13. (This usually taken mean that the
gas–air mixture will present for 1000h per year.6
Zener safety barrier.Basic Electrical Installation Work
272
● Zone area which explosive gas–air mixture likely to
occur normal operation.
13. However, many chemical and oil-processing plants
are entirely dependent upon instrumentation and data transmission for
their safe operation.)
If area not classified zone then deemed non-
hazardous, that normal industrial electrical equipment may used.4, and marked Ex‘d’ indicate flameproof enclosure. Therefore, very low-power instrumentation and data-
transmission circuits can used special circumstances, but the equip-
ment must intrinsically safe, and used conjunction with ‘safety
barrier’ installed outside the hazardous area. (This usually taken mean that the gas–air mix-
ture will present for less than 10h per year.)
● Zone area which explosive gas–air mixture not likely
to occur normal operation and does occur will exist for a
very short time. Intrinsically safe equipment
must marked Ex‘ia’ Ex’s’, specially certified for use zone 0.
Ordinary electrical equipment cannot installed Zone even when it
is flameproof protected.
Definition
An intrinsically safe circuit one in
which spark thermal effect is
capable causing ignition given
explosive atmosphere.
The intrinsic safety the equipment hazardous area assured by
incorporating Zener diode safety barrier into the control circuit such
as that shown Fig. normal operation, the voltage across a
Fuse R2
Z1 hazardous area
Supply
in
safe area
FIGURE 13.6.
In Zone all electrical equipment must flameproof, shown Fig.
. Electrical equipment this
area should marked Ex‘o’ for oil-immersed Ex‘p’ for powder-filled. For example, normal sockets and switches cannot installed in
a zone area, but oil-filled radiators may installed they are directly con-
nected and controlled from outside the area.
The electrical equipment used zone will contain minimum amount of
protection.
Intrinsic safety
By definition, intrinsically safe circuit one which spark ther-
mal effect capable causing ignition given explosive atmosphere
