The basics of dust-explosion protection - STAHL - #26

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Plugs and Sockets, Plug Connectors In Zone 20, plugs and sockets are not permitted. In Zone 21 and Zone 22, for electrical equipment from Categories 2D and 3D, the following require­ments apply:

> Plugs and sockets shall either be interlocked mechanically, or electrically, or otherwise designed so that they can not be separated when the contacts are energized or in such a way that they can only be separated when the power is off, or

> In accordance with 9.2 of EN 50014, plug connectors must be fixed together by special fasteners and a warning label affixed to them: "DO NOT SEPARATE WHEN ENERGIZED".

Supplementary Requirements EN 50281-1-1 contains several supplementary requirements for specific electrical equipment from Category 2D which must be taken into account. Here are some important examples:

> Rotating electrical machines, such as shaft driven external fans used for cooling, must be surrounded by a fan hood.

> Switch gear with contacts immersed in flam­mable dielectrics is not permitted. Enclosures must be locked with isolators or labeled with a warninglable:"DO NOTOPEN WHEN ENERGIZED", if contactors or other remote-controlled com­ponents are included in them. If a disconnector is built-in, it must disconnect all poles and be set up so that its contact position is visible, or their position is reliably indicated. Any interlock between such disconnector and the cover

or door shall only permit the door to be opened when the separation of the disconnector contacts are effective.

Radiation-Emitting Equipment Optical radiation is a peripheral phenomenon in the field of dust explosion protection. The first factor to be considered when determining the limit value is: How high is the power density?

If the power density is =s 0.1 W/cm², no further assessment is necessary. Pulse-shaped signals are assessed according to the energy density. In addition, however, the energy density as an average of the pulse-pause ratio must be taken into account. Besides this, an assessment of possible faults is to be carried out in order not to exceed the limit values for equipment from Categories 1D and 2D. The figure for the pulse-pause ratio, which is the basis of the calculation, is particulary im­portant. Radiation in the optical range (especially if it is focussed) may ignite the dust/air mixture. Laser radiation can cause ignition from a great distance, even when it is not focussed.

Limit values to be ensured:

> Power density =s 5 mW/mm² for continuous wave lasers

> Pulse energy density =s 0.1 mJ/mm² for pulsed lasers

In the case of ultrasonic transmitting (e.g. sensors), a large percentage of the energy emitted by the ultrasonic transducer is absorbed by dust. This causes heating of the dust particles, which in extreme cases even reach the ignition temperature. Limit values to be observed:

> Power density =s 0.1 W/cm²

> Pulse energy density =s 2m J/cm²

Additional internal supplementary enclosures for parts which remain energized when the enclosure is opened.

R.STAHL Explosion protection