In article ,
"Chris Oates" none writes:

Think of them as voltage, rather than current, sensitive fuses

Yes - they short-circuit during spikes, absorbing some of the
energy (rest lost in cable impedance). They have a fixed amount
of energy they can absorb before they are destroyed, and each
spike absorbed (except very small ones) uses up some of the
device.

Big ones would be mounted in an armoured case

A large spike which would require more energy to be dissipated
than the device is capable of can result in it exploding, and
they can also fail to a permanent short circuit which might
have the same effect. They can be destroyed by the back-emf
from inductive loads too.

--
Andrew Gabriel