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

Knowledge about Safety Relays

Sep 21 , 2023

Is it necessary to use safety relays on mechanical equipment?

Safety relays are used when equipment presents a risk of personal injury or risk of property damage. It does not enter the PLC, but uses hardware methods to disconnect the power supply, so that the equipment is in a state of complete shutdown; and the safety relay itself has several contacts to ensure that it can be effectively disconnected when it fails.

If your equipment wants to enter the international market, it must at least have CE certification, so some equipment that needs to be exported, especially to the EU and Japan, are usually equipped with safety relays.

As the international community pays more and more attention to safety, some technical standards and implementation will gradually be in line with the global ones. At this time, as a forward-looking equipment manufacturer, we can say with confidence that it is necessary to use intrinsically safe relays.

The safety relay is automatically activated. As long as the opening conditions are met (sensor and feedback circuit closed), the device will automatically open the starting circuit.

How much capacitance should be installed in the control circuit of the safety relay?

The parallel capacitance of the coil of the ordinary relay is to prevent the high voltage at both ends of the relay coil from breaking down the contacts of the relay and generating an arc, but because of the addition of capacitance, the action delay of the relay will increase.

Therefore, most of the relay applications now use the freewheeling diode to eliminate the arc. The safety relay is an electronic module with internal freewheeling diodes for arc suppression. In actual use, there is no need to install capacitors.

How long is the service life of the safety relay?

When the SIS system was first put into use, the security level was relatively high. But with the passage of time, the possibility of safety failure, the value of safety-related parameters may gradually become larger. If it is used for a long time and is not tested, the safety level will drop.

Moreover, the key part of the safety relay is the performance of the selected relay, and the intrinsically safe relay has its mechanical life. Therefore, regular fault diagnosis of the system is required. In the whole life cycle of safety-related systems, it should also be pointed out that there must be a maintenance management plan.

When doing PROOF TEST, it is to detect whether there is a safety failure in the safety system. If there is no fault, its safety level is still within the original range. If a fault is found, it needs to be dealt with accordingly, including replacement.

Therefore, the recommendation is a 5-year inspection test, or shorter, which is usually given in the "Safety Manual" of the safety relay product.

How to diagnose safety relay contacts?

Safety relay contacts can be manually and systematically tested. Some safety relays provide normally closed contacts. Take out the input power of the safety relay, that is, the power supply. In the case of power failure, the normally open contact of the safety relay will be broken, and the normally closed contact will be closed.

Due to the mechanical interlock of the safety relay, the normally open and normally closed contacts will not be closed at the same time. If the system detects that the normally closed contact has been connected, it can be fully considered that the normally open contact of the safety relay can be disconnected and the safety relay is intact.

Why should we pay attention to the power-on start-up current of the safety relay?

Power-on start-up current refers to the phenomenon that the current in industrial electronic instrument will be far greater than the normal working current at the moment of power-on. There are two indicators for the performance of the power-on start-up current: the maximum power-on current and the duration.

There are capacitors or inductive capacitors inside industrial electronic instruments. At the moment when the safety relay is powered on, the capacitor needs to be charged, and the capacitor shows an instantaneous short circuit phenomenon, which makes the start-up current instantly increase.

Therefore, when designing industrial electronic instruments, engineers must minimize the start-up current to reduce adverse effects on the system.

However, because the formal experiments of industrial electronic instruments are often carried out for one instrument, or the experimental power supply is ideal, this hazard is often ignored, so that the system will not work reliably if multiple instruments are used together on site.

Any circuit of the safety instrument system will carry out its own diagnosis. For example, the power supply circuit of the field instrument has short-circuit diagnosis and overload diagnosis. It will determine whether the circuit is short-circuited or overloaded by detecting the current of power supply circuit of the field instrument.

If it is diagnosed that the current is greater than the internal set value, it will send a fault signal to the system, enter the fault state, disconnect the power supply circuit, and ensure the safety of the entire system. If the power-on start-up current is too large, the system may not work properly. To mitigate the adverse effects of power-on start-up current in industrial electronic instruments, it is crucial to incorporate safety relays such as mat relay, intrinsically safe barrier relay, and e stop relay, which diagnose and safeguard the system by detecting and managing excessive current conditions.