“Methodology for determining safety requirements during the design and manufacture of guards and protective devices”
1. Introduction
Mechanical hazards may be prevented by:
- designs safe in themselves
This solution consists in eliminating risk, e.g. by placing a moving element beyond the reach of a machine operator or any other person. It may involve, for example, placing this element at considerable height. Another example is the replacement of a rotating spoke wheel with a full wheel.
- application of guards
This solution involves preventing access to the hazard zone by permanently separating a moving element with the use of casings, screens, covers, mesh, doors, fencing, etc.
- application of protective devices
Protective devices ensure technical safety by means other than guards. They are used to limit the possibility of access to the hazard zone by stopping hazardous operations in the event that a breach into the protective device detection area is detected.
Basic requirements for protective devices and guards are provided in Appendix 1 to Directive 2006/42/EC, chapter 1.4. The requirements are defined in more detail in the following type B harmonised standards:
EN 953:1997+A1:2009 Safety of machinery – Guards – General requirements for the design and construction of fixed and movable guards
EN ISO 13858:2008 Safety of machinery – Safety distances to prevent hazard zones being reached by upper and lower limbs.
EN ISO 14119:2013 Safety of machinery – Interlocking devices associated with guards – Principles for design and selection
EN ISO 13855:2010 Safety of machinery – Positioning of safeguards with respect to the approach speeds of parts of the human body
In the situation where guards are connected to a control system and where protective devices are applied, the following standards shall be taken into consideration:
EN ISO 13849-1:2006 Safety of machinery – Safety-related parts of control systems – Part 1: General principles for design
EN ISO 13849-2:2012 Safety of machinery – Safety-related parts of control systems – Part 2: Validation.
EN 62061:2005 Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems.
In the light of the foregoing, when determining safety requirements relating to the application of guards and protective devices, it shall be necessary to verify basic requirements by referring to a relevant standard – one of those mentioned above.
2. Application of guards
Another method, in addition to designs safe in themselves, used to separate hazard zones from accessible zones, is the application of guards. Guards are any type of barrier, baffle, casing, wall, mesh, door, etc. Guards prevent access to the hazard zone by forming a material barrier between a hazard zone and an accessible zone. Guards should:
· be of robust construction,
· be difficult to remove or turn off,
· be located at an adequate distance from the danger zone,
· hinder the work process to the smallest possible degree,
· not contribute any additional hazardous or detrimental factors,
· enable execution of necessary works relating to the installation and/or replacement of tools, or maintenance, if possible – without their removal, with access restricted only to the area where these works are to be executed.
Generally speaking, guards are grouped in terms of their attachment and operation, adjustability, degree of perforation and the degree to which they cover a hazardous factor. A guard may be attached to the installation area in two ways:
· permanently, inseparably (e.g. by welding), or by means of separable connectors (e.g. screws), in a manner preventing the removal or opening of a guard without using tools; this type of guard is referred to as a fixed guard,
· by means of mechanical elements that enable the opening of a guard without using tools (e.g. hinges, guides); this type of guard is referred to as a moving guard.
The position of a guard may be adjustable or not. Wherever access by the operator to the hazard zone during normal operation is not required, fixed guards shall be used. If the operator is required to frequently access the hazard zone, it shall be necessary to apply a moving guard or protective device. Alternatively, it is acceptable to use adjustable or automatically closing guards.
The effectiveness of a guard depends on its positioning in relation to the hazard zone. It is of particular importance to maintain a distance that will prevent a person from reaching the hazard zone over or around a guard, or through guard openings. The principles for determining these distances are provided in standard PN-EN ISO 13857:2008.
When guards are used to separate the accessible zone from the hazard zone, it is significant to design guards with such dimensions and location as to prevent operators from reaching the hazard zone over the guard.
Safety distance c shall be defined according to PN-EN ISO 13857:2008, section 4.2.2. It applies to guards with heights between 1,000 mm and 2,500 mm (low risk) or 2,700 mm (high risk). The reason for this is that guards lower than 1,000 mm are regarded as too low to limit movement of the body, while guards that are higher than 2,500 mm (2,700 mm) completely restrict access to hazard zones. Pursuant to EN ISO 13857:2008, a hazard zone may be efficiently separated by increasing the distance or lifting a guard.
2.2. Reaching around the guard
The fundamental principle for using guards is to locate them in such a way as to completely separate the area through which a person may reach the hazard zone. However, on many occasions, due to process reasons (e.g. materials delivery zone or product collection zone) or technical reasons (e.g. structural limitations), it is impossible to fully separate the zones to be protected. The sizes of the zones that may be reached adjacent to guards depend on applied access-restricting structures. Standard PN-EN ISO 13857:2008 defines safety distances to prevent hazard zones from being reached by the upper limbs, by restricting movement in a variety of ways.
