What is the difference between safety relay and general relay?

Update:17-09-2021
1. Larger appearance
The dimensions of safety relays are different from ordinary relays, and safety relays are larger.
2. Modules instead of components
The safety relay is a module, not an electronic component like an ordinary relay.
3. Color
The safety relay has its own color, which is generally yellow, red, or two colors of red and yellow as the main color (just like when the explosion-proof safety design, the dangerous side and the intrinsically safe side are usually marked in blue). Of course, these are just superficial phenomena.
4. [Core] More secure
Due to the difference in design principles: ordinary relays cannot achieve high diagnostic coverage (DC), and can only be used in lower safety level machinery industries such as Cat.B/1 and process control industry SIL1. However, Anji has passed the requirements of relays and auxiliary design of logic circuits, and its diagnostic coverage rate is relatively high, and some can even reach 99.9%. And in some occasions with high safety requirements, there are also regulations that must use certified safety relays to meet safety requirements!
5. Mandatory oriented structure
"Safety relay" is not a "relay without failure", but a regular action when a failure occurs. The safety relay has a forced-oriented contact structure (or other protection methods), which can ensure safety in the event of contact welding phenomenon, which is completely different from general relays.
Safety relay: The most core component is the forced oriented relay. From the perspective of the relay, the difference between the safety relay and the ordinary relay can be seen.
Ordinary relay: the contacts are fused, which will cause two pairs of contacts (NO/NC) to be ON at the same time.
Forced guide relay: Under the action of the internal forced guide rod, even if the contacts are fused, it will not make the two pairs of contacts (NO/NC) turn ON at the same time. The forced guiding structure is a simple and effective way to detect contact welding failures.