Interlock switch circuit with single fault detection

1. A testing system to test a plurality of series connected safety switches to ensure they are operating normally, each safety switch having a safety output which when closed enables a power supply, characterized in that the testing system comprises an individual diagnostic means on each switch to perform a self check of that switch and which controls that switch's safety output, and a test loop circuit which connects the diagnostic means in series and which is adapted to send a signal to each diagnostic means to open its switch's safety output if any one safety switch is activated to open its respective safety output, and wherein each diagnostic means is also adapted to check its switch to ensure that it is operating normally and which is activated when the test loop circuit sends said signal.

2. A testing system according to claim 1 , wherein the test loop circuit must be intact for the switches safety outputs to be closed.

3. A testing system according to claim 1 , wherein the diagnostic includes opening and closing the switches when the power supply is disabled.

4. A testing system according to any claim 1 , wherein the diagnostic means is adapted to ensure that the power supply cannot be re-established if a fault is detected in at least one switch.

5. A testing system according to claim 4 , wherein the detected fault is that at least one of the switches safety outputs remain closed.

6. A testing system according to claim 1 , wherein the test loop circuit cannot be re-established if a fault is detected by a diagnostic means.

7. A testing system according to claim 1 , wherein the testing system includes indicating means to indicate the condition of the switch.

8. A testing system according to claim 7 , wherein the indicator means indicates at least one of the conditions switch open, switch closed, switch test diagnostic completed and switch operating normally, and switch diagnostic completed and switch faulty.

9. A testing system according to claim 1 , wherein an indicator means is provided on each switch to provide an indication of the condition of that switch.

10. A method of testing a plurality of series connected safety switches to ensure they are operating normally, each safety switch having a safety output which when closed enables a power supply, characterized in that each safety switch has an individual diagnostic means which control its safety output, and in a test loop circuit which connects the individual diagnostic means in a continuous loop, the method comprising the steps of changing the condition of an individual safety switch by attempting to open its safety output, activating that individual switch's diagnostic means by said change in condition, said individual switch's diagnostic means then sending a signal round the test loop to the other switch's individual diagnostic means to instruct them to perform a self-check, and each individual diagnostic means then conducting a self check of its safety switch to ensure it is operating normally.

11. A method according to claim 10 , wherein if a fault in a safety switch is detected that switches diagnostic means sends a signal round the test loop to prevent power being enabled.

12. A method according to claim 11 , wherein the detected fault is that the switches safety output remains closed.

13. A method according to claim 10 , wherein each safety switch has indicating means to show the condition of the switch to indicate at least one of the conditions switch open, switch closed, switch test diagnostic completed and switch operating normally, and switch diagnostic completed and switch faulty.