2.3. Reaching through openings
Guards may be either closed or perforated with openings of various shapes. These openings are used, for example, to reduce weight or improve cooling. In such cases, it shall be necessary to take into account the possibility of reaching hazard zones through openings in guards. Standard EN-ISO 13857:2008, section 4.2.4 defines reaching through regular openings.
2.4. Interlocking devices as well as interlocking and locking devices relating to movable guards
Interlocking devices as well as interlocking and locking devices relating to movable guards should comply with the requirements of standard EN ISO 14119:2013. In cases that concern these types of devices, it is also recommended to consider standard PEN 957:1997.
An interlocking device (without locking) always enables the opening of a movable guard. When the guard is closed, the interlocking device generates an active signal allowing the activation of hazardous movements by the machine. A guard equipped with an interlocking device should perform the following safety functions:
- functions of a machine that cause a hazard, “controlled” by a guard, cannot be activated before closing the guard (activation interlock);
- opening of a guard when a machine is performing functions creating a hazard results in the generation of a signal activating the procedure of stopping the functions (automatic deactivation);
- functions of a machine causing a hazard, “controlled” by a guard, can be performed when it is closed, but the closing of a guard does not result in activation of the functions;
An interlocking device with guard locking keeps the guard closed. There are two types of these devices. Those where unlocking:
- may be initiated by the operator at any time (unconditional unlocking);
- is possible only if a condition of hazard disappearance is fulfilled (conditional unlocking);
A guard equipped with an interlocking device with locking should perform the following safety functions:
- functions of a machine that cause hazard, “controlled” by a guard, cannot be activated before closing and locking the guard (activation interlock);
- after unlocking, functions of a machine that cause the hazard should be stopped in a time that is shorter than the time needed to open the guard,
- functions of a machine that cause a hazard, “controlled” by a guard, can be performed when it is closed and locked, but the closing and locking of a guard does not result in activation of the functions;
Furthermore, in the case of conditional unlocking, a guard should be closed and locked until the hazard disappears.
If a stop signal is generated by a single sensor (switch), it is required to activate a forced change-over sensor. Change-over in non-forced mode is permitted only in association with a forced change-over sensor.
Electrical interlocking devices, and interlocking and locking devices employ, among other things, position switches (limit switches, with a roller) and devices with “a key” (interlocking, interlocking with guard locking) that may contain a single break contact changed over in a forced mode, or two contacts: break contact changed over in a forced mode and a make contact changed over in a non-forced mode. The application of non-mechanical change-over switches (proximity, magnetic) requires additional factors that increase the level of defect resistance (special systems that monitor operation). Manufacturers of these devices usually provide solutions that enable the attainment of systems with a certain category of resistance to defects.
3. Application of protective devices
Protective devices provide technical safety by means other than guards. Access to hazard zones is limited by the application of protective detection devices. By definition, protective detection devices include machine equipment that detects a person or parts of the human body, generates a relevant signal and transfers it to the control system in order to limit the risk of injury. The signal transferred to the control system may be generated when a person or parts of his/her body breach a designated limit of the hazard zone, or when a person or parts of his/her body are within the hazard zone, or in both cases.
Among the protective detection devices, the most common include electro-sensitive protective devices and pressure-sensitive protective devices. Their function is the detection of the presence of a person, or parts of his/her body, in the area close to the hazard zone, and the generation of a signal causing the hazardous movement to stop. In the case of guards, a protection measure prevents the reaching of the hazard zone by its physical separation from the accessible zone. The operation of protective devices is the reverse. They are used to prevent the reaching of the hazard zone by ceasing hazardous movements, thereby eliminating a hazard, and, as a result, a hazard zone.
In protective detection devices, a very important function is performed by a detection area, i.e. the zone where the presence of a person is detected. The detection area therefore constitutes an area where the access zone is under control.
The functions performed by protective detection devices are divided into two groups:
· automatic deactivation functions (stopping hazardous movements in case of a trespass into the detection area),
· activation of interlocking functions (preventing the start of a hazardous movement as long as there is a trespass in the detection area).
