Safety relay questions

Can anyone please explain me if there is no PWR light in Allen Bradley Guard master( Safety Relay )MSR138DP what is the meaning of it ?

A1 and A2 is the supply voltage input which is 24V DC and though voltage supply there but PWR is not lighting. Also please explain me about feedback loop circuit.

Thanks.

arocon said:

Can anyone please explain me if there is no PWR light in Allen Bradley Guard master( Safety Relay )MSR138DP what is the meaning of it ?

A1 and A2 is the supply voltage input which is 24V DC and though voltage supply there but PWR is not lighting. Also please explain me about feedback loop circuit.

Thanks.

Click to expand...

RTFM http://literature.rockwellautomation.com/idc/groups/literature/documents/in/440r-in063_-mu-p.pdf

"PWR| green, when unit is powered, flashing green in case of cross-loop faults"

So if it is not lighting up at all, it is not getting power, either externally or internally. If you have specifically MEASURED that there is 24VDC at A1 and A2, then the relay is broken and must be replaced.

The "Feedback Loop" to Y1 and Y2 are the status signals back to the relay from whatever it is you are controlling. This is what tells the relay that the device it is controlling has received the command and did in fact operate as it was supposed to. So for example if you are controlling Safety Contactors, you run a circuit from Y1 through aux contacts on the Safety Contactors, then back to Y2. If the Safety Relay is activated by a safety input device, it is going to drop out the coil power to the Safety Contactors. If one of those contactors is welded closed, the Feedback Loop (Y1 to Y1) is not in the correct state, so the relay will alarm and will not allow the system to re-start until after everything is put back into it's proper state, i.e. repaired.

Thank you for your reply. Could you please explain me how do S33 & S34 get power and also Y1 & Y2 ?

Also what is mean by getting power by internally ?

It sounds like you have an internal failure in the module if you have 24 Vdc going to the module, but no power light. S33 and S34 are for the reset button.

Will.

arocon said:

Also what is mean by getting power by internally ?

Click to expand...

Internally, the PC boards will not use 24VDC, they use TTL level (Transistor to Transistor Logic) voltages, typically 5VDC, up to 15VDC, but highly regulated. So inside of the relay, connected to the A1, A2 terminals, will be another board mounted DC-DC power supply circuit to provide and regulate that TTL voltage. It sounds as though that little DC-DC converter has failed.

9 times out of 10, that is typically the result of someone having crossed a wire in the 24VDC circuit with some other voltage level. It only takes a fraction of a second to fry that little power supply circuit.