When a load cannot be driven directly by a safety controller, an interposing relay becomes a requirement. For example, if the controller cannot handle the amount of power required to switch the load; or the voltage is different from 24 Vdc; or multiple contacts are required to be driven by the same control signal. SIL certified safety relays are a requirement to meet the necessary Risk Reduction Factor (RRF) when switching a load ON or OFF, in an IEC61508/61511 Safety Instrumented Function (SIF). SIL certified relays guarantee that the safety function is met with a suitable and known probability of failure (PFD).
