BEKA BA314NG, Manual

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4. SYSTEM DESIGN FOR GAS HAZARDOUS
AREAS.
When correctly installed in Zone 2 the BA314NG
Tachometer 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 BA314NG 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
BA314NG 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 BA314NG 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 BA314NG ATEX Type Examination
Certificate.
2. BA314NG terminals should only be connected
to circuits having safety parameters in normal
operation compliant with the BA314NG safety
parameters which are specified by the ATEX
Type Examination Certificate.
3. Hazardous area apparatus to which the
BA314NG 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 BA314NG.
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 BA314NG. See Fig 1.
4.1 Power supply
The BA314NG Tachometer 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 supply with a current limit
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 a
fuse with easily removable fuses which can be
extracted to achieve isolation as shown in Fig 2. If
an input current safety parameter Ii is specified, a
suitably rated fuse will ensure that it is not
continuously exceeded 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.
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