I am having trouble trying to get my head around using safety relays in PLC controlled machinery. I want to install a PLC into a machine that needs upgrading, it is straight forward enough, well i thought so until i ran into two safety relays im just not sure how or if i should tie them in to the PLC. The relays are controlling 4 estops and 12 guard door sensors, which i thought the PLC would be capable of doing. So how do i keep the two seperate or do they get hardwired to each other? The net seems to be very scarce on this info
Electronic – safety relays and PLCS
plcrelaysafety
Related Solutions
The "speed", rupture capability, and voltage spec of a fuse are each separate traits that aren't necessarily related. They will be specified according to the nature of the application.
Many cheap multimeters (i.e. <50$us) will only have glass fuses, because they're cheaper. But 'high rupture' capability (the ability to break a circuit up to a rated voltage, when a high fault current is flowing) is important in a multimeter (and in a circuit breaker), especially in a DC scenario, because DC >40V can easily form an arc and continue the current flow, even though the fuse has blown.
For the "20A" fuse in your meter, a HRC type would be wise, and rated for the max V rating of your multimeter, and same speed as the original.
I had edited my response to your earlier question to cover this lightly. Here's some more detail for a basic PLC.
Task execution order
Typically PLCs task execution order runs as follows:
- Read the inputs.
- Execute the logic.
- Write the outputs.
- Handle other tasks, etc.
The 'Read the inputs' and 'Write the outputs' stages are the 'mass I/O copying' stages. The reason the logic is executed this way is that it presents a consistent state for the inputs through the program and prevents output strobing during the program scan. It also means that updating of I/O can be handled as a background task.
Problem with direct input read
UP BUTTON MOTOR UP
X1 Y0
--| |---------+-------------( )-
UP BUTTON MOTOR DOWN
X1 Y1
--|/|---------+-------------( )-
In the above example if X1 were to turn off between the first read and the second read both MOTOR UP and DOWN outputs would be energised simultaneously. Normal practice is to read all the inputs into an internal register and the program looks at that rather than directly at the inputs. This gives consistent predictable behaviour of the code and avoids race conditions.
Problem with direct output write
UP BUTTON MOTOR UP
X1 Y0
--| |---------+-------------(L)-
UP LIMIT SW MOTOR UP
X2 Y0
--| |---------+-------------(U)-
This example might not be the best code but PLC coding allows it and it can prove to be a useful tool when used with skips / jumps. The first rung turns on MOTOR UP using a latching 'relay coil'. The second rung unlatches the coil when the UP LIMIT SW turns on. It should be clear that if X1, UP BUTTON, switch is held on that immediate output writes would cause Y0 to blink briefly on every scan. Writing to the internal output memory and copying the whole lot to the outputs when program execution is complete solves this problem as the state at the end of the logic scan is what gets written to the outputs.
Immediate read
Most PLCs allow an immediate read and write which can be useful to shorten the cycle time of a machine. Others allow a few inputs to be used as interrupts so that program execution can jump to a subroutine and execute immediately.
Best Answer
It is very important for safety, regulatory and legal reasons that you get this right.
Basics
Monitoring
Your safety relay will have auxiliary contacts one of which can drive an input to the PLC so that the PLC knows the status of the safety circuit.
Safety wiring
Safety circuits now almost all use dual channels and monitoring. E-stops have two contacts that are forced apart (not sprung apart) when the e-stop button is pressed. Similarly guard door switches have two contacts and again the act of opening the door will force the contacts apart. If one channel fails to open on e-stop the other will make the system safe but the monitoring will prevent a reset. This forces the user to investigate the problem.
See my answer to Symbol or marking on safety relay for an in-depth of how a safety relay works.
No. The e-stops and guard door switches are controlling the safety relay or, if you prefer, the safety relay is monitoring the switches. The safety relay is controlling the load to ensure energy (electrical, pneumatic, hydraulic, etc.) is de-energised when required.
Related: my answers to the questions below may be helpful.