ATEX Information for equipment suppliers, consultants or agents

We believe that this information will be of value to everyone who is involved in supplying end users with bulk materials handling equipment. Here are the main issues to consider when supplying bulk solids handling machinery or replacement parts such as motors, gearboxes, switches etc. for an above ground installation where dust (as opposed to gas) is the assumed to be the hazardous material in question.

In what zone will the machinery be used?

It is your customer's responsibility to Zone their process plant. It is worth informing them that a potentially explosive dust cloud is so thick that you cannot see someone 1 metre away from you.

Here are the three zones relevant to the ATEX dust regulations, and your customers' obligations for each zone:

Zone 20: a continuous presence of combustible dust. A high level of protection is required for both electrical and non-electrical elements. Two separate safety systems are required for each risk. Type testing and certification is required by a notified body.

Zone 21: an occasional presence of combustible dust during normal operation. Elements considered to be a risk must have one level of protection. Full risk assessment and technical details to be included in a technical file which must be stored with a notified body.

Zone 22: the presence of combustible dust is unlikely, and if so for very short periods. Elements considered to be a risk must have one level of protection. Full risk assessment and technical details to be included in a technical file and stored with the manufacturer.

What measurements need to be taken?

Before deciding what equipment will be suitable for your customers' zone, the manufacturer will need the following information about the product being handled.

Kst value. This is a measure of the explosivity value of the dust. Measured in bar.m/sec. Dusts may be classified as follows:

Dust explosion class	Kst	Characteristic
St 0	0	No explosion
St 1	>0 <200	Weak explosion
St 2	>200 <300	Strong explosion
St 3	>300	Very strong explosion

Minimum ignition temperature (MIT). This is the ignition temperature of a cloud of dust. Measured in °C.

Layer ignition temperature (LIT). The ignition temperature of a 5mm layer of dust. The maximum permissible surface temperature for any component part, whether electrical or mechanical, can be calculated from the MIT and LIT figures. Use the lowest of either:

Two-thirds of the MIT, or
The LIT minus 75°C.

Conductivity. This is used to assess whether dust ingress is a potential hazard on electrical elements. Measured in Ω. Here are the ATEX limits:

≥10⁵ Ω to be specified with IP5x enclosures.
<10⁵ Ω to be specified with IP6x enclosures.

Industry practice tends to be IP65 as the cost implications are minimal.

It is recommended that the product your customer handles is tested, although data is available on common products or assumptions can be made based on product properties.

References

ATEX Guidelines on the application of Council Directive 94/9/EC.
DTI - Equipment and protective systems intended for use in potentially explosive atmospheres.
UKASTA - Control of Fire & Explosion risks in animal feed mills.
HSE - Dust explosions in the food industry - Food sheet No 2.
BS EN 13463-1:2001 Part 1 non-electrical equipment for potentially explosive atmospheres basic method and requirements.

PLEASE NOTE:
The information on the following pages is based on our understanding of the ATEX regulations and associated issues at the time of publication. Always check with qualified experts before acting on any of this. We will update these pages if appropriate new information or case experience comes to light.

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