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5.3 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 clock it may be less expensive
and easier to use galvanic isolators especially
when a high integrity earth connection is not
already available.
Any certified Zener barriers may be used with the
BA364D providing their 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 manufacturers type
number, although alternatives may be used.
The basic system used for all BA364D hazardous
area clock installations is shown in Fig 5.1
Intrinsic safety requirements are explained in
section one of this manual which should be read
before the system is designed.
Fig 5.1 Clock installation using barriers
If clock synchronisation is not required and the
optional control outputs are not fitted, only a
power supply Zener barrier is required.
5.3.1 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 clock the BA364D
consumes approximately 15mA and requires a
minimum voltage of 10V between terminals 1 and
2. The supply voltage in the safe area must
therefore be between 16V and the maximum
working voltage of the Zener barrier – usually
about 26V.
5.3.2 Synchronisation
When terminals 7 & 8 are connected together
the time displayed by the BA364D will be set to
the preprogrammed reset time. The clock
function will be inhibited until the contacts are
opened when the clock will restart. To achieve
reliable resetting the contacts must close for at
least one second. This function enables one or
more BA364D clocks to be synchronised with a
master clock once every 24 hours.
Resetting may be accomplished by any
mechanically operated switch located in the
hazardous area providing it can withstand a 500V
rms insulation test to earth for one minute. No
Zener barrier is required.
The BA364D may also be reset from within the
safe area. Any switch may be used but a Zener
barrier is required to transfer the contact closure
into the hazardous area. A diode return barrier
may be used which can be combined with the
supply barrier so that only one dual channel
device is required. The system certificate lists
some of devices that may be used.
Figs 5.1 illustrates how the BA364D clock may be
synchronised from both the safe and the
hazardous area.
5.3.3
Optional control outputs
The BA364D can be supplied with two isolated
solid state control outputs. Each output may be
programmed to turn on and off twice during a
twenty four hour period. These outputs may be
used to switch certified intrinsically safe devices
within the hazardous area, or the contacts may
be transferred to the safe area via a Zener
barrier.