On Tue, 19 Nov 2019 23:53:40 +0000, Fredxx wrote:



Looking at some more info and their characteristics at raised voltages I
do wonder how effective these are at preventing high voltage transients.

Not very, they are simply 3 voltage dependent resistors which
"prevent" transients by failing (much as "surge protected" extension
leads. Having failed they sit there being completely useless until
someone spots the little light on them and remembers this means you
have to go out and buy a replacement cassette (or a completely new
device at about the same price.

Any lightning near bye will destroy them long before they have done
any good so they are a triumph of rules for the sake of rules.

Fortunately they are not required in singe dwelling houses (Section
443) .