safety relay and PLC

[JPLYNCH]

Hi guys

I am installing a Mitsubishi ALPHA 2 into a machine that needs to be upgraded i am sizing the project up from scratch but one of the things i am not really sure on is how or do i incorporate 2 safety relays into the PLC. The relays are monitoring 3 e stops and 10 guard door sensors with manual reset button. Do i keep this seperate of the PLC or what?

Thanks guys

 

[Gene Bond]

Depends...

I have used separate safety circuits (hard wired)and another output contact from the safety relay to a plc input for monitoring, alarm and control.

 

[JPLYNCH]

I'm not sure i fully understand this. First of all lets say i hard wire the safety relay excluding the PLC does this mean the PLC would not be controlling the e stops and guard door sensors? And if i did give the PLC an input from the safety relay do i not have to give it a corresponding output somehow then?

 

[jstolaruk]

Unless the plc is safety rated, outputs from the plc shouldn't be used as part of the safety function. Its allowed be used for monitoring, resetting, and other non safe functions. Using it as part of the safety circuit opens up all sorts of liability issues let alone the risk of a failure that could cause injury. You may want to refer to someone with experience in this area if you are unsure.

 

[karlek]

Maybe this can help you out (relay is Schneider Preventa XPS-AF5130, you can google its manual, internal logic and wiring scheme easily on the web), in this case PLC is used for resetting the relay after emergency stop (relay 30K2).

As relays 30K2 and 17K3 are wired in parallel, safety relay can also be reset using switch 17S3 (that has a keylock).

One other use of PLC can be (as GeneBond stated) monitoring if you wire contacts from relay 17K2 to PLC input, then you'll have information when safety relay gets energized/deenergized.

There are also standards on how safety relays have to be used and wired, I don't guarantee that this drawing is 100% correct and by standards, this is just some random project I got my hands on.

 

[jacoffey85]

I'm not sure i fully understand this. First of all lets say i hard wire the safety relay excluding the PLC does this mean the PLC would not be controlling the e stops and guard door sensors? And if i did give the PLC an input from the safety relay do i not have to give it a corresponding output somehow then?

The PLC can monitor the e-stops and door sensors, but the safety relay will control the outputs.

Maybe someone else can chime in, but I've seen in the past where safety relays have output the control voltage to PLC output cards. So if the safety circuit isn't satisfied, your outputs won't function on that card. Obviously there are cases where this wouldn't be the best option, but it worked for our application.

 

[karlek]

The PLC can monitor the e-stops and door sensors, but the safety relay will control the outputs.

Maybe someone else can chime in, but I've seen in the past where safety relays have output the control voltage to PLC output cards. So if the safety circuit isn't satisfied, your outputs won't function on that card. Obviously there are cases where this wouldn't be the best option, but it worked for our application.

Similar is in the project I posted, contactor 17K2 is placed on the output of the transformer that supplies AC control voltage.

 

[jstolaruk]

The PLC can monitor the e-stops and door sensors, but the safety relay will control the outputs.

Maybe someone else can chime in, but I've seen in the past where safety relays have output the control voltage to PLC output cards. So if the safety circuit isn't satisfied, your outputs won't function on that card. Obviously there are cases where this wouldn't be the best option, but it worked for our application.

Routine in my designs.

 

[GaryS]

the country you are in will determine what you need on the safety controls
to be qualified as a safety E-stop it must be 2 loop 4 wire monitored
all devices it controls must be directly connecter to the safety relay
in the drawing shown I don't see the 2nd redundant output relay it is required
when I do them I always, in addition to the required double loop safety I use a feed back from each device to the plc inputs that way I can post o the HMI exactly what device is tripped the operator doesn't have to spend time running up and down the line to figure out the exact device

 

[JPLYNCH]

So let me see if i have it right. if i want the PLC to monitor the status of the relay tell it that its on or off i give the PLC an input from the output side of the relay? If so how would i write this particular instruction in the programme?

 

[JPLYNCH]

also here is the link to the relay that is currently in use there are actually 2 of them in series. so what terminal would i be taking the signal to the plc from?

http://uk.rs-online.com/web/p/miscellaneous/2512246/

 

[Aabeck]

Run 24VDC to terminal 13 of the first, run a wire from terminal 14 to terminal 13 of the second. Then run from 2nd terminal 14 to a PLC input.

For controlling the outputs voltage run s similar loop through terminals 23 & 24 of each that allow your Control Voltage / MCR to be turned on.

 

[Timbert]

Maybe someone else can chime in, but I've seen in the past where safety relays have output the control voltage to PLC output cards. So if the safety circuit isn't satisfied, your outputs won't function on that card. Obviously there are cases where this wouldn't be the best option, but it worked for our application.

Since you asked for someone to chime in . I've seen this done, depends on the required SIL or PL. Not so much that this won't work under normal circumstances but rather you have to be careful with your diagnostic coverage.

Often the safety relay controls the voltage to PLC which in turn controls a power contactor. If you forget to monitor the contactor in addition to the safety relay, and if the contactor welds closed you can have an undetected dangerous failure.

In short, if implemented correctly, this approach will work.

 

[Timbert]

Shouldn't 17K2 be a 100S-C09EJ10? The 100S product group is for safety applications and uses positively guided contacts rather the 100 series which is just a standard relay.