Topic
Safety
Machinery functional safety: risk assessment, ISO 13849 and IEC 62061, emergency stop categories, safety relays and safety PLCs, light curtains and two-hand control.
9 articles
Cause and effect matrix: how to write one for control and safety
A cause and effect matrix lists each trip condition as a row and each resulting action as a column, with a mark wherever a cause produces an effect. Each row also states the tag, trip setting, delay, voting and reset type. Read across a row for what one cause does, down a column for what stops one device, and test every mark at FAT.
11 min readSIL or PL: which one does your machine need?
Performance Level (PL a to e) comes from ISO 13849-1 and is written for machinery. SIL (1 to 3 in this context) comes from IEC 62061 and is written for electrical safety related control systems. They are harmonised so the levels map: PL c is roughly SIL 1, PL d is SIL 2, PL e is SIL 3. For a machine built in Europe, ISO 13849 is the usual route because its arithmetic is simpler and its scope covers mechanical and hydraulic parts too.
8 min readCategories B, 1, 2, 3 and 4, in plain terms
Category B is a single channel built to good practice: a fault loses the safety function. Category 1 is the same with better components. Category 2 adds a periodic test. Category 3 is dual channel, so a single fault does not lose the function and most faults are detected. Category 4 is dual channel with a fault detected at or before the next demand, so faults cannot accumulate.
7 min readSafety relay or safety PLC: how to decide
Use a safety relay when there are a handful of independent safety functions and they do not interact: an E-stop, a guard, a light curtain, each stopping everything. Use a safety PLC when functions interact, when zones must stop independently, when muting or speed monitoring is involved, or when there are more than roughly six functions. The crossover is usually cost neutral around four to six relays.
7 min readStop categories 0, 1 and 2, and why an E-stop is not a guard
A category 0 stop removes power to the actuators immediately and lets the machine coast. A category 1 stop keeps power long enough to bring the machine to a controlled halt, then removes it. A category 2 stop halts the machine but leaves power on. These are IEC 60204-1 stop categories and have nothing to do with ISO 13849 architecture categories, despite sharing the word.
7 min readProgramming a safety PLC without inventing anything
You configure certified function blocks rather than writing safety logic from scratch. Each block, E-stop, guard monitoring, two hand control, muting, has been assessed as part of the controller's certification, and using it unmodified is what carries the rating. The safety program stays small, holds no sequencing, and communicates with the standard program in one direction only.
7 min readMuting a light curtain without defeating it
Muting temporarily and automatically disables a light curtain so material can pass through while people still cannot. It is made safe by muting sensors arranged so that only an object of the expected size, moving in the expected direction, at the expected time can satisfy them. Two sensors crossed at the curtain, or four in sequence, with a maximum muting time, is the standard arrangement.
7 min readTwo hand control: types, timing and why it is often the wrong answer
Two hand control requires both actuators to be pressed within about half a second of each other, held for the whole hazardous movement, and released before another cycle can start. ISO 13851 defines types: IIIA and IIIB differ in monitoring, IIIC adds Category 4 architecture. It protects only the person holding the buttons, which is why it is a poor primary safeguard on any machine somebody else can reach.
6 min readA machinery risk assessment that is not a formality
ISO 12100 sets the sequence: determine the limits of the machine, identify the hazards, estimate the risk, evaluate whether it is acceptable, and reduce it if not, then repeat. Reduction follows a strict hierarchy: design it out first, then guard it, then warn about it and provide procedures. Protective devices come third, not first, and jumping to them is the most common way an assessment becomes paperwork.
8 min read