Light curtains (ESPE) — type 2 versus type 4, and the safety-distance formula

The guide on guard interlocking and locking switches covers guarding of a movable guard (door, flap). This article covers a different type of guarding that uses no physical guard at all: the light curtain, a form of electro-sensitive protective equipment (ESPE) per IEC 61496, which monitors a hazardous zone with an optical curtain of infrared beams.

Type 2 versus type 4

Among others, IEC 61496 distinguishes two commonly used light curtain types, which differ in the maximum achievable reliability level and in their self-test approach:

Which type is required does not follow from a free choice but from the machine's risk assessment (per ISO 12100) and the resulting required performance level or SIL — just as described in the ISO 13849-1 guide. A type 2 light curtain must not be used where the risk assessment requires a higher level (PLd/PLe or SIL3).

The safety-distance formula (ISO 13855)

A light curtain must be mounted far enough from the hazard that the machine has come to a complete stop before a body part moving through the light curtain can reach the hazard. ISO 13855 gives the formula:

$$S = K \times T + C$$

• S — minimum required safety distance (mm).
• K — approach-speed constant (mm/s): 2000 mm/s for the fastest achievable hand/arm movement, or 1600 mm/s when the calculated distance is already ≥ 500 mm and a full-body approach is more realistic than a fast hand movement.
• T — total response time: the machine's stopping time plus the light curtain's own response time.
• C — additional distance, depending on the light curtain's resolution (the smallest object the curtain can still detect). For hand detection, C = 8 × (d − 14) is generally used, with d the resolution in mm.

The coarser the light curtain's resolution (i.e. the larger d), the larger the additional distance C must be — a light curtain that only detects large objects must be mounted further from the hazard than a finer curtain that already detects a finger.

Practical relevance

When assessing or installing a light curtain, check whether the chosen type (2 or 4) matches the required performance level/SIL from the risk assessment, and whether the actual mounting distance to the hazard is at least equal to the calculated S from the ISO 13855 formula — including the machine's real (measured, not assumed) stopping time in T.

Common mistakes

1. Using a type 2 light curtain where the risk assessment requires PLd/PLe or SIL3 — this type does not reach that level.
2. Estimating rather than measuring the machine's stopping time when determining T — an overly optimistic assumed stopping time results in a safety distance that is too small and unsafe.
3. Using the wrong K value — using 2000 mm/s for a full-body approach (instead of 1600 mm/s at an already calculated distance ≥ 500 mm) generally results in an unnecessarily large but not unsafe deviation, whereas the opposite mistake (using 1600 mm/s where 2000 mm/s applies) undermines the safety margin.
4. Not accounting for the light curtain's resolution in C — a lower resolution (coarser detection) requires a larger additional distance, which, if overlooked, leads to a safety distance that is too small.

Related

• Guard interlocking and locking switches (ISO 14119)
• ISO 13849-1 — performance levels
• Two-hand control (ISO 13851)
• Emergency stop categories (IEC 60204-1)