When exactly does the alarm of a heartlimit sound on the fenix 5x?

When I set a heartlimit the alarm does not always sound when immediately I'm going over the limit.  But sometimes it does.  At least that's my impression.  Does anyone know what the rules are for the sounding of the alarm a part from going over the limit?  Is the limit for example checked every 30 seconds?  Meaning the alarlm would sound alomost immediately when you go over the limit after 5'29".  The alarm would then sound after 5'30".  And while it take a "long" time if you go over the limit after 5'31".  The alarm would then sound after 6'00".

And when is the alarm repeated when I stay above the limit?  Also every 30" in my example or is the time between alarms shorter when you stay above the limit?