PL calculation in Step 3 When you calculate the PL for a safety function, it is easi-est to split it into separate, well defined blocks (also called subsystems). It is often logical to make the breakdown according to input, logic and output (e.g. switch - safety relay - contactors), but there may be more or fewer than three blocks depending on the connection and the number of components used (an expansion relay could for example create an additional logic block) .For each block, you calculate a PL or PFHD-value. It is easiest if you obtain these values from the component manufac-turer, so you do not have to calculate yourself. The manu-facturer of switches, sensors and logic devices often have PL and PFHD-values for their components, but for mechani-cal devices (such as key switches or contactors) a PL-value cannot be supplied since it depends on how often the component will be used. You then need to calculate yourself according to EN ISO 13849-1 or use default values from the standard, if provided.To calculate PL or PFHD for a block, you need to know its category, DC and MTTFd. In addition, you need to protect the system against systematic errors and ensure that an error does not knock out both channels, and generate and vali-date any software used correctly. The following text gives a brief explanation of what to do.Safety function (SF)+ + PFHD, InputInputPL/PFHDPFHD, LogicLogicPL/PFHDPFHD, Output OutputPL/PFHDPFHD, Tot al =The relationship between categories, the DCavg, MTTFd for each channel and PL. The table also shows the PFHD-range that corresponds to each PL. PFHDPL10-4a10-5b3x10-6c10-6d10-7e10-8DC noneDC noneDC lowDC mediumDC lowDC mediumDC highCat. BCat. 1Cat. 2Cat. 2Cat. 3Cat. 3Cat. 4MTTFdlowMTTFdmediumMTTFdhigh11– 14SAFETY PRODUCTS CATALOG ABB JOKAB SAFETY
