Factory safetyReading time 12 min

How far back does a light curtain have to sit?

You do not pick a safety device by reaching for the highest rating. The rating falls out of a risk assessment, and that assessment has to be written down.

01Start with the risk assessment

ISO 12100 defines the loop: determine the machine limits, identify hazards, assess the risk, reduce it, then reassess until the residual risk is acceptable. Risk reduction follows a strict order that cannot be skipped:

Inherently safe design. Remove the hazard, change the mechanism, lower the energy, open up the gap.
Guards and protective devices. Fixed guards, interlocked doors, light curtains, safety mats.
Information for use. Warning labels, manuals, training.

02PLr: how much safety performance is required

ISO 13849-1 uses three variables to look up the required performance level (PLr, a through e):

Variable	Options	What it asks
S, severity of injury	S1 slight / S2 serious or fatal	Is it recoverable?
F, frequency of exposure	F1 rare / F2 frequent or prolonged	How often does someone enter the zone?
P, possibility of avoidance	P1 possible / P2 scarcely possible	Can you see it coming and get clear?

S2 + F2 + P2 = PLr e, the highest level.

In practice you also have to account for the whole safety chain's category, MTTFd, DCavg and CCF. One high-rated component does not make the chain PL e, the architecture (single or dual channel, with or without diagnostics) is what decides it.

03The safety distance is a calculation

ISO 13855 gives the base equation: S = K × T + C

S, minimum distance from the hazard zone to the detection device (mm)
K, approach speed, normally 2000 mm/s for hand movement (1600 is allowed once S exceeds 500 mm)
T, total stopping time: detection plus control plus mechanical stop (seconds)
C, intrusion allowance, derived from the device resolution

Curtain resolution d	C value	Detects
14 mm or finer	0	Fingers
14 ～ 30 mm	8 × (d − 14)	Hands
30 ～ 40 mm	850 mm	Arms / body
40 mm and above	850 mm	Body passage only

Finer resolution means a smaller C, so the curtain can sit closer, at a higher price.

04Common installation mistakes

You can reach under or around the curtain. Add a supplementary curtain or a physical barrier.
Reflective surfaces let the beam bypass a person. A stainless panel beside the curtain can bounce the beam around someone standing in it. Mount far enough from reflective surfaces.
Someone can stand inside the field. Once past, the person no longer breaks the beam and the machine can restart. Add a safety mat, or require reset from outside the guarded area.
The safety relay gets bypassed. Shorting the contacts for convenience is the most common and most serious violation on the floor.

05Keep the paperwork

Risk assessment sheet, hazard list, S/F/P, PLr, countermeasures, residual risk
List of safety functions with the PL verification calculation for each
Safety distance calculations and measured stop-time records
Periodic inspection records, how often each device gets function tested
Change records, a modified machine needs its risk assessment redone