BEKA BA378C, Руководство пользователя, Страница: 7 / 27

3.6 4/20mA output - terminals 5 & 6
The BA378C transmitter is powered via these
terminals, and the current drawn is the 4/20mA
output signal. The transmitter is normally powered
from the safe area so a Zener barrier or galvanic
isolator is required to protect the circuit. Any single
channel Zener barrier or galvanic isolator certified
EEx ia IIC by an EEC approved body with output
safety parameters equal to, or less than, those
shown below may be used.
Uo = 30V
Io = 280mA dc
Po = 0.85W
4. ELECTRICAL SYSTEM DESIGN FOR
INSTALLATION IN HAZARDOUS AREAS
4.1 Using Zener barriers
There are three basic requirements when designing
a loop incorporating the BA378C:
1. The intrinsic safety parameters of the Zener
barrier must be equal to or less than:
Uo = 30V
Io = 280mA
Po = 0.85W
2. The voltage between terminals 5 & 6 of the
BA378C must be between 10 and 30V.
3. The maximum supply voltage must not
exceed the maximum working voltage of the
Zener barrier.
Fig 2 illustrates the simplest configuration in which a
BA378C is powered from an isolated (floating)
power supply. Only one barrier is required as the
other wire is earthed at the barrier busbar.
Fig 2 BA378C powered from floating supply
If a common power supply is used to operate more
than one loop the negative side of the supply is
normally connected to earth. To enable the
negative side of the 4/20mA load also to be
earthed it is necessary to have a Zener barrier in
each wire going into the hazardous area as shown
in Fig 3. Any certified diode return barrier may be
used in the return wire, for guidance System
Certificate Ex96D2505 lists some of these devices.
Fig 3 BA378C powered from a common supply
When designing any loop it is necessary to
establish that the sum of the voltage drops caused
by the BA378C transmitter, both Zener barriers, the
load and the cable resistance is less than the
minimum supply voltage.
For the transmitter loop shown in Fig 3:
Minimum operating voltage of BA378C 10.0V
Maximum voltage drop caused by 28V 6.8V
300ohm Zener barrier
(340ohms x 20mA)
Maximum voltage drop caused by 1.3V
diode return barrier
Maximum voltage drop caused by
250ohm load
5.0V
(250ohms x 20mA)
Maximum voltage drop caused by cable 0.2V
resistance (10ohms x 20mA)
_____
Total maximum voltage drop 23.3V
The power supply must therefore be above 23.3V,
but below the maximum working voltage of the 28V
300ohm Zener barrier which is typically 25.5V.
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