See also: LHPEw-10/2-B pull-wire switch
LHPEw-10/2-BSL(R) pull-wire switch

Devices’ operating safety
and advices for material’s transport according to
PN-EN 418, PN-EN 1050, PN-EN 954-1 norms

We often come across questions that are related to protective devices which should be installed on a conveyor belt to meet current safety norms. To be convinced of those kinds of devices, one should ask another question – “which protections are already implemented and what is the safety category of the particular belt?”. Unfortunately, the users do not know how to answer that question most often. Without the answer, the matter of protection according to current norms is left unsolved. There is an exemplary belt in the picture, for which current norms are as follows:

"EN 1050:1999 Safety of machinery. Principles of risk assesment."
"EN 954-1:2001 Safety of machinery, Safety-related parts of control systems "

On the basis of the above norms one could state what risk could occur and what is the category of the device.

In the table below, there are the particular risk’s categories introduced. The starting point of evaluating risk of the particular part of the control system is the choice of bodily injury threat level (S1 and S2). In our case: near the conveyor belt there could occur serious bodily injury (permanent effects) and even loss of life, (S2):

Bodily injury threat

S1 Light bodily injury (impermanent effects)

S2 Serious bodily injury (permanent health impairment) or even loss of life

The next stage is an evaluation of a frequency and or time of risk’s occurrence. If the belt is going to operate; for example, at a certain height where people’s access will be limited to the exceptional situations, one could assume that he has F1 category.

With our example, the access to the belt is free and people who have a direct access are constantly endangered. Therefore, we choose F2 category.

Frequency and time of risk’s occurrence

F1 Rarely to frequently or short time of risk’s occurrence

F2 Frequently to constantly or long time of risk’s occurrence

Possibility of avoiding risk

P1 Possible in a certain conditions

P2 Practically impossible

In our situation we should choose P2 because we are in endangered area and the belt is not equipped with a sufficient protective casing.

In this way (on the basis of the table) we have established safety category of this control system as „4”. Establishing a category should be conducted collectively; best in a group that contains: device’s operator, device’s producer, independent person and designer. Committee should take into consideration every technical possibility and every safety matter that influences the category of a device’s choice. Furthermore, they should take into account device’s place in the technological process.

For the "4" category in PN-EN 954-1 norm there are conditions mentioned according to which B category’s restrictions must be fulfilled, safety’s rules must be implemented and control systems’ protective parts must be designed in the way that assures of single fault of any part of the system not being the cause of losing protective function. Fault must be detected before or during the next use, for example, during device’s switching on and off. If the fault’s detection is impossible, failures’ accumulation cannot lead to protective function’s loss.

Therefore, what results form „4” category restrictions is the fact that safety circuits must be at least doubled and mutually conjugated. In this case, the safety blocks are used which you plug into device’s safety circuit.

For the emergency stopping of a conveyor belt there are pull-wire switches used most often. They are fulfilling the condition of belt’s emergency stopping from every endangered place. The construction of the pull-wire switches is regulated by the PN-EN 418:1999 norm "Safety of machinery. Emergency stop equipment, functional aspects ”. The norm tells about functional aspects, about emergency stopping and about devices that are applicable. In this moment, we should clearly state it is about device’s emergency stopping, not only its switching off. In some situation, belt which is switched off could be still moving for a while, and another pulling a wire does not cause its full stopping. If the switched off belt is moving using the power of inertia, the application of an braking device is essential. This device will be put into operation just after emergency stopping initiation. Only after an actual stopping of the belt, power supply can be cut off. Obviously, the type of stopping device also matters in that case. Useful device in the discussed situation will be the one which is active in a potential-free status – braking is being conducted after cutting off the feeding potential. The applied safety device, pull-wire switch, is the control element, and the wire serves for an emergency stopping activation.

According to the norm, the control of an emergency stopping (wire) should be red. The norm says that the wire could be additionally marked with the elements that are clearly visible (e.g. colored pennants). Pull-wire switch must be equipped with the wire-breaking control. We frequently encounter a normal wire, non-taut, uncolored, and without breaking control. Unfortunately, those kinds of devices are still applied in the new projects.

We recommend a wide range of pull-wire switches that fulfill the restrictions determined by EN 418:1999 norm for different branches of industry. Especially for longer distances and external environment or for interiors with frequent temperatures’ changes, we recommend application of LHPE type switches. These particular switches need relatively greater expenditures in relation to other types applied in the light industry. However, thanks to its construction, there is no accidental emergency stopping, e.g. causes by temperature’s change or vibrations. The costs connected to another starting of the belt could be higher with the first unintentional emergency stopping. Durability and quality of workmanship of these devices are appreciated by our previous clients.

Let’s now get back for a moment to safety circuits and ways of pull-wire switch plugging in according to "4" category. Because of safety circuits being doubled, equipping pull-wire switch with two disengaged contacts with disengage extortion according to PN-EN 60 947-5-1 norm is essential. Disengage extortion means that both contacts are mechanically joined. Every contact has one safety circuit plugged in. This circuit ends in a control safety module and it could be plugged in more control devices. For "4" these circuits must be close to each other; in other words, the contacts in control devices cannot be connected in line, but one socket of one control device must be in one circuit. For this category, wiring’s costs are two-times higher than for others. For the circuit closing, there are different types of safety modules produced. These modules are able to conduct the control the particular circuits and evoke safe initiation of belt’s emergency stopping also in the case of faults’ accumulation in the system (short circuit, etc.). This system, in a case of fault, prevents from system’s putting into operation.

As you can see, problems connected with safety devices (in our case pull-wire switches) influence electrical and mechanical designing. The belt conveyor, which is produced as a complete product, should be equipped with a safety system, at least with emergency stop switches.

Other devices that are used to increase safety are rotary motion sensors (prevent from drum’s sliding and belt’s breaking), misalignment sensors, sensors installed in a dumping, and other mechanical sensors. Those sensors are used as elements that supplement technological process’ protection system. They can be additionally plugged in similarly to emergency stopping. If they are to be implemented to emergency power supply circuit, they should fulfill EN 418:1999 norm restriction too.

Thus, we introduce to you in short the project of safety circuit of conveyor belt’s emergency stopping. In case of further questions feel free to contact our company who will advise you with pleasure and recommend proper products to ensure safety.