Rockwell MSR127TP Safety Relay-Looking for a way to monitor the Safety Relay Voltage

Good Afternoon ,

We have a number of MSR127 Safety Relays on various machines. We had an issue a few months ago that we had a EStop contact that was not making good contact and caused a Safety Relay to drop out . Checking resistance , it seemed high and would not allow the Safety Relay to reset.

What I would like to find , is a way I can monitor the health of the safety relay into a CompactLogix , so I can show the status , voltage , etc. on an HMI.

The EStops have 2 contacts going to the MSR127 Safety Relay . I'm upgrading a machine and I'm going to bring individual wires from each EStop back to a input on the new PLC. But that would only show if the EStop was pressed . If we have a bad , or weak contact , the Safety Relay drops out , and we still would not see which one.

Do you guys know a way we can do this ? So you guys say there is a pulse going out on the channel and that is why we only read around 16 volts when checking the channel with a meter ?

Thanks so much ,

Not sure if this will work, it might drop the voltage too low for the ESTOP circuit.

Depending on the size of your project (and to extend what version of the software you have) the CompactLX now have some great value safety version, which makes diagnostics much easier.

If its a simple e-stop circuits though, that maybe overkill, only you can make that call.

We have used these before:

https://www.routeco.com/en-gb/shop/safety/safety-relays/440c-cr30-22bbb

Which are nice for the price and expandable, each e-stop can have it own circuit to make pin pointing down where the failure is. Again depends on how many e-stops / safety circuits you have. Also programmable within Studio 5000.

Both types will tell you of a failure, if only 1 channel drops out. (Doesn't have to be AB, PILZ et al do ethernet/IP SRs)


For a bog standard, common or garden safety relay your options are limited, aside a N/O contact and thats about it. If a channel drops out, then you get a red LED. So make the LED seen:

The simplest and very effective way is I have seen is, the electrical cabinet has a cut out made in front of the safety relay, in the panel door, with clear perspex, which means the safety relay can be observed easily from outside the cabinet, so that a fault is evident straight away. Which saves the operator and the maintenance guy slamming doors and twisting e-stops for a day before realizing something is wrong!

If you had one contact with high resistance did you check the rest? I would be a bit concerned that age or exposure to the environment had compromised that contact and that others might need replacing. I would consider replacing all the contacts. One, it will help maintain safety; two, it may well save unwanted down-time.

That said, do you have multiple buttons to each safety relay? If not and you don't use all the contacts I'd wire the safety relay to the PLC. Let it tell you of failures. If you have multiple buttons to each relay consider adding more relays or using a compact GuardLogix safety controller.