Passive safety up to SIL3/PLe: simple & cost-effective alternative by Bihl+Wiedemann

In principle, machine safety can be achieved in different ways but passive safety is becoming increasingly more important in fieldbus solutions – mainly due to the associated cost advantages and reduced complexity.

What is so special about passive safety?

What distinguishes passive safety technology from classic fieldbus solutions for functional safety? With classic fieldbus solutions, a safe output is required for every actuator. The advantage: All actuators can be safely switched off individually. However, this level of flexibility is not always required, especially when entire groups of actuators have to be de-energized simultaneously. In this case, passive safety is a good and cost-effective alternative. This is because having many safe outputs greatly increases the total cost of the application. With passive safety, all outputs can be switched off and controlled without safety individually or, if required, entire groups of actuators can be switched off safely using a single safe output — simply by switching off the supply voltage to all connected actuators in the drive string.

How can passive safety be realized with ASi?

To establish passive safety, you must make sure that the actuator cannot be externally supplied with power when the supply is switched off. In a best case scenario, the power supply for the actuators is galvanically isolated from the supply for communication and sensor systems.

This supply principle has always been used with ASi — especially the B+W modules. In the smart wiring system, the profile cables are separated from each other — as a system string in the lower part of the module — with the yellow ASi cable transmitting control signals and energy, and the black profile cable providing auxiliary energy. This can be realized up to a safety level of SIL3 or PLe.

Thanks to the smart wiring system ASi, communication and power supply of the module via ASi are always maintained if the periphery is selectively switched off via AUX line. This means that none of the participants in the bus communication fail — as happens often with Ethernet-based systems — and there are no unforeseen effects on other system parts. Through the bus, the module can additionally detect and report that the AUX voltage has been de-energized. This information can also be evaluated for diagnostics through the existing communication with the control system via ASi.

Through Bihl+Wiedemann's broad product portfolio, you can easily integrate passive safety into your system — and use different IP protection classes. With our IP20 products, passively safe junctions can be utilized in a cost-optimized way inside the switching cabinet. It is also possible to shut down the AUX line locally in the field by integrating our dust and splash-proof IP67 modules into your system, while benefiting from a drastically reduced wiring effort with ASi. What's more, ASi can be seamlessly integrated into almost all commonly used automation systems.

For the simple setup of passive safety, you only need one safety monitor with local outputs combined with a contact extension. Operational switching of the actuators can be achieved with non-safe standard Bihl+Wiedemann products that are suitable for passive safety. This is because almost all of our new ASi-5 modules, such as the various ASi-5 modules with integrated IO-Link master, and many of our ASi-3 modules, meet the requirements for passive safety.

Passive safety can only be established if the following technical requirements are met:

• Safe disconnection must be implemented in accordance with the requirements of the Machinery Directive 2006/42/EC and safety standards such as EN ISO 13849-1 or EN 62061.
• The galvanic isolation of passively safe switched potentials as well as other potentials must always be ensured or prevented by excluding faults (e.g. protected laying of cables, protected installation of passive safety devices).

What this means, in concrete terms for ASi systems, is that it is the easiest way to exclusively use ASi modules where the compliance with passive safety is explicitly stated in the data sheet. Furthermore, all modules with ASi and AUX should be supplied in an isolated manner via the yellow and black profile cables.