In article , Dave
writes

I found out about this at school about 2 years ago, but one point I
can't understand is that if a loo cistern starts to vent water because
of a faulty ball valve, how does the loo cope with all this water if it
happens to have a blocked waste/soil pipe?

That's two faults and unless it's a real safety critical system then system
designers and legislators generally rely on the single fault principle ie. no
single technical failure, erroneous action or mistake should lead to
damage, fatalities or serious injuries.

I think there's a general rule that the single fault should be noticeable so
that it's your own fault if you ignore it and allow a subsequent fault to
cause damage, injury etc.
--
fred
Plusnet - I hope you like vanilla