Page 225 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 89

Table 8.3 Test Requirements for resistance to
impact

Mechanical details Impact energy
in Joules
Risk of mechanical danger High Low

1 Guards, protective covers, 7 4
fanhoods, cable entries
2 Plastics enclosures 7 4
3 Light metal or cast metal 7 4
enclosures
4 Enclosures of other 7 4
materials than 3 with
wall thickness of less
than 1 mm
5 Light transmitting parts 4
without guard
6 Light transmitting parts 2
with guard (tested
without guard)

(from BSEN 500241)
Note: Impact test is normally with a 1 kg mass with a hardened
steel 25cm ball at its impact point. The height for dropping
purposes is one tenth of the required impact energy in metres.

and apparatus for use in the presence of conducting dusts is likewise almost
always required to be IP65.
As far as residual charge or temperature after isolation is concerned, the
only way to overcome this problem if it cannot be avoided by design is to
identify the time taken for decay, and label the apparatus with a warning
indicating a delay which should be observed before opening. For residual
charge, figures are given as follows:

Group IIA - The residual charge must not exceed 0.2mJ when the enclo-
sure is opened;
Group IIB - The residual charge must not exceed 0.06mJ when the
enclosure is opened;
Group IIC - The residual charge must not exceed 0.02mJ when the enclo-
sure is opened.

These figures allow the calculation of the necessary time to be included on
the warning label.
Where the charge cannot be dissipated in a sensible time, or the temper-
ature likewise is retained for a very long time, an alternative approach is to