Shanghai Chenzhu Instrument CO., LTD.
Shanghai Chenzhu Instrument CO., LTD.

Why Are Safety Relays Safe?

Jun 07 , 2022

Ⅰ. Definition of safety relay

A safety relay is a module composed of multiple relays and logic circuits, and is a circuit component unit. Its purpose is to complement each other's defects in fault state, so as to achieve correct and low malfunction function, reduce its error and failure value, and improve the safety factor. To be more specific, it should be called a safety relay module.

Due to different mechanical equipment and different process links, there are different safety risks and levels of danger. Therefore, when designing products, a variety of safety relays are designed to protect different levels of mechanical equipment. The main purpose is to protect machinery operators exposed to different levels of dangers.

Ⅱ. What is the difference between safety relays and general relays?

1. Larger shape: The size of the safety relay is different from that of the ordinary relay, and the safety relay is larger.

2. It is a module instead of a component: a safety relay is a module, not an electronic component like an ordinary relay.

3. Red and yellow: Safety relays have their own colors, usually yellow or red. Or use these two colors as the main color (just like in our explosion-proof safety design, the dangerous side and the intrinsically safe end are usually marked in blue.)

4. The core of the safety relay system is safer: due to the difference in design principles: ordinary relays cannot achieve high diagnostic coverage (DC), and can only be applied to lower safety levels, such as Cat.B/1 in the machinery industry and SIL1 in the process control industry, so it is widely believed that ordinary relays are not safe enough.

Safety relays have a high diagnostic coverage rate through the requirements of relays and the auxiliary design of logic circuits, and some can even reach 99.9%. Moreover, in some occasions with high safety requirements, it is also stipulated that certified safety relays must be used to meet safety requirements!

5. Force-oriented structure: "safety relay" is not a "failure-free relay", but a regular action when a fault occurs. The intrinsically safe relay, like light curtain safety relay, has a forced-guided contact structure (or other protection methods), which can ensure safety in the event of contact welding, which is completely different from general relays.

When the NO is in the closed state, the NC contact must be in the open state. When the NC is in the closed state, the NO contact must be in the opening state. That is to say: no matter whether the coil is energized or de-energized, and whether the contacts are normal or not, one group of NO contacts and NC contacts is in a closed state, and the other group of contacts is forced to be in an opening state.

6. From the perspective of relays, the difference between intrinsically safe relays and ordinary relays can be seen:

Ordinary relay: The contacts are fused, which will cause the two pairs of contacts (NO/NC) to appear ON at the same time.

Forced guide relay: Under the action of the internal forced guide rod, even if the contacts are fused, the two pairs of contacts (NO/NC) will not be "ON" at the same time. Force-guided structures are a simple and effective way to detect contact fusion failures.

To put it simply: safety relays are safer and more reliable, and are suitable for industrial safety occasions.

Ⅲ. The role of safety relays

1. The monitoring function of the safety relay on the input

As soon as the safety conditions change, the safety relay safety outputs react instantaneously.

2. The feedback monitoring function of the safety relay on the output control object

If the control object has malfunctioned in the last cycle, the safety relay cannot be activated immediately. It's better to eliminate hidden dangers, check again, and start it only after no abnormal function is found. Please do not allow security risks to accumulate!

3. The role of safety relays on safety output

An internal fault of the safety relay will not cause the safety outputs to be closed.