A safety PLC is designed such that a failure will (probably) not cause an unsafe condition.
and what are the similarities?
They can both control things, although the conventional PLC will probably offer more features, cost less, be more readily available, etc.
>can conventional plc use as a safety plc?
No, that is why we have safety PLCs. However, most PLC applications do not require safety PLCs as safety is provided by other means.
I said that a safety PLC will usually use redundancy while a regular PLC will usually not. However, redundancy is not by itself sufficient to make something a safety PLC. Some conventional PLCs will use redundancy (in some configurations it’s called “hot backup”) to improve reliability.
To look at it another way, some conventional PLCs may use certain features such as redundancy to make make it less likely that your plant will shut down if something goes wrong. Safety PLCs on the other hand will be designed to make it more likely that your plant will shut down (safely) if something goes wrong. If you are baking biscuits you are mainly worried about whether your day’s production of biscuits will be ruined if something goes wrong. If you are making chemicals you are probably more worried about whether you will wipe out Bhopal if something goes wrong.
Auditing and certification doesn’t by itself make something a safety PLC. What these can do is prove that it is a safety PLC. Where I live we must by law provide a report from a qualified engineer (and not just any engineer) that a piece of equipment meets all the applicable safety requirements (including laws, regulations, and industry standards) before we are permitted to place it into operation. If the design of the system is such that the PLC must be operating correctly in order for the equipment to detect a problem and shut down safely, then that engineer is going to want to see some appropriate paperwork that says that another qualified person has studied the design of the PLC and said that it is capable of doing so. However, just sticking a safety PLC into a machine doesn’t by itself make everything safe. The entire system design must be taken into account, and the PLC is just one component in the system.
For the type of equipment that I have been involved in, safety PLCs are rarely used. Typically, the control system safety is provided by safety relays and other similar devices which operate outside of and independently of the PLC. In those applications the safety relays (and light curtains, etc.) will over-ride the operation of the PLC so a failure of the PLC will have no effect on the safety of the equipment. This is a very common situation, which is why most PLCs are not safety PLCs. This is very convenient from the point of view of the equipment owner as this tends to be cheaper overall and much more flexible. However, not all production processes lend themselves to this sort of simple design which is why safety PLCs exist.
I won’t try to cover the design details of how a safety PLC works internally, because there is probably more than one way to do things. However, if you look at how safety relays, emergency stop circuits, light curtains, and other such safety circuits work you will get a good idea of what the design issues are.
Credits - M Griffin