48
3.3.2 Inputs
When the BA364D is programmed as a timer
both input A and input B are operational and may
be independently programmed to operate with
almost any pulse output transducer.
The following table shows the switching
thresholds for the various transducers. For
reliable operation the input signal must fall below
the lower threshold and rise above the upper
threshold.
Input
Switching
thresholds
Switch
100
1000
Proximity
detector
1.2mA
2.1mA
Open
collector
2k
10k
Magnetic
pick-off
0mV
20mV
peak
Voltage
pulse
1.0V
3.0V
The intrinsic safety requirements are explained in
section 1.4.3 Almost any mechanically or
magnetically activated switch, intrinsically safe
NAMUR 2-wire proximity detector or intrinsically
safe voltage may be directly connected to these
two inputs.
If the BA364D is located in a hazardous area but
the transducer(s) 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.
3.3.3 Resetting the timer
When terminals 7 & 8 are connected together the
timer will be reset to zero if counting up and to
the set time (‘SEt t’) if counting down. 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 timer 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 may be combined with the supply barrier
so that only one dual channel device is required.
The system certificate lists some of devices that
may be used.
Fig 3.1 illustrates how the timer may be reset
from both the safe and the hazardous areas.
3.3.4 Options
When the BA364D is configured as a timer,
optional control outputs may be used. The
intrinsic safety features of these are described in
section 1.9.5.2 of this manual.
3.4 USE WITH GALVANIC ISOLATORS
Galvanic isolators are probably the easiest
intrinsically safe interface to use as they provide
isolation and do not require a high integrity earth
connection.
Any EEx ia IIC certified intrinsically safe galvanic
isolators may be used with the BA364D providing
the output parameters do not exceed the input
parameters of the terminals to which they are
connected.
To assist with system design, the galvanic
isolator system certificate lists some of the
isolators that may be used and defines the
maximum permitted cable parameters.
Fig 3.2 illustrates the basic circuit that is used for
all BA364D timer installations protected by
galvanic isolators. Intrinsic safety requirements
are explained in section one of this manual which
should be read before the system is designed.
If the timer is reset from the push-button behind
the control cover, or from an external switch in
the hazardous area, the reset galvanic isolator is
not required.
Fig 3.2 Basic timer circuit using isolators
In Fig 3.2 the timer has one optional control
output switching an intrinsically safe solenoid
valve. The inputs are shown in the hazardous
area, but each may be transferred from the safe
area via a galvanic isolator or intrinsically safe
relay.