ON-OFF key switch. Dimension: 86(L) X 86(H) X 2(W) mm. The Override Key Switch is designed to be installed in a secure location outside, wired in parallel or series with the power supply and the electromagnetic lock (EM lock). If the EM lock malfunctions due to the failure of the reader or other reasons, the user can switch between normally closed (N.C.) and normally open (N.O.) contacts by using the key, thus forcing the door to open. Override Key Switch is used as a backup to an access control system.