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4
.3 USE WITH ZENER BARRIERS
Zener barriers are the least expensive
intrinsically safe interface between a safe and
hazardous area. However, they do not provide
isolation and require a high integrity earth
connection that may be expensive to install. For
a single BA364D totaliser it may be less
expensive and complicated to use galvanic
isolators when a high integrity earth connection is
not already available.
Any certified Zener barriers may be used with the
BA364D providing their output parameters do not
exceed the input parameters of the terminals to
which they are connected. Only one polarity of
Zener barrier i.e. positive or negative, may be
used with each instrument.
To assist with system design, the Zener barrier
system certificate specifies the maximum output
parameters of the Zener barriers that may be
connected to the BA364D and defines the
maximum cable parameters. Diode return
barriers are identified by manufacturers type
number, although alternatives may be used.
The basic system used for all BA364D hazardous
area clock installations is shown in Fig 4.1
Intrinsic safety requirements are explained in
section one of this manual which should be read
before the system is designed.
Fig 4.1 Tachometer installation using barriers
4.3.1 Power supply
A BA364D is powered from the safe area via a
positive polarity 28V 300
Zener barrier which
will have an end-to-end resistance of about
340
. When operating as a tachometer the
BA364D consumes approximately 15mA and
requires a minimum voltage of 10V between
terminals 1 and 2. The supply voltage in the
safe area must therefore be between 16V and
the maximum working voltage of the Zener
barrier – usually about 26V.
4.3.2 Input
When the BA364D is programmed as a
tachometer only input A is operational. This input
may be programmed to operate with almost any
pulse output transducer.
The following table shows the switching
thresholds for the various transducers, plus the
maximum operating frequency and the input
terminal numbers. For reliable counting the
input signal must fall below the lower threshold
and rise above the upper threshold.
Switching
thresholds
Freq
max
Input
terminals
Switch
100
1000
100Hz
3 & 4
Proximity
detector
1.2mA
2.1mA
5kHz
3 & 4
Open
collector
2k
10k
5kHz
3 & 4
Magnetic
pick-off
0mV
20mV
peak
5kHz
3 & 4
Voltage
pulse
1.0V
3.0V
5kHz
3 & 4
The intrinsic safety requirements are explained in
section 1.4.3 Almost any mechanically or
magnetically activated switch, or intrinsically safe
NAMUR 2-wire proximity detector may be directly
connected to the input terminals.
If the BA364D is located in a hazardous area but
the transducer is in a safe area, a Zener barrier is
required to transfer the transducer output to the
instrument. The system certificate defines
barriers which may be used.
4.3.3 Resetting the run-time display
When terminals 7 & 8 are connected together the
run-time display will be reset to zero. To achieve
reliable resetting the contacts must close for at
least one second.
Resetting may be accomplished by any
mechanically operated switch located in the
hazardous area providing it can withstand a 500V
rms insulation test to earth for one minute. No
Zener barrier is required.
The BA364D may also be reset from within the
safe area. Any switch may be used but a Zener
barrier is required to transfer the contact closure
into the hazardous area. A diode return barrier
combined with the supply barrier so that only one
dual channel device is required is an economic
solution. The system certificate lists some of
devices that may be used.
Fig 4.1 illustrates how the BA364D tachometer
run-time display may be reset from both the safe
and the hazardous area.
