On 30/05/2015 22:11, John Rumm wrote:

So a user has to know what design decisions have been made so he can
avoid doing something that compromises them.

The user will have to trust that the designer and installer had a clue,
or else they might risk having shorter than expected service life from
their cable or sockets.

But the users doesn't know the rules and may well put several high
demand loads on for days at a time, say heaters and dehumidifiers after
a flood as you appear to think no one would ever have a reason to do so.


Much the same as the user will trust that the mechanic made a proper job
on the brakes of their car, or that the surgeon really does know what he
is doing.

A mechanic doesn't install brakes that only work if the user doesn't
press them for more than 90% of the time orat least not without telling you.


They will need to trust that the plumber did not override the safety
interlocks on their immersion heater, and that the gas fitter remembered
to still a screw in the flue coupling on their boiler.

But you can't design a final ring that can't be abused by the user
either intentionally or occidentally. The fact that the regs tell you
they are unsuitable for heavy loads tells you that they aren't suitable
for heavy loads but you don't stop them from being plugged in, you just
blame the user for not knowing better.


That's the problem, you don't know what a user will do.
All you have been able to say is that typically a user wont compromise
the design but they still can, either by design or by accident.

If you think long and hard enough you can contrive ways in which a user
can abuse any system. No circuit topology or design is immune from
abuse. Hence you have to pick a compromise that is good enough in all
likely cases.

So how does a user overload a radial without the breaker tripping?
He can do that on a ring just by plugging stuff in and he may not know.