What is the need of feedback loop in a safety relay?

 

Unlike other relays, safety relay is crucial in the system. So one must know the actual status of this relay. A very common problem of heavy load relays is stuck contacts because of high current.

Another good technique which I follow is fail safe.

 

I'm sorry, I don't understand you. Would you explain more detail please?

 

gotumal is quite correct, and makes the distinction between a safety relay and an ordinary relay that is used in a design such that it fails safe.

The problem relays can have is that the contacts can be arc-welded shut by current surges. With a normal double-pole relay there can exist the unfortunate condition whereby one set of contacts is welded shut, while the other set of contacts can still move, despite being operated by the same armature (the armature can twist - I've seen this happen). Safety relays are a bit special in that the armature is arranged so that the contacts are mechanically linked in such a way that they are forced to move together or not at all. Additionally, there is a feedback output - normally a switch for each contact, arranged in series, that gives a reliable indication of the position of the main relay contacts. If a contact is stuck, the feedback loop will be open, and the control system won't allow progression to the next stage in the process cycle.

Using an ordinary relay in a design such that it fails safe is difficult, but possible. I designed a gas boiler controller many years ago that had such an arrangement - if they weren't designed this way then there is a failure mode whereby the boiler fills your house with gas, and then sparks the ignitor. Boom!

 

Simply stated the feedback loop is intended to monitor the final switching point (FSP) to detect if it is working correctly. The FSP is usually a higher current contactor (to serve the higher current loads)