...that the Performace Level (PL) is reduced with a series connection of safety switches with mechanical contacts?

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Performance Level (PL)

 

HOLDX Application Example

 

In order to reduce costs, safety switches for multiple safety doors are often connected in series to a safety relay.

However, when door switches with mechanical contacts are connected in series, fault diagnostics become significantly less effective. This makes it difficult to determine the achievable performance level. EN ISO 14119, section "8.6 Logic series connection of interlocking devices", adresses this issue and refers to the technical report ISO/ TR 24119.

In the past, manufacturers often incorrectly assumed the same diagnostic coverage (DC) for mechanical safety switches in series connection, frequently specifying a DC of 99%. In reality, DC in such configurations often drops below 60%, reducing the achievable performance level from PLe drops to PLc. As a result, many machines unknowingly operate with an insufficient PL, compromising safety.

According to ISO/TR, these faults are categorized as fault concealment. However, EN ISO 13849-1 requires for Cat. 3 or Cat. 4 that every first fault must be detected by the system without impairing the protective function. Consequently, machines with undetected faults cannot be classified as Cat. 3, preventing them from achieving PLe, even if DC exceeds 60%.

 

All doors are closed, No error in the safety circuit, Motor running

Figure 1: All doors are closed, No error in the safety circuit, Motor running

All doors are closed, Error in the safety circuit (cross cirguit), Fault due to safety relay not detected, Motor running

Figure 2: All doors are closed, Error in the safety circuit (cross cirguit), Fault due to safety relay not detected, Motor running

Door 1 opened, Error in the safety circuit, 2-channel error is detected by the safe relay (only one channel switches off), Motor stopped

Figure 3: Door 1 opened, Error in the safety circuit, 2-channel error is detected by the safe relay (only one channel switches off), Motor stopped

All doors are closed, Error in the safety circuit, 2-channel error is detected by the safety relay, Motor stopped

Figure 4: All doors are closed, Error in the safety circuit, 2-channel error is detected by the safety relay, Motor stopped

Door 2 is opened, Error in the safety circuit, Errors are cleared in the safety relay by opening both channels, Motor stopped

Figure 5: Door 2 is opened, Error in the safety circuit, Errors are cleared in the safety relay by opening both channels, Motor stopped

All doors are closed, Error in the safety circuit, But no error detected in the safety relay (error overwritten by opening both channels), Motor running

Figure 6: All doors are closed, Error in the safety circuit, But no error detected in the safety relay (error overwritten by opening both channels), Motor running 

 

The SAFIX 3 safety sensors and the HOLDX R process guard locking feature safe OSSD outputs in the output circuit. The use of OSSD outputs does not affect the wiring category or diagnostic coverage (DC) under EN ISO 13849-1. Every detected fault triggers a safe shutdown, allowing multiple safety switches to be connected in series up to PLe without issues.When cascading safety sensors, only the PFHD value of the entire circuit needs to be calculated. SISTEMA libraries for validation software are available for download from the SSP website.

 

The built-in EDM function monitors downstream, positively driven contactors. A safety relay is no longer necessary

The built-in EDM function monitors downstream, positively driven contactors. A safety relay is no longer necessary.

Error detection in the system thanks to the built-in OSSD outputs Further information on page 2

Error detection in the system thanks to the built-in OSSD outputs Further information on page 2

 

 

Products

 

Safety Sensor SAFIX

Safety Sensor SAFIX

Process Guard Lock with RFID HOLDX R

Process Guard Lock with RFID - HOLDX R