BEKA BA307NE, Руководство пользователя

Страница: 6 / 31

6
4. SYSTEM DESIGN FOR USE IN ZONE 2 GAS
HAZARDOUS AREAS.
4.1 Transmitter loops
When correctly mounted in Zone 2 both indicators
may be connected in series with almost any
4/20mA current loop with apparatus in the safe
area, or with Ex n, Ex e, Ex p or Ex d protected
apparatus located in Zones 1 or 2. The indicators
are transparent to HART
® signals.
Because the BA307NE and BA327NE are not
certified intrinsically safe they should not be
connected to an intrinsically safe system.
There are four design requirements:
1. The indicator must be installed in a panel
enclosure complying with the requirements
for Ex n protection as shown in section 5 of
this manual.
2. The certificate specifies that the indicator
should be connected to a limited energy
4/20mA circuit having a maximum output
current of 200mA. A
low voltage supply,
usually 24V, that is safe in normal operation
and suitable for live connection i.e. CE
marked, is usually considered acceptable.
3. Wiring must comply with Clause 9 of
BS EN 60079-14.
4. The loop must be able to tolerate the
additional 1.2V required to operate the
indicator. This increases to 5.0V if the
indicator is fitted with an optional backlight
which is loop powered. See 9.5.1
Figs 2 illustrate a typical application in which a
BA307NE or BA327NE located in Zone 2 is
connected in series with a 2-wire Ex d transmitter
located in Zone 1. BEKA Application Guide
AG310, which can be downloaded from
www.beka.co.uk, contains examples of other Ex n
applications.
Fig 2 Typical Zone 2 transmitter loop
To comply with the requirements of
BS EN 60079:
14:2008 Electrical installations
design, selection and erection , each of the wires
entering the hazardous area should be individually
fused and contain a means of isolation. These two
requirements are frequently satisfied by using
switch fuse holders with easily removable fuses
which can be extracted to achieve isolation as
shown in Fig 20. This is a satisfactory method at
the low voltages and currents common in
instrumentation systems. Clear identification of,
and easy access to the means of isolation is
essential for their effective use. It is also necessary
to ensure that the maintenance procedure makes
sure that unauthorised re-closure of the switches
does not occur. It is not considered necessary to
have a means of isolation or electrical protection
for the screen.
For some applications Ex nA instrumentation
energised by a current limited power supply or
instrument that can be switched off, is often
considered adequate and to comply with the
requirements of the standard.
4.2 Remote indication
The BA307NE and the BA327NE may also be
driven directly from a safe area instrument with a
4/20mA output to provide a remote display within a
Zone 2 hazardous area.
There are four design requirements:
1. The indicator must be installed in a panel
enclosure complying with the requirements
for Ex n protection as shown in section 5 of
this manual.
2. The certificates specify that the indicator
should be connected to a
limited energy
4/20mA circuit having a maximum output
current of 200mA. An instrument located in
the safe area that is safe in normal operation
and has a 4/20mA output suitable for live
connection i.e. CE marked, is usually
considered acceptable.
3. Wiring must comply with Clause 9 of
BS EN 60079-14:2008.
4. The output from the safe area 4/20mA
instrument must be able to supply the 1.2V
required to operate the indicator. This
increases to 5.0V if the indicator includes an
optional backlight which is loop powered.
See 9.4.1
Fig 3 shows a typical application.