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.
- 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.
- 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.