SIM 03/2009/14
This SIM alerts inspectors to anomalies which occur between calculations of separation distances for light curtains fitted to presses manufactured to comply with PM41, EN693:2001 and HSG180, although in practice there is no increased risk.
1. Light curtains fitted as safeguards for presses should be positioned in accordance with the relevant European Standards. These standards have varied slightly over time, and each press should have been fitted with a light curtain set at the distance calculated from the standard current at the time of manufacture/supply of the press. However, some manufacturers anticipated new standards, and while technically these presses do not conform to the EN standard, in practice there is no increased risk to the operator or others affected by the operation of the press.
2. Members of the Safety Assessment Federation (SAFed) have queried with HSE the action they should be taking if a press and light curtain combination does not comply with the standard current at the time of manufacture/supply but meets a more recent standard. SAFed have been advised that provided the press/light guard combination meets, either the standard current at the time of manufacture/supply, or a subsequent standard no further action would be expected.
3. PM 41, “Application of photo-electric safety systems to machinery”, (1984) set out the appropriate separation distance for the application of light curtains to presses, including press brakes. This guidance note was to be used in conjunction with BS 6491:1984 “British standard specification for electro-sensitive safety systems for industrial machines”. This guidance was withdrawn in 1999 and replaced by HSG 180 “Application of electro-sensitive protective equipment using light curtains and light beam devices to machinery”. The separation distances contained in HSG 180 were taken from BS EN 999: 1999 (ISO 13855) “Safety of machinery - The positioning of protective equipment in respect of approach speeds of parts of the human body”, which sets out different separation distance requirements than had been contained in PM41. Subsequently this standard was incorporated within EN 693:2001 “Machine tools – Mechanical presses – Safety” the ‘C’ Standard relevant to the manufacture of presses. Some manufacturers of presses (who were aware of, and involved in, the standards revision process) started using the values contained in it (and by default in HSG180), prior to the delayed publication of EN 999:1999.
4. The incentive for using the draft standard was that compliance with the separation distances was, in most cases, less onerous than those contained in HSE guidance note PM41 (but see paragraph 5).
5. Variations between the criteria for calculating the separation distance from each of the standards suggest that a problem may occur with some presses whose design pre-dates EN 693:2001 and HSG 180, i.e. between 1984 and approx 1998. Some have been fitted with a light guard with a detection capability of 14 mm and marked with a minimum separation distance of 150 mm between the light guard and the trapping point at the tools. While this separation distance complies with EN 693:2001, EN 999:1999 and HSG 180, a separation distance of 174 mm is required to comply with PM 41. 
[Exemption: S43 Commercial Interests]
6. The opposite problem may also occur, for presses fitted with older light curtains with a detection capability greater than 30 mm, i.e. that they are able to comply with PM 41, but not able to meet the requirements of EN 693 or HSG 180.
7. Further details of how these anomalies arise, and the effects, are contained in the appendix.
8. There is no risk based need for Inspectors to proactively investigate separation distances between light guards and the trapping point at presses, but if there is a reactive need, or if there are other queries, Inspectors are advised to consider the information in the Appendix and/or seek advice from SG Mechanical Specialists or the Manufacturing Sector Engineering Section.
9 Inspectors should be aware of who (in terms of diversity e.g. men, women, disabled etc) is the target group in the sector they are dealing with. Give consideration to, and factor into the approach, any issues that may surround this audience such as literacy issues, English as a second language and disability (access needs).
The Diversity pages give more information on these areas.
HSG 236 'Power presses: Maintenance and thorough examination'
HSG 180 'Application of electro-sensitive protective equipment using light curtains and light beam devices to machinery'
INDG 316 'Procedures for the daily inspection and test of power presses and press brakes'
BS EN 999 - Safety of machinery - The positioning of protective equipment in respect of approach speeds of parts of the human body (ISO 13855)
BSEN 692 – Machine tools - Mechanical presses - Safety
BSEN 693 – Machine tools – Safety - Hydraulic presses
BSEN 12622 – Safety of machine tools - Hydraulic Press brakes
1. The anomalies at paragraph 5 arise from the calculation of the Penetration Factor (P)
In PM 41 the Penetration Factor, P =3.4(S - 7) where S = Object Detection Capability (ODC).
In EN 693 (from EN 999) the Penetration Factor C = 8(d - 14) where d is the ODC.
For light guards with ODC of 14 mm:
PM 41 formula gives a Penetration Factor of 3.4 x 7 = 23.8mm
EN693 (From EN 999) gives a Penetration Factor of zero.
2. The charts below show the various separation distances for stopping time up to 200 ms for ODCs of 14mm, 20mm, 30mm and 40mm. The charts show the relationship between stopping time and separation distance for the three standards for each ODC, based on the formulae applied by PM 41, EN 693 and HSG 180. It should be noted that the variation between the values derived from EN693 and HSG180 are very small. This is due to the fact that EN693 uses figures given in Table A.1 of Annex A of the standard whereas HSG180 uses the formula 8d-14 to calculate C (the additional distance, in millimetres, based on intrusion towards the danger zone prior to actuation of the AOPD), where d is the ODC.
3. 14mm ODC - The requirements for HSG 180 and EN 693 are virtually identical. The small amount of variation between them is due to the fact that EN693 uses figures taken from table A2 in Annex A of the standard in . PM 41 requires a larger separation distance for a given stopping time than the other two standards up to about 120ms when the curves cross over.
4. 20mm ODC - The requirements are more disparate. Once again PM 41 requires a greater separation distance up to about 50ms when it crosses the curve for EN 693. The curve for HSG 180 generally runs parallel to the EN 693 curve. For most practical purposes EN 693 requires the highest separation distance.
5. 30mm ODC - The requirements for HSG 180 and EN 693 are virtually identical again. PM 41 requires a greater separation distance up to about 50ms. There after the separation distance is lower.
6. 40mm ODC - HSG 180 and EN 693 both require a higher separation distance than PM 41 throughout, with EN 693 requiring a slightly larger separation distance than HSG 180. The kinks at the upper end of the curves for HSG 180 and EN 693 are the result of the ability to use a slower approach speed, 1.6 m/s, when the separation is greater than 500mm if using an approach speed of 2.0 m/s.
