Light Curtain, Muting and the Safety Distance
A light curtain with 14 mm resolution protects operator access to a press brake in a palletising cell. The curtain's own response time is 20 ms, the safety relay adds 30 ms, and the press brake needs 300 ms to come to a standstill. The curtain is mounted 550 mm from the nearest hazard point. The cell builder now proposes muting: two inductive sensors detect an approaching pallet and suppress the curtain while the pallet passes, so the robot can load without a stop. The sensors are standard PNP units, single-channel, rated PL c. The risk assessment assigns the hazard S2 (serious, irreversible injury), F2 (frequent exposure) and P2 (little possibility of avoidance), which yields PL d. The production manager argues that because muting lasts only 2.5 s per cycle, 'the distance no longer matters'.
1. Calculate the required minimum safety distance according to ISO 13855 and evaluate the installed 550 mm.
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Formula S = K × T + C
Step 1 - total response time T T = 0.020 s (curtain) + 0.030 s (relay) + 0.300 s (brake stop) = 0.35 s
Step 2 - intrusion factor C For a detection capability d ≤ 40 mm, C = 8 × (d − 14). With d = 14 mm, C = 8 × 0 = 0 mm (no allowance for reaching through, because the resolution prevents it).
Step 3 - approach speed K First trial with K = 2000 mm/s: S = 2000 × 0.35 = 700 mm. Because 700 mm > 500 mm, the standard requires the calculation to be repeated with K = 1600 mm/s, which is valid for larger distances. S = 1600 × 0.35 + 0 = 560 mm, and the value must not be taken below 500 mm, so 560 mm governs.
Evaluation The curtain is installed at 550 mm - 10 mm short. The installation is non-compliant, and the shortfall has no margin at all: brake wear, relay ageing and measurement tolerance all eat into stopping performance, so a compliant design would need a deliberate buffer (for example 600 mm) or a shorter stopping time.
Cheapest correct options
- Move the curtain back to at least 600 mm, if floor space allows; or
- Reduce T, typically by fitting a faster brake or a brake-monitoring relay - each 10 ms saved buys 16 mm.
2. Does the proposed muting concept meet the required Performance Level? Justify your answer and state what would have to change.
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No - the concept as described does not reach PL d.
Reasoning
- Muting is not an add-on; it is part of the safety function. While the curtain is suppressed, the muting sensors are the protective device. Their failure mode is therefore safety-relevant: a failed sensor that stays 'on' would mute the curtain permanently.
- Two single-channel PNP sensors with no testing, no cross-monitoring and no plausibility logic give, at best, a Category 1–2 architecture and PL c. That is one level below the PL d the risk graph requires.
- The architecture must match the required PL, so the sensors alone already disqualify the concept.
What would have to change
- Redundancy: two-channel muting sensors with complementary or cross-monitored signals, or a certified muting module.
- Plausibility logic: sequential activation within a defined time window, correct order (sensor A then B for one direction), and a maximum muting duration (typically 60 s or less) so the function cannot stay active indefinitely.
- Arrangement for people as well as pallets: four-sensor layouts or a muting bar are used where a person could walk through with the pallet. Muting must not be triggerable by a single hand or a single body.
- Indication and diagnostics: muting lamp visible from the hazard zone, plus diagnostics of sensor faults.
- Documentation: the whole muting function has to be verified as part of the safety function, not just the curtain.
Key exam point The PL applies to the function, not to the components. A PL d curtain with a PL c muting arrangement yields a PL c function.
3. Write the reply to the production manager in three or four sentences explaining why a 2.5 s muting window does not remove the need for the safety distance.
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Reply
While the curtain is muted, it detects nothing, so the protection during those 2.5 s comes entirely from the assumption that the pallet occupies the opening - an assumption that fails the moment someone reaches in beside or behind the pallet. The safety distance is what guarantees that the press can still stop before a person who has passed the detection zone reaches the closing edge, so it remains the basis of the design at all times, muted or not. Muting therefore does not remove the distance requirement; it adds a second protective function whose sensor arrangement, geometry and performance level must themselves be assessed against PL d. In short, a shorter muting window changes nothing, because the risk occurs in the window, not after it.
Why this framing works with a non-specialist audience
- It starts from the physical fact the manager can picture ('it detects nothing') rather than from the standard.
- It separates the two issues - distance and muting performance - instead of letting one be traded against the other.
- It ends with a conclusion about the existing installation, which is what the manager actually has to decide about.