# Explanation of structure and function of a safety relay

I am reading a safety guide from Pilz. This guide contain a chapter about safety relays.

A circuit given in this guide is given below:

I know the purpose of all switches and buttons defined in it. But I am unable to understand the circuit completely.

How does it working, and how does the short circuit occur?

What I understand is that positive-guided always wants its switches to be open, but it gets closed for some reason (I don't know which,) and as a result a short-circuit occurs. But I am not sure.

• See if my answer to electronics.stackexchange.com/a/240352/73158 helps. Jan 19 '21 at 16:02
• Nice explanation. But in case of my circuit as shown above, Which side is "load" and which side is "Power". And what is start relay K3 in my circuit. Jan 19 '21 at 17:50

The Pilz unit, being referred to, is known as an 'E-stop safety relay'.

At the outset, the E-stop relay circuit comprised a relay controlled by an 'E-stop' push button switch and a 'Start' push button switch. It was generally referred to as the 'Control On' circuit.

This circuit could fail, should the 'NC' contact of the E-stop push button switch get stuck. Hence it was supplemented by a second 'NC' contact in series. It could also fail, should the 'NO' contact of the E-stop relay get stuck. So a second relay was added, with it's 'NO' contact wired in series with the first one. The resulting circuit was fail-safe but the fault could remain unnoticed. Hence the third relay was added to monitor the other two and prevent their subsequent turn-on.

The 'NO' contacts of the first two relays, wired in series with the 'NC' contact of the third one, ensured E-stop of the machine and prevented subsequent restart in case of a fault (with the status of the third relay also being monitored).

This dual-redundant circuit is the basis of the Pilz E-stop safety relay.

It is also used to render fail-safe functioning of machine guards, light curtains, motor clutch / brake units etc.

The page following the diagram you added is quite clear about things: -

The typical design of a first generation safety relay in relay technology is based on the classic 3 contactor combination. The redundant design ensures that wiring errors do not lead to the loss of the safety function. Two relays (K1, K2) with positiveguided contacts provide the safe switch contacts. The two input circuits CH1 and CH2 each activate one of the two internal relays. The circuit is started via the start relay K3. There is another monitoring circuit between the connection points Y1 and Y2 (feedback loop). This connection is used to check and monitor the position of actuators which can be activated or shut down via the safety contacts. The device is designed in such a way that any faults in the input circuit are detected, e.g. contact welding on an emergency off/emergency stop pushbutton or on one of the safety contacts on the output relay. The safety device stops the device switching back on and thereby stops the activation of relays K1 and K2.

Kindly guide me How is it working, and how the short circuit occur?

The shorts (introduced failure faults) are showing that even with a fault on the E-stop button and a short in one of the output contactors, the relay will still safely work and turn off a 3-phase device connected to pins 13, 23 and 33.

• What is start relay K3, as there are two K3 relays? And safely working of relays (inside ch1 and ch2)?? And how did you know it is a 3-phase device?? Basically I am new to this system, that's why I need some help. Jan 19 '21 at 15:48
• No, there's only one K3 relay but, it comprises sub two-sections; the activation coil and the associated contacts. Ditto the other two relays. It's a 3 phase device because (a) safety relays are commonly used for 3 phase devices and (b) the triple contacts provide that clue. Jan 19 '21 at 15:50
• I didn't get your point of K3 relay... Is this K3 relay has connection with S12 and resistor K1?? And if its one what's the purpose of two relays K3? Jan 19 '21 at 16:14
• There are not 2 K3 relays, there is only 1. What is "resistor K1"? I see no resistors?? Jan 19 '21 at 16:17
• A rectangular block (K1) between UB- and K3 (CH1) isnt a resistor?? Jan 19 '21 at 16:21

How does it working, and how does the short circuit occur?

Figure 1. Two possible fault conditions.

"Short circuit" may not be the correct term as it implies that current is taking a short-cut and bypassing part of the circuit. In this case we're looking at two potential faults that are detected by the safety relay. The concept of redundancy is important here.

1. One of the e-stop button contacts has failed to open. This could be due to switch malfunction, cable abrasion or salt-water ingress, etc. The result is that K1 will remain energised when the e-stop is pressed. In this event K2 will drop out and make the circuit safe. With K1 remaining energised the reset button ("ON" in the diagram) can't reset K3 so the safety relay can't be reset. This forces maintenance attention to diagnose and repair the problem.
2. In the second example one of the output contacts has welded on K1. This would be a reasonably likely fault if the load was high current and switching was frequent (such as an access guard door). The same reset rules apply.

What I understand is that positive-guided always wants its switches to be open, but it gets closed for some reason (I don't know which,) and as a result a short-circuit occurs. But I am not sure.

You should have a better understanding by now. The positive contact guiding just makes sure that none of a relay's NC contacts can close unless all of the NO contacts are open.

Which side is "load" and which side is "Power".

Contacts 13/14, 23/24 and 33/34 are used to switch the load on and off. Power would normally be fed in the top and the load (solenoid valves, motors, etc.) fed from the bottom. There is no technical reason why you couldn't do the opposite.

And what is start relay K3 in my circuit.

Notice that K1 and K2 latch themselves on once energised. K3 does the energising when the reset (ON) button is press provided K1 and K2's NC contacts are closed. It provides a vital part of the redundancy monitoring. Without it the user would have no indication that the redundancy had been lost if one channel stuck on. C1 provides a short drop-out delay so that K3 remains energised long enough for K1 and K2 to pick-up properly so that their latching contacts keep them on.

The Y1-Y2 loop can be used to monitor other devices such as a safety air solenoid valve to ensure that it has dropped out during e-stop also.

Figure 2. Omron's Safety Relay: How Does a Forcibly Guided Contact Work.

If we label the contacts in Figure 2 A, B and C from left to right we can see that the spring (between A and B) is pushing the blue guide to the left. NO contact B has welded so the guide can't move any further. The guide contact has enough slack in it that contact C has opened but not enough that contact A can close. The Omron article gives some specifications on the clearance distances.