Before talking about “proven components” and “proven safety principles”, according to the requirements of the standard, we know that the actuator applied in the safety loop, such as intermediate relays, contactors, etc., and the contacts used as monitoring functions must meet the “forced guidance logic”, today we will talk about the relevant content of forced guidance logic.

What is forced oriented logic?

According to the definition in Appendix L of IEC 60947-5-1, a mechanical contact is a combination of normally open contacts and normally closed contacts (components) designed in accordance with the following requirements, that is, they cannot be closed at the same time. Contact combinations that meet such requirements may be marked as follows in the component or documentation

Meaning of forced guide contact

Why do we need to use relays and contactors that force guide contacts instead of regular components? Many customers often ask about the difference between these two types of products. In fact, they differ only in whether the NO and NC contacts can be closed at the same time.

In the safety loop, we generally use NC contacts to access the feedback loop (feedback loop) to judge whether NO contacts effectively cut off the dangerous loop. In other words, if the safety controller cuts off the relay coil, the NC contact of the ordinary relay will close with the armature controlled by the coil; If we choose a forced guide relay/contactor, in the case of NO contact fusion welding, due to the characteristics of NO and NC can not be closed at the same time, the NC contact will be in the disconnected state, which can correctly reflect the fault in the loop.

Considering that Cat.1 requires “proven components” and “proven safety principles”, and Cat.2-4 both require “proven safety principles”, we can conclude that:

Relays and contactors used in safety loops that meet Cat.1 and above architectural categories shall meet mandatory guide construction requirements.

Need to expand too many contacts, how to break?

For medium and large production lines, if more frequency conversion or servo motors are involved, more security outputs need to be extended to cut off the STO port of the driver. In order to meet the high performance level, it is necessary to use two contact points of the forced guide relay to form a redundant structure.

In such a scenario, a wide range of intermediate relays greatly increase the workload of wiring and occupy valuable space in the electrical cabinet.

The Sigma series of safety extension modules in the PILZ family of control systems can effectively solve this problem. Not only contact redundancy, small size, and a single chip can meet PLe applications. If you are still using a safety PLC with many forces-guided relays, PNOZ Sigma is recommended as a solution for contact expansion, both from a cost and design justification point of view.

The PNOZ s7 has four sets of redundant normally open and one set of normally closed contacts with a width of just 17.5mm