Safety Relays - Principle of Operation

Typical Circuit
Explanation of Circuit Operation
Closing The Controlled Circuit
Opening The Controlled Circuit
Emergency Stop Relay Fault Table

The diagram below shows the typical circuitry of an Emergency Stop Relay.
The controlled circuit is opened or closed via the safety contacts on the relay, contact chains 13-14, 23-24 and 33-34. The circuit may only be closed when the Emergency Stop Button is in the reset position (contact closed), the "On/Reset" button is depressed (contact closed) and the Emergency Stop Relay is in a healthy condition.

Back to top

The auxiliary supply is applied to A1(+) and A2(-). When all button contacts are closed a 24V dc supply appears at X4 via S11 and S12.

Relay K1 energises:
Contact K1-1 closes energising relay K2 via S12.
Contact K1-2 closes energising relay K3 via S22.
Contact K1-3 closes (remote start signal if required).
Contact K1-4 opens to inhibit closing of controlled circuit.

When relay K2 energises:
Contact K2-1 opens to de-energise relay K1.
Contact K2-2 closes to latch in relay K2.
Contact K2-3 opens (see Note 2).
Safety contacts K2-4 close in the controlled circuit.

When relay K3 energises:
Sequence as for K2.

Note 1
When relay K1 is de-energised there is a 200 ms delay on contact response to permit the operation of K2 and K3.

Note 2
K2-3 and K3-3 can be used for remote signalling. These contacts must not be used for disconnection purposes.

Back to Top

If internal relays K1, K2 and K3 function correctly then all contacts in chains 13-14, 23-24 and 33-34 will close, thereby permitting closure of the controlled circuit.

Activating the OFF or the Emergency Stop buttons will remove power from S12 and S22 and de-energise K2 and K3. Contacts K2.4 and K3.4 will open

Back to top