Electro-Magnets resist damage

Considering the importance of electro-magnets in situations where their reliability can result in life preserving actions, such as the release of fire doors in an emergency, their location and indeed their composition, has always made them vulnerable to accidental damage.
Chipping or denting can damage the flat surfaces which the magnet needs to develop the force required to keep doors open. In an emergency, when the fire alarm cuts the power and the magnets are de-activated to close doors and prevent airflow, damaged magnets may not perform correctly.