Protective Devices: Functional Safeguards

Protection against unexpected start-up in accordance with EN 1037.

When an operation is in progress, the same question always arises: when a machine is brought to a halt via an operational stop command, how safely is the machine prevented from starting up unintentionally: What happens in this situation should a fault occur in the control system and a drive is started up unexpectedly? This is an issue which is just as important as the consideration of functional safety associated with “more obvious” safeguards. A key point to consider is the issue of converter-controlled drives. These drives are often stopped by signals such as “Zero Speed” or “Controller Inhibit”. The desire is often to avoid shutting down the power supply so as not to lose any data about the current drive status. In some cases, spontaneous shutdown of the connection between the mains and the converter or even between the converter and the drive is linked to device defects and so cannot be considered.

In cases such as these the machine designer has two options: If isolation from the energy supply is possible without damaging the unit and without initiating other hazardous movements, standstill monitoring can be used. Although the converter controlled drive is stationery it is still active, so it is monitored to check it does not move. Should any movement occur on account of an error, the supply to the whole branch is shut down via a contactor. This solution assumes that the slight drive movement which occurs in the event of an error does not cause a hazard. The movement itself consists of two parts: the part which activates the sensor technology for monitoring and the part occurring before the protection circuit has reacted and a contactor has switched. These influences must be examined in a risk assessment.

If an unintended movement such as this is unacceptable, safe drive technology must be used, which will prevent such faulty behaviour from the start.

Two-hand Control Device Requirements

Two-hand control devices are used on a workstation to keep both of the operator's hands committed to a two-hand circuit; while the devices are operated, the hands are kept away from the danger zone. Various types of two-hand circuits are defined and can be applied to suit the necessary level of protection.

Requirement levels for two-hand control devices:

Laser Scanners, Safe Camera Systems and Pressure Sensitive Mats

Laser scanners

A second ESPE installed horizontally or at an angle is often used to protect against encroachment from behind. Often this only covers a small area, so a scanner can be used for additional optical monitoring of encroachment from behind. A laser beam scans the area to be monitored. If the beam is reflected by a foreign body, this will be detected and the hazardous movement will be shut down.

Safe camera systems

The latest developments on the market are safe camera systems for monitoring freely configurable zones. In contrast to simple sensors, they are able to record and analyze detailed information about the whole monitored zone. This way potentially hazardous work processes are safely monitored, protecting man and machine.

Pressure sensitive mats

Many pressure sensitive mats operate in accordance with the normally open principle: They require the use of special evaluation devices, which account for this actuation principle and guarantee appropriate fault detection. Pressure sensitive mats that operate to the normally closed principle are also available, however; where a low safety level is required and the electrical loads are low, these can be used to activate contactors directly.

The most popular material used on pressure sensitive mats is EPDM (Ethylene-Propylene-Diene-Monomer), but as this is not permanently oil-proof, it has limited suitability for use in a machine environment. Other materials such as NBR (Nitrile Butadiene Rubber) are available, but they reduce the sensitivity of the sensor.

What is Muting?

Muting is the safe, temporary, automatic suspension of electrosensitive protective equipment (ESPE), so that material can be transported into and out of a danger zone. Special sensors are used to ensure the muting controller only starts the muting cycle when the material is being transported through the protected field The sensors must be positioned in such a way that persons cannot activate the muting sensors. If anyone should access the protected area, the potentially dangerous movement is shut down immediately.

The industry has developed special safety relays with muting function specifically for this case. Some light curtains also provide the option to mute the protected field only partially (blanking). In this process for example, the precise section through which the item is being transported is rendered passive. However, under no circumstances should anyone be able to reach the danger zone undetected via this deactivated section of the protected field. A design measure (e.g. a cover for the remaining free space) should be used to ensure that nobody can reach the danger zone from the side, in between the item and the protective device.

Protective Devices: How to calculate safe distance

Protective Devices

Protective devices (electrosensitive protective equipment, abbreviated to ESPE below) are always used when access to the corresponding hazard zone is to be particularly easy to achieve and there are no hazardous repercussions to be anticipated from the machine itself (example: welding or grinding processes). To ensure that a potential hazard can be shut down quickly enough, the protective device must be installed at an appropriate distance.

This distance or safety distance (S) is defined in EN 999 and depends in particular on the following factors:

·         t1 = Response time of the protective device itself.

·         t2 = Response time of the machine, i. e. the machine's stopping performance in response to the signal from the protective device

·         C = Potential approach towards a danger zone undetected by the protective device, e. g. reaching through two beams of a light curtain undetected, depending on the distance of these beams

·         K = Anticipated approach speed of the human body or parts of the human body. This factor is defined in EN 999 as 1.6 m/sec for walking speed and 2 m/sec for hand speed

The distance to be implemented is therefore

S = K* (t1 + t2) + C

If the ESPEs form horizontal or inclined protected fields above an accessible area which requires safeguarding, the fields must be positioned at a minimum height, as predetermined by the application and ESPE. Here too, the safety distance between the outer edge of the protected field and the danger point to be safeguarded should be such that the possibility of injuries resulting from the hazardous movement in the danger zone is excluded, bearing in mind the machine’s stopping performance.