There is normally a overheat fusible link, which should fail if the heater
is covered when it is taking on heat.

Same happened to mine 22 years ago when I covered one of
them when it was in the "Charging" cycle which was 00:00 hours to 08:00
hours. The thermal fuse at the top blew, the replacement was a couple
of £'s I seem to remember. There are different temperature ratings for
these thermal fuses.

The problem did not occur again as I installed a full C/H system
as they were SOOooo expensive to run.

Chris.