuse itself and then onto the Granny unit.
Over repeated use and lengthy charging sessions, heat builds up around the area of the live pin itself and has to pass through those thin fuse retaining clamps.
Heat weakens / fatigues the material of the clamps and its ability to apply the same level of clamping force onto the fuse, and naturally it becomes weak and then compromised.
When this process commences, it’s only a matter of time before burning occurs to the casing of the three pin plug body.
Very rarely is this picked up by the owner, unless the plug is removed periodically from the outlet.
We are dealing with constant high load / demands over long periods of time.
It’s a similar situation on other high load appliances like microwaves / fast boil kettles and irons of course.
However, these items are only intended to be used for much shorter periods of time.
As battery packs get larger, then they need to be changed for longer periods of time of course.
This places a much higher strain on the electrical system used to provide the energy to the charger in the car.
Just to be clear here, so am not bashing owners who use Granny units.
This situation is not just isolated to the use of Granny units, although charging large packs over greater periods of time, will place a LOT strain on that unit without a doubt.
Wall boxes are better equipped to deal with the higher loads, but can also be vulnerable to the problems caused by heat build up equally.
There is enough evidence to support wall box overheating/ failure problems.
Purely speaking for myself here, I consider that if you intend to charge using a Granny unit regularly over longer period of time, you have to be super vigilant and do not become complacent with your regular safety checks.
If you are a low user case, or have a smaller battery pack, or a PHEV owner, then these Granny units are going to be mainly okay fundamentally.
If you are a higher user case and have a larger pack, then of course your risk of failure is increased massively.
Many manufacturers do not supply Granny units with their cars anymore, stating that the demand has fallen away due to the length of time they take to charge the car.
Although this is true, I can’t help but think there is other reasons why they are not encouraging there use anymore, if you get my drift.
