BEKA BA458C, Manual

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8
4. SYSTEM DESIGN FOR HAZARDOUS
AREAS
4.1 System Certificates
In addition to the EC-Examination Certificate,
ITS have issued two System Certificates for
the BA458C batch controller. One defines
how the BA458 may be used with Zener
barriers and the other with galvanic isolators.
Both are divided into two sections to cover
input signal sources in the hazardous and the
safe area.
Interface Certificate No.
Zener barriers Ex03E21380
Galvanic isolators Ex03E21381
System Certificates are Certificates of
Conformity which are primarily issued for
installation guidance in the UK. They do not
form part of the certification to the ATEX
Directive.
4.2 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 BA458C batch controller
installation it may be less expensive and
complicated to use galvanic isolators when a
high integrity earth connection is not already
available.
Any Zener barrier certified EEx ia IIC by an
EEC approved certification body may be used,
providing that the safety parameters are within
the limits specified by the BA458C EC-
Examination Certificate. Only one polarity of
Zener barrier, i.e. positive or negative, may be
used with each BA458C batch controller
installation.
To assist with system design, the Zener barrier
System Certificate specifies the safety
description of barriers that may be used and
lists the maximum permitted cable parameters.
Fig 3 illustrates the basic circuit that is used for
all BA458C installations protected by Zener
barriers. Although the BA458C batch
controller will operate from a wide range of
input sources, only one source may be
connected at any time.
Alternatively the input source may be located
in the safe area. Fig 4 shows how an
additional Zener barrier may be used to
transfer the signal from the safe area to the
batch controller in the hazardous area. For
switch contact and open collector inputs the
additional end-to-end resistance of the Zener
barrier must not prevent the input at the batch
controller terminals passing through the batch
controllers input switching thresholds.
Fig 3 Batch controller powered via a Zener
barrier with input in hazardous area.
Fig 4 Batch controller protected by galvanic
isolators with Input in safe area