5
4.
SYSTEM DESIGN FOR HAZARDOUS
AREAS
4.1
4/20mA loop using Zener barriers
The required operating voltage depends on which
options are fitted as shown below.
Options Voltage range Terminals
BA405C
6.1 to 30V
1 & 3
Without
Options.
BA405C with
10 to 30V
1 & 13
Loop powered
(3 & 12
display
joined
backlight. together)
Fig 2 illustrates a typical application in which a
BA405C is being used to adjust the septpoint of a
speed controller. To enable the negative side of
both the load and the power supply to be earthed,
a Zener barrier is required in series with both wires
going into the hazardous area.
Any Zener barrier or barriers certified by an EEC
approved body to EEx ia IIC, having output
parameters equal to or less than the maximum
BA405C certified input safety parameters may be
used. For this type of application a 28V, 93mA
0.65W (28V 300
Ω
) device is normally used to
power the loop, plus a diode return barrier. These
two devices are available as two channels in one
package.
The maximum permitted cable parameters are
those specified on the Zener barrier certificate less
the set point stations equivalent input capacitance
or inductance Ci & Li.
When designing the loop it is necessary to
establish that the maximum voltage drop caused
by the manual set point station, both Zener
barriers, the load and cable resistance is always
less than the minimum supply voltage.
Fig 2 Typical application using barriers
In this example:
Minimum operating voltage of BA405C 6.1V
Maximum voltage drop caused by 28V 6.8V
93mA, 0.65W Zener barrier
(340
Ω
end-to- end resistance x 20mA)
Maximum voltage drop caused by
1.3V
diode return barrier
Maximum voltage drop caused by
5.0V
250ohm load
(250ohms x 20mA)
Maximum voltage drop caused by
0.2V
Cable resistance
(10ohms x 20mA)
______
Total maximum voltage drop
19.4V
The power supply voltage must therefore be above
19.4V, but below 25.5V which is the maximum
working voltage of a typical 28V Zener barrier. If
the optional loop powered backlight was included
in the circuit this introduces a further 3.9V drop so
that the supply voltage would have to be regulated
between 23.3 and 25.5 See section 4.3.2.
4.2
4/20mA loop using Galvanic Isolators
Galvanic isolators, although more expensive than
Zener barriers, do not require a high integrity earth
connection. For small systems where a high
integrity earth is not already available, the use of
galvanic isolators often reduces the overall
installation cost.
The example described in section 4.1 can be
simplified by the use of galvanic isolators as shown
in Fig 3.
Fig 3 Typical application using a galvanic isolator
