Mine effectiveness

[Andrés]

Another problem with mines is that you can only self destruct a whole minefiled.
E.g. if your minefield is made of 50 obsolete old mines and 50 new mines and you want to upgrade it to 100 new mines you have to self destruct them all and lay new ones.
It could be explained that once a mine self destructs it starts a chain reaction that destructs the whole minefield, but then why isn't the whole minefield destroyed when they hit a ship?