BEKA BA368C, Manual

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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 BA368C 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
BA368C 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 BA368C 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 BA368C 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 BA368C is powered from the safe area via a
positive polarity 28V 300
Ω
Zener barrier which will
have an end-to-end resistance of about 340
Ω
.
When operating as a clock the BA368C 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 BA368C 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 the BA368C clock 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 BA368C 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 and 5.2 illustrate how the BA368C clock
may be synchronised from both the safe and the
hazardous area.
5.3.3 Optional control outputs
The BA368C 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.