5
This allows all the models to be installed in all gas
Zones and to be used with most common industrial
gases except carbon disulphide and ethyl nitrite
which have an ignition temperature of 95°C.
3.3
4/20mA input
The input safety parameters for the 4/20mA input,
terminals 1 and 3 are:
Ui
=
30V dc
Ii
=
200mA
Pi
=
0.84W
The
maximum
equivalent
capacitance
and
inductance between the two 4/20mA input
terminals 1 and 3 is:
Ci
=
5.4nF
Li
=
0.016mH (0.02mH)
The maximum permitted loop cable parameters
can be calculated by adding these figures to Ci
and Li of other instruments in the loop and
subtracting the totals from the maximum cable
capacitance Co and cable inductance Lo permitted
for the Zener barrier or galvanic isolator powering
the loop.
Although the indicators do not themselves comply
with the requirements for
simple apparatus
, the
EC-Type Examination Certificate states that for
intrinsic
safety
considerations,
under
fault
conditions the output voltage, current and power at
terminals 1 & 3 will not exceed those specified by
clause 5.7 of EN 60079-11 for
simple apparatus
.
This simplifies the application and intrinsic safety
documentation for a loop into which an indicator is
connected.
3.4
Certification label information
The certification information label is fitted on the
top surface of the instrument assembly. It shows
the ATEX and IECEx certification information and
BEKA associates name and location.
Non
European certification information may also be
shown. The instrument serial number and date of
manufacture are recorded on a separate label
inside the instrument enclosure.
Typical certification label
4.
SYSTEM DESIGN FOR GAS
HAZARDOUS AREAS.
4.1
Transmitter loops
All the models 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. The indicators
are transparent to HART
®
signals.
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, defined by the Zener barrier or
galvanic isolator powering the loop, must be
reduced by 5.4nF and the maximum
permitted cable inductance by 0.02mH.
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 9.2.1
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
