Reed Relay basics
Normally open reed relays are by far the most common configuration of reed relay. However, normally closed relays can also be supplied where the blade is biased so it is normally closed and the application of a magnetic field opens the relay contacts.
The contact bias is created by adding an internal permanent magnet to hold the reed switch in a normally closed state. When the relay coil is energised it cancels out the magnetic field bias and the contacts open. If the coil voltage is increased substantially beyond its nominal voltage (typically greater than 1.5 times nominal) there is a risk that the contact will reclose.