The basics of dust-explosion protection - STAHL - #17

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material to become brittle which would eliminate the required IP protection.

For electrical equipment of Category 3D, it is sufficient for the material to have a TI at least 10K higher than the temperature at the hottest point of the enclosure. Proof of a continuous operating temperature (COT) which fulfils the same requirement as the TI is also sufficient. No thermal endurance test is carried out in this case.

6.1.1 Enclosures of Moulded Materials

Thermal Endurance

Plastic materials must certainly fulfil the most complex requirements. For electrical equipments of Categories 1D and 2D, the temperature index "TI" must be known, according to EN 50281. This figure allows conclusions about the long-term mechanical performance of moulded materials to be drawn. The temperature index corresponds to the 20,000-h point on the thermal endurance graph without loss of flexing strenght or tenside strenght excluding 50%. This figure must be 20 K higher than the temperature at the hottest point of the enclosure. In addition, the moulded material must be proven to have sufficient thermal resistance for the intended application.

Enclosures or parts of enclosures made of moulded materials for electrical equipment from the Categories 1D and 2D must be subjected to thermal endurance tests according to EN 50014 (artificial ageing). The ageing process caused by extreme temperatures must not cause the moulded

Table 9: Types of Protection for Use in the Presence of Combustible Dust in the Current Standards

Symbol

Principle

Type of Protection

Current Status at IEC

Future Status at IEC

Status at CLC

pD

Pressurization

IEC 61241-4 (2001-03)

IEC 61241-2

iD

Intrinsic safety

31H/171/CDV

IEC 61241-11

mD

Encapsulation (moulded compound)

31H/153/CDV

IEC 61241-18

R.STAHL Explosion protection