BEKA BA368C, Руководство пользователя

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3.3.2 Inputs
When the BA368C 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
Ω
1000
Ω
Proximity
detector
1.2mA 2.1mA
Open
collector
2k
Ω
10k
Ω
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 BA368C 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 BA368C 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 BA368C is calibrated as a timer, optional
control outputs may be used. The intrinsic safety
features of these are described in section 1.9.3.2
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 BA368C 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 BA368C 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 instrument front panel,
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.