8
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
Ω
1000
Ω
100Hz
5 & 6
Proximity
detector
1.2mA
2.1mA
5 kHz
5 & 6
Open
collector
2k
Ω
10k
Ω
5kHz
5 & 6
Magnetic
pick-off
0mV
20mV
peak
5kHz
3 & 4
Voltage
pulse
1.0V
3.0V
5kHz
3 & 4
4.2.3 Switch contact input
Any mechanically activated switch contact located
in the hazardous area may be directly connected
to terminals 5 & 6 providing it can withstand a
500V rms insulation test to earth. This includes
most magnetically activated reed relays used in
turbine flowmeters. The BA338C contains filtering
to prevent contact bounce being counted, this
limits the maximum operating frequency for a
switch contact to 100Hz.
4.2.4 2-wire proximity detector input
Any certified intrinsically safe 2-wire proximity
detector complying with NAMUR switching
thresholds may be used, providing the input safety
parameters are greater than the output safety
parameters of terminals 5 & 6. The system
certificates list some of the acceptable devices.
When programmed to operate with a proximity
detector, the BA338C maximum input frequency is
5kHz.
4.2.5 Voltage pulse input
Voltage pulse sources should be connected to
terminals 3 and 4. These terminals comply with
the requirements for simple apparatus and may be
directly connected to any certified intrinsically safe
voltage source within the hazardous area which
can withstand a 500V rms insulation test to earth
for one minute and has output parameters equal to
or less than:
Uo
=
28V dc
Io
=
100mA dc
Po
=
0.7W
This enables the BA338C to be connected directly
to most flowmeters incorporating a certified
intrinsically safe magnetic pick-off, or a certified
intrinsically safe amplifier producing a high level
pulse output. The BA338C has two programme
selectable voltage switching thresholds, ‘COIL’ for
magnetic pick-offs and ‘UOLtS’ for higher voltage
pulses.
Maximum voltage pulse counting frequency is
5kHz.
Fig 3 Pulse source in safe area
4.2.6 Remote reset
The BA338C total display is reset to zero when
terminals 7 & 8 are connected together for more
than one second. Permanent interconnection
inhibits totalisation. Remote 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.
No Zener barrier is required.
The BA338C may be reset to zero from the safe
area. Any switch may be used but a Zener
barrier is required to transfer the contact closure
into the hazardous area. Any one of the diode
return barriers specified on the system certificate
can be used for this application, it may be
combined with the supply barrier so that only one
package is required. Fig 2 illustrates how the
BA338C may be reset from both the safe and the
hazardous area.
Note:
The BA338C can be programmed to reset
the total display to zero when the Up and Down
push-buttons are operated simultaneously for more
than two seconds – see 6.12.