On 5 June, 11:34, "Andrew Mawson"
wrote:

They are only allowed by law to sound for 20 mins

No, a _system_ is only allowed to sound for 20 minutes. A damaged or
deliberately tampered with system is allowed to sound indefinitely, as
there's no reasonable way the law can require it to shut up when it's
damaged or comproimised from how it's supposed to work.

So the panel has a timeout after 20, but a severed bell box will keep
on tweeting while the power lasts.