Reference ID: MET-4B26 | Process Engineering Reference Sheets Calculation Guide
Introduction & Context
ISO 13855 provides a deterministic safety-distance rule that converts the worst-case stopping time of a cutting blade into the minimum guard distance required to prevent operator contact. It is the primary sizing method for fixed and interlocking guards on industrial slicers, saws, and milling cutters inside food, wood, and plastics plants.
Methodology & Formulas
Convert the measured stop-time from milliseconds to seconds for dimensional consistency:
\[
t_{\mathrm{s}} = \frac{t}{1\,000}
\]
Compute the distance travelled by a hand moving at the standard approach speed:
\[
s_0 = k_{\mathrm{s}}\,t_{\mathrm{s}}
\]
where \( k_{\mathrm{s}} \) is the hand-speed constant (e.g., 1600 mm/s for normal approach).
Add a fixed margin to account for penetration depth and potential system over-travel:
\[
s = s_0 + C
\]
where \( C \) is the additional safety distance.
Validity Limits
Parameter
Symbol
Lower Limit
Upper Limit
Stopping time
\( t \)
20 ms
500 ms
Guard distance
\( s \)
100 mm (absolute minimum)
600 mm
Outside these ranges, or if the calculated distance \( s \) is less than 100 mm, ISO 13855 mandates a case-specific risk assessment and the application of the 100 mm absolute minimum.
OSHA 1910.212 and ANSI B11.21 limit the opening to 6 mm (¼ in.) when the blade is at its closest approach to the guard. Measure the gap with a feeler gauge at five equally-spaced points across the blade width; the largest reading must not exceed 6 mm. Re-adjust or replace worn wear-strips if any reading fails.
Use dual-channel solenoid interlock switches with integrated positive-guided locking mechanisms and monitored safety outputs. Typical solutions are based on principles like trapped-key systems or coded magnet interlock switches. Wire them to a safety PLC that performs cross-monitoring and external device monitoring (EDM). These devices, when properly selected and integrated, can meet ISO 13849-1 requirements for Category 3 / PL e. The system must stop the 45 kW saw motor within the measured stopping time used in the ISO 13855 safety distance calculation.
Re-validate annually plus after any modification that could influence safety distance or stopping time. The procedure must include:
re-measure safety distance per ISO 13855
cycle test the interlock switches per manufacturer specifications
record stopping-time with a calibrated timer suitable for safety validation
archive results in the machine technical file for the required retention period (often ≥10 years)
If any value deviates >5 % from the baseline validation, replace components or re-calculate guard position before returning to production.
Light curtains are acceptable only if they meet ISO 13855 safety distance requirements and environmental suitability tests prove the optical system remains reliable. Install Type 4 curtains with appropriate resolution (e.g., 14 mm) at a calculated minimum distance from the nearest hazard point. Add a zoned muting system for non-hazardous operations like sheet loading and consider an enclosed back shield to block unintended beam paths. If dusty, steamy, or high-reflectivity conditions persist, supplementing with a physical barrier may be necessary to maintain the required Performance Level (PL) or Safety Integrity Level (SIL).
Worked Example: Guarding Requirements for an Industrial Bread-Slicer
A process engineering assessment is conducted for an industrial bread-slicer with a 300 mm diameter blade powered by a 3 kW motor and an electro-mechanical brake. The plant safety standard specifies that the safety distance (minimum distance to the hazard behind the guard) must not exceed 200 mm. The following calculation determines the required guard distance based on the machine's stopping performance.
Knowns (Input Parameters):
Measured worst-case stopping time, \( t = 85.0 \, \text{ms} \)
Hand-speed constant for normal operation, \( k_{\mathrm{s}} = 1600.0 \, \text{mm/s} \)
Additional safety distance, \( C = 100.0 \, \text{mm} \)
Step-by-Step Calculation:
The stopping time is converted to seconds: \( t_s = 85.0 \, \text{ms} / 1000 = 0.085 \, \text{s} \).
Add the safety distance \( C \) to obtain the required guard distance: \( s = s_0 + C = 136.0 \, \text{mm} + 100.0 \, \text{mm} = 236.0 \, \text{mm} \).
Final Answer: The calculated required safety distance \( s \) is \( 236.0 \, \text{mm} \). This exceeds the ISO 13855 absolute minimum of 100 mm and is therefore valid. However, since this exceeds the plant's internal limit of 200 mm, the current braking system does not comply with the stricter in-house standard. This indicates a need to upgrade the brake to reduce stopping time or to re-assess the guard design and its opening through a detailed risk assessment.
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