The basics of dust-explosion protection - STAHL - #25

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In the future, manufacturers will have the option, during the type testing, of determining a surface temperature T_L under a deposit of thickness L (orientated toward the thickness of the layer during actual use), which may be stated on labels (Figure 8). Section 5.2 of IEC 61241-0 states: "In addition to the maximum surface temperature T_L required in 5.1, the maximum surface temperature may be stated for a given depth of layer, T_L, of dust surrounding all sides of the apparatus, unless otherwise specified in the documentation, and marked according to 29.2(8)". For the determination of the temperature T_L, 23.4.5.2 requires: "...the electrical apparatus to be tested shall be mounted and surrounded by a layer depth "L" as stated by the manufacturers specification. The measurement for the maximum surface temperature shall be made .using a dust having a thermal con­ductivity of no more than 0.003 kcal/m °C h." The user must take note of Clause 6.3.3.4 of the installation standard 61241-14: "Where the apparatus is marked TL for a layer depth the ignition temperature of the combustible dust, at a layer depth L, must be applied in place of T_5mm."

The following verification from 1 and 2 must be made by the user!

The maximum permissible surface temperature, in relation to the ignition temperature of a dust cloud T_ic and/or the ignition temperature of a dust layer T_il of the surrounding dust is determined as follows:

1. Maximum permissible surface temperature in case of dust clouds T_max = 2/3 T_ic

2. Maximum permissible surface temperature in case of dust layers (5 mm thickness)

^Tmax = ^Til ^{- 75 K}

Example: flour

a 380°C and T_N a 300°C Tmax(i) = 2/3 x 380°C = 253°C Tmax(2) = 300°C - 75K = 225°C

Accordingly, the surface temperature of the electrical equipment in this particular case must not exceed the value of 225°C; this must be guaranteed by the manufacturer (see "Determi­nation of the Maximum Surface Temperature" above).

Figure 9: Explosion Protected Luminaire 6600 Series for Use in Zone 21 (Category 2D) and Zone 22 (Category 3D).

R.STAHL Explosion protection