# A safety relay systems circuit

Can someone help explain the operation of a safety relay system. I know that they can be connected to e-stops for emergency stops but I don't understand the relays and how they operate.

• See my answer to electronics.stackexchange.com/a/240352/73158 for a detailed analysis of a safety relay. Mar 30, 2020 at 6:43
• Michael, please do not vandalise your posts in such a way as to make the answers already written look stupid. If the image which has been referenced in the answer is no longer applicable or relevant to your question then edit your question to explain why but leave the image there so that the answers make sense. You have done this on other posts too so it shows a repeating lack of respect to those giving freely of their time to educate you. Mar 30, 2020 at 18:41

In general , safety relay contacts are mechanically linked or "force guided". in other words and i quote phoenixcontact " Forced guidance means that the N/O and N/C contacts of an elementary relay are linked to one another mechanically. This prevents N/O and N/C contacts from closing at the same time. When used together with a suitable circuit, failure to open is reliably detected. This is the most reliable way of ensuring maximum safety for both person and machine "

Offcourse addition circuits and interlocks are added to a safety relay module for diagnostics

EDIT after the new image you posted: In the figure K1 and K2 are 2 separate contacts connected in series , the horizontal line with 4 dots near letter K1 and K2 means the contacts are mechanically linked

13-23-33 are connected to 14-24-34 when the relay is energized through A1-A2 . while 41 to 42 will be open. Y32 is not clear what it does , it should be mensioned in the data sheet. S34 appears to be a reset pin if fault occuered.

for more details check this answer : Safety Relay Symbol

• i updated the image if that helps. So when say and e-stop is pressed the N/O contacts close but whats the purpose of the contacts at 41 and 42?
– user244756
Mar 29, 2020 at 22:42
• 41 -42 is the NC contact , if the NO contact is welded this contact will become open ( because of mechanical linkage ) , if you test for continuty on 41-42 you will no that it is open while it should be closed Mar 29, 2020 at 22:46
• offcourse you can do whatever you want with this signal , for example use 41-42 to feed a LEd or PLC input or shut off supply for something . Mar 29, 2020 at 22:47
• consider this contact a way to know the state of the other relay contacts . In a normal relay you cannot do that , because there is no guarantee that NO and NC will not engage at the same time . I hope this helps Mar 29, 2020 at 22:48

A 'safety relay' is a dual-redundant safety device meant to shut down a machine even in the event of failures like

1. An E-stop mushroom pushbutton switch 'NC' contact not opening.
2. An E-stop relay 'NO' contact not opening.

After the shutdown the 'safety relay' is to prevent a machine restart till the fault is cleared.

For the purpose, the 'safety relay' has 2 relays monitoring each other and other machine E-stop mushroom pushbutton switches via hard-wired logic utilising the 'NO' & 'NC' of these devices.

Earlier this safety function was hardwired in machine controls (especially in cam-operated and hydraulic presses).

Later it became mandatory to incorporate exclusive stand-alone safety relay units, certified by international occupational safety and health monitoring agencies.