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3.2 SYSTEM CERTIFICATES
In addition to the ATEX EC-Type Examination
Certificate, ITS have issued two system
certificates. One defines how the BA364D may
be used with Zener barriers and the other with
galvanic isolators.
Interface Certificate No
Zener barriers
Ex01E2007
Galvanic isolators
Ex01E2008
This section interprets these system certificates
and illustrates how to design timer systems with
Zener barriers and galvanic isolators.
System certificates are Certificates of Conformity
which are primarily issued for guidance in the UK.
They do not form part of certification to the
European ATEX Directive.
3.3 USE WITH ZENER BARRIERS
Zener barriers are the least expensive
intrinsically safe interface between a safe and
hazardous area. However, they do not provide
isolation and require a high integrity earth
connection that may be expensive to install. For
a single BA364D timer it may be less expensive
and complicated to use galvanic isolators when a
high integrity earth connection is not already
available.
Any certified Zener barrier may be used with the
BA364D providing its output parameters do not
exceed the input parameters of the terminals to
which they are connected. Only one polarity of
Zener barrier i.e. positive or negative, may be
used with each instrument.
To assist with system design, the Zener barrier
system certificate specifies the maximum output
parameters of the Zener barriers that may be
connected to the BA364D and defines the
maximum cable parameters. Diode return
barriers are identified by manufacturer’s type
number, although alternatives may be used.
The basic system used for all BA364D hazardous
area timer installations is shown in Fig 3.1
Intrinsic safety requirements are explained in
section one of this manual which should be read
before the system is designed.
Fig 3.1 Timer installation using barriers
If the timer is reset from the push-button behind
the control cover, or from an external switch in
the hazardous area, a reset Zener barrier is not
required.
In Fig 3.1 the timer has one optional output
switching an intrinsically safe solenoid valve.
Inputs A and B are in the hazardous area but
each may be transferred from the safe area via a
Zener barrier.
3.3.1 Power supply
A BA364D is powered from the safe area via a
positive polarity 28V 300
Zener barrier, which
will have an end-to-end resistance of about
340
. When operating as a timer with two
proximity detector inputs, the BA364D consumes
approximately 20mA and requires a minimum
voltage of 10V between terminals 1 and 2. The
supply voltage in the safe area must therefore be
between 16.8V and the maximum working
voltage of the Zener barrier – usually about 26V.
