Pilz: Can safety gate switches be used in series?

Recently, the "core shortage problem" has been plaguing major industrial control brand manufacturers, from safety PLCs and inverters, to safety door locks, etc., generally because of the lack of chips, resulting in longer delivery times, and some even unable to deliver the goods at all. This has created a "problem" for the friends in the electrical design department, and they are racking their brains every day to find alternative manufacturers and alternatives.

"Standard Answers" for "Classic Questions"

Due to the comparative, the current delivery time of safety relays is okay, some friends can't help asking: "Put the safety door interlock switches on our equipment, the same group are connected in series, and then connected to the safety relay, can't you solve the urgent need?"

This is actually a classic issue that has been discussed in the field of machinery safety for many years, and has been unanimously recommended and documented: "ISO/TR 24119 Safety of Machinery – Evaluation of Fault Masking Problems in the Application of Passive Contact Structural Interlocks in Tandem". The standard makes it clear that tandem applications will reduce Diagnostic Coverage and, in turn, the Performance Level.

In the vernacular, it means that the safety door switch is used in series, and there is a high probability that the "safety level" will be reduced.

Faultmasking: A brief analysis of the problem of fault masking

Pilz: Can safety gate switches be used in series?

　　As shown in the figure above, three safety gate switches (B1, B2, B3) are connected in series as an example:

5.2 Fig., B2 is turned on, and the safety relay is cut off normally.

Figure 5.3, B1 is open, and the contact B1.2 cannot be broken, but at this time, because both input channels are in a disconnected state because of B2, the safety relay does not detect the fault that occurs in B1.

5.4 Fig., B1 closes, B1.1 and the two sets of contacts of the faulty B1.2 are closed again. At this time, the safety relay still thinks that the two input channels are normally disconnected, and does not give an alarm, and is still in a normal disconnected state.

5.5 figure, when B2 is turned off, only need to reset normally, and the safety relay is about to restore the output.

Faults in B1 were masked throughout the maintenance interval. This is the term called fault masking. At this point, the matter is far from over, if such fault masking phenomenon continues to occur, and finally, as shown in Figure 5.6, the B1.1 contact of B1 also has adhesion, open the B1 door, the safety relay will not react, it means that the safety door has a dangerous failure.

Therefore, the standard concludes that when a safety gate switch is connected in series, it reduces the probability that the fault will be detected by the safety logic unit (in this case, the safety relay) and also reduces the diagnostic coverage (DiagnosticCoverage). Obviously, the more safety doors are opened and closed in series, and the more doors need to be opened per maintenance job, the more likely it is that such a fault will occur. Changes in DC will also further affect the performance level of the security chain.

Due to space limitations, I will not elaborate on the correspondence between DC and PL. A simple tool is provided in the standard that can be used to quickly and simply assess the impact of the number of door openings on DC. The first two columns are "Number of doors that need to be opened frequently" and "Total number of remaining doors".

　　"Innovative Answers" to "Classic Questions"

The safety PLC can easily and flexibly access multiple input signals, but it is difficult to deliver at present; When the number of safety switches is large, if the safety relay is used for one-to-one individual monitoring, the hardware circuit design is not very reasonable, and the workload of project implementation will be very large.

So, is there a suitable product with good design flexibility and relatively short delivery time?

myPNOZ may be the best option at the moment

The modular safety relays of the myPNOZ series are the latest safety controllers from Pilz. Compared with traditional safety relays, it has the advantages of simple design and implementation. We can easily "connect these safety gate switches in series" logically rather than hardware. The configuration tool myPNOZ creator automatically generates the required module configuration based on the set logic, so you don't have to worry about booking the wrong or missing components. Customers who place a complete order will receive a product that has already been condition-tested on the test rack by Pilz, which reduces project risk.

　　For retrofit users, the myPNOZ series has a narrow width, which can make maximum use of the space of the original electrical cabinet of the equipment; If the original design used myPNOZ, it is also very convenient to expand the existing module without having to change the wiring of the previous module, as is the case with normal safety relay circuits.

The most important point is that the supply of the myPNOZ series is quite reliable, compared to the order cycle of the safety PLC, which can often be half a year or more, myPNOZ still maintains a safety stock, which can ensure that the delivery time is shorter.

　　Source: Pilz

Pilz: Can safety gate switches be used in series?

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