The basic safety relay arrangement uses an internal latching sequence to set up two or more output relays into an energized condition when all circuits are healthy and after a reset contact has been closed. The relays remain latched in until the input circuit is broken either by the guard door switch or by the E-stop.

A typical hardware-based implementation of a guard door safety function will link the guard door switches in series with an E-stop switch to provide an input to a latching relay. The latching relay will trip when the guard door is opened or when the E-stop is pressed.

To improve the safety of the circuits an additional relay is used to prevent the latching relay from being reset unless the safety control circuits are healthy (i.e. free of dangerous faults). For example, in figure below a simplified safety relay is shown where K3 is a relay that must be energized before the latching relay K1 can be set. K3 will not energize unless the power control contactor(s) C has been released, proving that it is not held in by another stray circuit or by a mechanical defect.

In practice relay K1 is usually duplicated by a second channel or redundant relay K2 and both relays must be energized and latched to close the output circuits. K3 is often arranged with multiple contacts and expansion units to enable many drives to be interlocked from the same logic.

The example shown in figure above uses a safety monitoring relay unit to perform the essential logic functions required to provide safety integrity. These are:

• Checks on the state of input signals. They must be safe before latching reset is allowed.
• Detection of stuck contactors through monitoring of the auxiliary contacts.
• Wiring faults in the input and output circuits.
• Timing and logic for controlling the reset actions, etc.

The safety monitoring relay modules ensure that the safety interlocks and E-stop functions are able to operate independent of the basic control system actions at all times. This is one of the most essential features of any safety control system.

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