BEKA BA334D, Руководство пользователя

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4.3 Use with Galvanic Isolators
Galvanic isolators are probably the simplest
intrinsically safe interface as they provide
isolation and do not require a high integrity
earth connection.
Any certified galvanic isolator with output
parameters less than the input parameters of
the BA334D may be used. The BA334D
system certificates list some of the suitable
devices together with the maximum permitted
cable parameters
Fig 4 BA334D used with galvanic isolators
Fig 4 illustrates the basic circuit that is used for
all BA334D installations protected by galvanic
isolators. For simplicity, connections for the
optional pulse output, 4/20mA output and
backlight are included in the appendix to this
manual.
Alternatively the pulse source may be located
in the safe area. Fig 5 shows how an
additional galvanic isolator is used to transfer
the signal to the rate totaliser in the hazardous
area.
Fig 5 Pulse source in safe area
4.3.1 Power supply
The BA334D galvanic isolator system
certificates list a wide range of certified
galvanic isolators that may be used to power
the rate totaliser. If using the entity concept to
select an isolator, in addition to matching the
safety parameters the isolator must be capable
of supplying at least 25mA at 10Vdc to the
BA334D.
4.3.2 Pulse input
As shown in Fig 4 the BA334D will count
pulses from a wide variety of sources.
Terminals 5 and 6 are for sources that need
powering e.g. a switch contact or a 2-wire
NAMUR proximity detector. Terminals 3 and 4
are for voltage inputs.
Note: Only one input may be used at a time,
i.e pulses can not be counted at terminals 3 &
4 and at 5 & 6 at the same time. No galvanic
isolator is required in series with the input if the
pulse source is located within the hazardous
area.
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.