The automatic deactivation function may be activated when a hazard occurs (operating machine), i.e. when trespass into the detection zone results in activation of a protective device and, in consequence, initiates activities leading to hazard cessation (stopping of the machine). Hazard cessation never occurs immediately – it is accompanied by a delay; a time known as “system after-running time”. This is the maximum period lasting from the moment the protective device is activated until complete cessation of the hazard. This time is a sum of two values:
T = t1 + t2
where:
t1 – activation time of a protective detection device
t2 – maximum response time of a machine
The protective device must not be located at the edge of an actual hazard zone, since protective operation of the safety function is always delayed by the after-running time. For this reason, a protective detection device should be activated before a hazard zone is actually entered. This is achieved by locating protective devices at a certain minimum distance from the actual hazard zone, referred to as safety distance and marked as S. The safety distance depends on the type of the device, its parameters and its location by the machine.
3.1. Electro-sensitive protective equipment (ESPE)
Electro-sensitive protective equipment (ESPE) is a very technically advanced and, currently, frequently used group of protective devices. This concept refers to a set of devices and/or interactive elements that execute automatic protective deactivation or detect the presence of a certain factor, and are made up of, at least: a detection device (proximity-type); control/monitoring devices and output signal switching devices, as well as, optionally, an auxiliary change-over device. These devices may use various physical phenomena (electromagnetic radiation related to microwaves; IR; visible light; acoustic waves, including ultrasounds; variation of capacitance and inductance, etc.) in order to detect a person or parts of his/her body. At present, only active opto-electronic protective devices using infra-red radiation are approved for use. There is ongoing research into other methods of person detection and one may expect that new types of protective devices will soon be introduced to the market.
This group of devices includes:
· active opto-electronic protective devices, i.e. safety light curtains (barriers) and light beam safety devices;
· devices where a sensitivity function is performed by opto-electronic sending-receiving elements that detect reflections of dispersed optical radiation generated by these devices, caused by the presence of an object in the two-dimensional detection area. These devices are commonly known as laser scanners.
The application of protective devices is justifiable and proper in machinery requiring the operator’s frequent access to the hazard zone (e.g. to the working area), or the interaction between an operator and a machine during process operations – when it is necessary to have good visibility of the machine and on-going process – as well as in machines in which it is difficult to install fixed guards. Certain distinctive features of a machine may prevent the application of protective devices as the only means of protection; therefore, it may be necessary to use other additional means of protection.
The process of selecting protective devices should be carried out by choosing the most suitable protective device and related auxiliary means of protection, and should take into account:
· characteristics of a machine,
· characteristics of the environment,
· safety features,
· characteristics of the human body,
· typical parameters of electronic protective devices.
Characteristics of a machine may prevent the application of certain protective devices, for example, due to:
· the risk of throwing off processed material, chips or parts of elements,
· the occurrence of thermal or any other type of radiation,
· the machine exceeding acceptable noise levels,
· the risk of the machine’s operation being detrimentally affected by the environment,
· impossibility of achieving a safe status for the machine during a process cycle, caused by: the nature of the process (e.g. its stoppage might cause additional hazard), the method driving the machine (e.g. a coupling with a rotary wedge, which can be detached only after the end of a working cycle, is used to drive the machine) or stored energy (e.g. in the form of pneumatic or hydraulic pressure in tanks).
· the machine’s stopping time (time of achieving a safe status) not complying with requirements for the installation of protective devices, which is an effect of machine design solutions (it may be related to: manufacturing of the machine stopping system with the use of technology that is not suitable for interaction with protective devices, or the system’s significant delay time, as well as insufficient stoppage of machine movements, resulting from variable speed, load or inertia).
When selecting protective devices it shall be necessary to consider features of the human body in terms of the following:
· the speed and direction of approaching the hazard zone;
· the detected part of the body (e.g. detection of a finger, hand, upper or lower limb, whole body);
· interactions of a person with the machine.
Also, positioning of protective devices is significantly dependent on the aforementioned factors. It is also important to select and position the devices in a manner that minimises the risk of exposing persons to the hazard by avoiding them. For example:
· access to the hazard zone over, under or around the detection zone;
· reaching over the detection zone;
· crossing over the detection zone;
· change of position of the protective equipment sensing elements;
· reflection of light rays by means of reflective surfaces causing changes to the detection area in electro-sensitive protective devices;
· remaining in the hazard zone beyond the detection area.
4. Summary
The process of determining safety requirements regarding applied guards and protective devices shall involve the selection of proper standards regarding the relevant type of device. Thus, subsequent steps of the procedure should involve:
1. Selection of a type of guard or protective device – EN 953:1997+A1:2009
2. Determination of requirements for positioning of guards – EN ISO 13858:2008
3. Determination of requirements for positioning of protective devices – EN EN ISO 13855:2010
4. Determination of requirements for resistance of safety functions to defects – EN ISO 13849-1:2006 or EN 62061:2005
5. Validation – EN ISO 13849-2:2012, EN 62061:2005