The basics of dust-explosion protection - STAHL - #16

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6. Types of Dust Explosion Protection

The goal of explosion protection is to prevent excessive temperatures and energies in the form of sparks, arcs and so forth in an equipment.

Currently four types of dust ignition protection exist (see Table 9):

6.1 Type of Protection "tD"

In Europe, this is generally regarded to be the most important method for power engineering equipment.

EN 50281-1-1 defines the type of protection for electrical equipment: "Protection by Enclosure." Two degrees of dust protection are defined for dust explosion protection:

1. Dust-tight: for the use of equipment in Zone 20, Zone 21 areas and even in Zone 22 areas, in the case of the presence of conductive dust.

2. Dust-protected: for the use of equipment in Zone 22 areas, in the case of the presence of non-conductive dust.

The type of protection "tD" requires the limitation of the surface temperature of the enclosure and the limitation of dust ingress into the enclosure (dust-tight and dust-protected enclosures):

> Dust-tight enclosure An enclosure which prevents the ingress of all observable dust particles (IP6X). This means that a safe area is established inside the enclosure. Please note that as com­bustible dust particles are approx. 20 to 40 urn in size and thus below the gap of flameproof joints required by EN 50018 for flameproof enclosures. Flameproof enclosures are not in themselves dust-tight and must be separately tested and certified according to this condition.

> Dust-protected enclosure

An enclosure which the ingress of dust is not totally prevented, but does not enter to interfere with the safe sufficient quantities to operation of the equipment. Dust shall not accumulate in a position within the enclosure where it is liable to cause an ignition hazard (IP5X).

The material used for the enclosure is central significant. It must be subjected to specific material tests. Despite the ageing process under­gone by the material and the expected mechanical stresses, the enclosure must ensure the required dust protection. Possible materials are:

> Metals (such as coated steel, stainless steel, light metal)

> Glass (for enclosure parts, eg. inspection windows)

> Moulded plastic

Metals used for this purpose may have to be sub­jected to an impact test at low temperatures, as some metals (light metals) have less favourable mechanical stress at low temperatures than at higher ones.

In addition, light metal may contain a maximum of 6 % magnesium, as it otherwise tends to form sparks upon impact with materials such as rusty iron. Glass must withstand a thermal shocktest without cracks or without such extensive damage that it breaks during a subsequent impact test.

R.STAHL Explosion protection