BEKA BA307E-SS, Руководство пользователя

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4. SYSTEM DESIGN FOR GAS HAZARDOUS
AREAS
4.1 Transmitter loops
Both indicator may be connected in series with almost
any intrinsically safe 4/20mA current loop and
calibrated to display the measured variable or control
signal in engineering units. There are three basic
design requirements:
1. The intrinsic safety output parameters of the
4/20mA loop, which are defined by the Zener
barrier or galvanic isolator powering the loop,
must be equal to or less than:
Uo = 30V dc
Io = 200mA
Po = 0.84W
2. The maximum permitted cable capacitance of
the loop must be reduced by 13nF. The
maximum permitted cable inductance is not
reduced by the inclusion of an indicator.
3. The loop must be able to tolerate the additional
1.2V required to operate the indicator. When
fitted with an optional backlight this increases to
5.0V if the backlight is loop powered. See 9.4.1
Figs 2a and 2b illustrate typical applications in which
an indicator is connected in series with a 2-wire
transmitter powered by a Zener barrier and
alternatively by a galvanic isolator.
Fig 2a Loop powered by a Zener barrier
Fig 2b Loop powered by a galvanic isolator
4.2 Remote indication
The indicators may be driven via an intrinsically
safe interface from a 4/20mA safe area signal to
provide a remote display within a hazardous area.
The type of intrinsically safe interface is not critical,
either a Zener barrier or a galvanic isolator may be
used, providing that Ui, Ii and Pi of the indicator are
not exceeded and the voltage capability of the
4/20mA signal is sufficient to drive the indicator
plus the interface.
When a high integrity earth connection is already
available, a Zener barrier is usually the least
expensive option. If an earth connection is not
available or isolation is required, a galvanic isolator
is the correct choice.
If one side of the 4/20mA current loop may be
earthed, a single channel Zener barrier provides
the lowest cost protection. If the 4/20mA signal is
not isolated, then two Zener barriers, a two
channel Zener barrier or a galvanic isolator must
be used.
Fig 3 shows the alternative circuits which may be
used.
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