If this product operates more than one strikes, each of them should have a varistor. The varistor controls the overload produced by the strike coil – back emf. If you are using a « Shear Lock » electromagnetic lock, it is recommended to use a separate power supply than the one connected to the PROMI 1000 ! PROMI 1000 STAND ALONE ACCESS CONTROL SYSTEM Edition 10/2003 PROMI 1000 USER MANUAL I. PRESENTATION DU PRODUIT..................................................................................