BEKA BA334NG, Руководство пользователя

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4. SYSTEM DESIGN FOR GAS HAZARDOUS
AREAS.
When correctly installed in Zone 2 the BA334NG
Rate Totaliser may be connected to almost any
apparatus in the safe area and to Ex n, Ex e, Ex p
and Ex d protected apparatus located in Zone 2.
Because the BA334NG is not certified intrinsically
safe it should not be connected to an intrinsically
safe system.
BEKA Application Guide AG310, Guide for
Installation of [extra low voltage d.c.] Ex nA
instrumentation
, which can be downloaded from
www.beka.co.uk, contains explanations and
recommendations for the installation of Ex nA
equipment.
In addition to being able to be connected to other
equipment in the safe area and in Zone 2, the
BA334NG may also be connected to suitably
protected and certified equipment located in Zone 1.
This is illustrated in Fig 5 and explained in
Application Guide AG310.
There are four design requirements:
1. The BA334NG should be powered from a
circuit that has output safety parameters in
normal operation equal to, or less than, the
input safety parameters for terminals 1
and 2 specified by the BA334NG ATEX Type
Examination Certificate.
2. BA334NG terminals should only be
connected to circuits having safety
parameters in normal operation compliant
with the BA334NG safety parameters
which are specified by the ATEX Type
Examination Certificate.
3. Hazardous area apparatus to which the
BA334NG is connected should be protected
by a technique suitable for the Zone in
which the equipment is located such as Ex n
or Ex e if located in Zone 2. Equipment
protected by intrinsic safety should not be
connected to a BA334NG.
4. Wiring should comply with Clause 9 of
EN 60079-14.
When designing a system it is important to
remember that terminals 2, 6 and RS2 are
interconnected within the BA334NG. See Fig 1.
4.1 Power supply
The BA334NG Rate Totaliser requires a minimum
of 10V between terminal 1 & 2 and consumes:
10mA without optional backlight
plus 6mA when terminals 3 & 4 are linked
plus 16mA with optional backlight
A 24V dc regulated, current limited supply located
in a safe area is suitable.
The power supply should meet the requirements
for personnel safety so that ‘live maintenance’ can
safely be performed. The implicit requirement for
galvanic isolation from the mains supply ensures
that the possible difficulties from circulating earth
currents caused by mains faults is minimised. In
European terms if the power supply is CE marked
it is almost certainly acceptable.
To comply with the requirements of EN 60079:14
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 may
be satisfied by using DIN rail mounted terminals
incorporating easily removable fuses which can be
extracted to achieve isolation as shown in Fig 2.
The fuse also limits the current to comply with the
instruments Ii safety parameter in normal
operation.
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.
Fig 2 DIN rail mounting terminals incorporating a
fuse.
For some applications Ex nA instrumentation
energised by a current limited power supply or
instrument that can be switched off, is considered
adequate and to comply with the requirements of
the standard.