5
3.3 Pulse input terminals
The BA338C rate totaliser has two alternative pairs
of input terminals enabling the instrument to count
pulses from a wide variety of sources. Note:
Only one pair of input terminals may be used at
one time.
3.3.1 Voltage pulse inputs
Terminals 3 and 4 comply with the requirements
for simple apparatus. In Europe, sources of
energy which do not generate more than 1.5V;
100mA and 25mW are, for intrinsic safety
purposes, considered to be simple apparatus
(Clause 5.4 of EN50 020:1994).
Although the BA338C indicator does not itself
comply with the requirements for simple apparatus,
the EC-Type Examination Certificate specifies that
under fault conditions the voltage, current and
power at terminals 3 & 4 will not exceed those
specified for simple apparatus in Clause 5.4 of
EN50020:1994. This allows these input terminals
to be connected to any certified intrinsically safe
apparatus or circuit providing that the output
parameters of the apparatus or circuit do not
exceed:
Uo = 28V dc
Io = 100mA dc
Po = 0.7W
Therefore the certified intrinsically safe voltage
pulse output of a flowmeter, or the output of a
certified magnetic pick-off may be connected
directly to these terminals providing:
The output parameters of the device do
not exceed the figures shown above.
The device can withstand a 500V rms
insulation test to earth.
The BA338C EC-Type Examination Certificate
specifies that the maximum equivalent capacitance
and inductance between the two pulse input
terminals 3 and 4 is:
Ci = 20nF
Li = 20µH
To determine the maximum permissible cable
parameters these figures must be subtracted from
the maximum permitted cable parameters
specified for the Zener barrier or galvanic isolator
connected to terminals 3 and 4. The two system
certificates specify maximum permitted cable
parameters when 28V 300Ω devices are used
3.3.2 Contact, 2-wire proximity detector
or open collector input
Terminals 5 and 6 are intended for connection to a
switch contact, a certified open collector output or
a certified intrinsically safe 2-wire NAMUR
proximity detector. The output safety parameters
are:
Uo = 10.5V dc
Io = 9.2mA dc
Po = 24mW
and the maximum permitted external capacitance
and inductance is:
Co = 2.0µF
Lo = 248mH
Mechanically operated switch contacts comply with
the requirements for simple apparatus. Providing
the switch and the BA338C are both located in the
same hazardous area, the switch may be
connected directly to terminals 5 & 6. This also
applies to most magnetically operated reed
switches used in turbine flowmeters.
Similarly, certified intrinsically safe open collector
outputs may be directly connected to terminals 5
and 6
A certified intrinsically safe 2-wire proximity
detector may also be connected directly to
terminals 5 & 6 providing the input safety
parameters of the detector are less than the output
parameters of terminals 5 & 6. For guidance the
BA338C system certificates specify some of the
intrinsically safe 2-wire proximity detectors that
may be used.
In all cases the device and circuit connected to
terminals 5 & 6 must be capable of withstanding a
500V rms insulation test to earth for one minute.
The maximum input safety parameters for
terminals 5 & 6 are:
Ui = 28V dc
Ii = 100mA dc
Pi = 0.7W
and the internal capacitance and inductance is:
Ci = 20nF
Li = 20µH
which allows Zener barriers and galvanic isolators
to be connected to these terminals. The BA338C
system certificates specify the maximum permitted
cable parameters when a 28V 300Ω Zener barrier
or galvanic isolator is used to transfer a pulse from
the safe area to the BA338C in a hazardous area.
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