2N Helios IP Safety Installation Manual page 42

Warning
When you connect a device containing a coil, such as a relay or an
electromagnetic lock, it is necessary to protect the intercom against voltage
peak while switching off the induction load. For this way of protection we
recommend a diode 1A / 1000V (e.g., 1N4007, 1N5407, 1N5408) connected
antiparallel to the device.
2N TELEKOMUNIKACE a.s., www.2n.cz 42/65