VEGA Z728 Zener barriers - Manual

1
Zener barriers - operating instructions
Operating principle
Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany
Pepperl+Fuchs Group • Tel.: Germany +49 621 776-0 • USA +1 330 4253555 • Singapore +65 67799091 • Internet http://www.pepperl-fuchs.com
Da
te of issu
e
05/2
3
/03
1 Zener barriers - operating instructions
Application
• Zener barriers are used in control and instrumentation
systems for the processing of standardised signals, such as
20 mA and 10 V. Zener barriers contain intrinsically safe
circuits that are used to drive intrinsically safe field devices
within hazardous areas. The manufacturers data sheets
must be consulted.
• The relevant regulations and directives governing the
intended application must be observed.
Installation, commissioning
• Zener barriers are constructed to a protection classification
of IP20 and accordingly must be appropriately protected
from adverse conditions such as splashing water and soiling
in excess of pollution severity 2.
• Zener barriers must be installed outside the hazardous area!
Only those circuits identified as intrinsically safe may be
located within the hazardous area.
• When intrinsically safe field devices are interconnected with
the intrinsically safe circuits of the related Zener barriers, the
respective highest values (safety parameters) for the field
devices and the Zener barriers - in the sense of explosion
protection - must be observed (demonstration of intrinsic
safety). The EU certificate of conformity or EU prototype test
certificate must be followed. Particular importance is
attached to maintaining the "Special conditions" contained
in these certificates.
• When intrinsically safe circuits are employed in an explosive
dust atmosphere (zones 20 and 21), only appropriately
certificated field devices are permitted to be incorporated.
Installation and commissioning within zone 2
• The devices must be installed in switch boxes or distributor
boxes to protection category IP54 or better.
• The devices may be installed within zone 2. Only those
circuits identified as intrinsically safe are permitted to be
installed in zone 1 or zone 0 and in accordance with their
ignition protection category approval. The actual installation
of the intrinsically safe circuits is to be carried out in
accordance with the applicable installation regulations.
• When interconnecting intrinsically safe field devices with the
intrinsically safe circuits of the associated Zener barriers,
the respective highest values (safety parameters) of the field
device and the associated device, in the sense of explosion
protection, must be taken into account (demonstration of
intrinsic safety). The conditions stated on the EU certificates
of conformity or EU prototype test certificates must be
observed.
• In addition, for operation within zone 2, the statements of
conformity of the certifying authorities/declarations of
conformity of the manufacturer must be observed. Particular
importance is attached to maintaining the "Special
conditions" contained in these certificates.
• When intrinsically safe circuits are employed in an explosive
dust atmosphere (zones 20 and 21), only appropriately
certificated field devices are permitted to be incorporated.
Servicing and maintenance
The transmission characteristics of the devices remain stable
over long periods, so that regular adjustments or other
precautions are not required. This also means that no
maintenance work is required.
Fault elimination
No modifications may be made to devices that are operated in
connection with hazardous areas. Repairs must only be carried
out by specially trained and authorised personnel.
1.1 Operating principle
The Zener diodes in the barriers are connected in the reverse
direction. The breakdown voltage of the diodes is not
exceeded in normal operation.
Figure 1.1 Circuit diagram
If this voltage is exceeded, due to a fault in the non-Ex-area,
the diodes start to conduct, causing the fuse to blow, thus
preventing the transfer of unacceptably high energy into the
hazardous area.
Terminals 7 and 8 are connected to the devices in the non-
hazardous area. The single condition that these devices must
satisfy, is that they must not contain a source whose potential
relative to earth is greater than 250 V/253 V
eff
AC or 250/
253 V DC.
Terminals 1 and 2 are connected to the intrinsically safe
circuits in the hazardous area. If they are used in the
hazardous area, active intrinsically safe apparatus must be
certificated unless the electrical values of such apparatus do
not exceed any of the following values: 1.5 V; 0.1 A; 25 mW.
Pepperl+Fuchs Zener barriers are identified in terms of
voltage, resistance and polarity, e. g. 10 V, 50 Ohm, positive
polarity. These figures correspond to the Zener voltage U
z
and
the total resistance of all barrier components. They therefore
represent the safety values. The values stated on the type
identification label correspond to the "worst case" data for
U
z
(U
o
) and I
k
(I
o
) determined during certification.
I
k
is obtained by dividing U
z
by the resistance R1. It should be
noted once again, however, that these values do not
correspond to the operating range of the Zener barrier.
Ideally, Zener diodes would not allow any current in the reverse
direction until the Zener voltage has been attained. In practice,
Zener diodes do allow a small leakage current, the value of
which increases as the applied voltage is increased.
Hazardous area Safe area
ZD1
Zener diodes
Resistor
R1
Fuse
F1
ZD2 ZD3