Reason a Safety Relay is Required in Safety Circuit

Submitted by OMRON Corporation
on Tue, 05/24/2011
Safety Relays have forcibly guided contacts to prevent NO and NC contacts from operating at the same time during contact welding, which enables detection of contact welding in the Safety Relay itself. This can be used in building safety circuits. General-purpose Relays do not have forcibly guided contacts.
Sufficient care must be taken, however, because some circuits can be dangerous even with Safety Relays, depending on how the circuit is constructed. The circuit in Figure 1 below, for example, is dangerous because the power cannot be cut.
Build redundancy and a self-monitoring function that uses the welding detection function provided by the forcibly guided contacts into the circuit, as shown in Figure 2.
Figure 1:
Figure 2: Part of a G9S-301 Safety Relay Unit Circuit (Extract)
K1, K2, and K3: Safety Relays built into the G9S-301
For more information about OMRON parts and products, please visit their website at