VEGA Z728 Zener barriers - Manual (Page 5)

5
Zener barriers - operating instructions
Zener barrier specifications
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
1.5 Zener barrier specifications
Nominal data
The following are typical data used in the description of a
barrier:
28 V, 300 Ohm, 93 mA. These values relate to the maximum
voltage, the minimum value of the built-in resistance and the
resulting maximum current.
The maximum voltage stated is not representative of the
operating range, it is the maximum value that can be attained
in a failure case, before the fuse responds. The resistance
value is not identical to the maximum series resistance. These
values merely provide an indication of the maximum values
that can apply in the case of a failure.
Series resistance
This is the resistance that can be measured between the two
ends of a barrier channel. It is obtained from the sum of the
resistance R and resistance value of the fuse at an ambient
temperature of 20 °C.
Polarity
Zener barriers are available in various versions. On Zener
barriers for positive polarities the anodes of the Zener diodes
are grounded. On barriers for negative polarities it is the
cathodes which are grounded. On barriers for alternating
polarities, interconnected Zener diodes are employed and one
side is grounded. These can be used for both alternating
voltage signals and direct voltage signals.
Maximum voltage in the intrinsically safe circuit. (U
z
)
This is the maximum value of voltage that can occur in the
intrinsically safe circuit in the failure case.
Maximum current in the intrinsically safe circuit (I
k
)
This is the maximum current that can flow in the intrinsically
safe circuit in the failure case.
Maximum input voltage (max. U
in
)
The maximum voltage (correct polarity) that can be applied
between the contacts in the safe area and the ground without
the fuse responding. This value is determined for an open
intrinsically safe circuit and an ambient temperature of 20 °C.
Input voltage (U
in
at 10 (1) µA)
The maximum voltage (correct polarity) that can be applied
between the contacts in the safe area and the ground at a
defined leakage current (as a rule 10 µA). This is the upper
value of the recommended operating range.
Maximum connectable external capacitance C
max
This is the maximum capacitance that can be connected to the
terminals of the barrier intrinsically safe circuit. This value is
determined from the sum of the wiring capacitance and the
input capacitance of the field device.
Maximum connectable external inductance L
max
This is the maximum inductance that can be connected to the
terminals of the barrier intrinsically safe circuit. The value is
determined from the sum of the inductance of the wiring and
the input inductance of the field device.
Note:
The designations of the values given in the specifications
above are not those of the relevant standards, but those
specified on certificates of conformity (e. g. in EN 60079-14,
Section 3, I
K
is now I
O
).
1.6 How to select the correct barrier
For very many applications the standard solutions are given in
this catalogue, in the section on Example Applications.
However, in the event that a particular application has not been
covered, the following information may be helpful.
1. First decide whether it will be necessary to have a floating
circuit, or whether the intrinsically safe circuit can be
connected directly to ground. Check whether any existing
instrumentation is grounded. If the answer is yes, then
check whether additional grounding could lead to faults.
Bear in mind that the floating circuit offers a better common-
mode rejection characteristic than the grounded circuit. On
the other hand, it is more expensive. If a floating circuit is
employed, the barriers will normally resist a ground fault.
2. Select the required polarity. This is either determined by the
circuit itself, or by any other existing grounds in the circuit. In
most applications barriers for positive polarities are used. In
order to achieve greater system standardisation, barriers
suitable for alternating polarities can be used in place of
unipolar ones.
3. Decide the nominal voltage of the Zener barrier. Then
determine the maximum output voltage of the device in the
safe area during normal operation. Normally the required
value is the next highest nominal voltage of a Zener barrier.
If these values are close together, it could be that the
recommended operating range of the Zener barrier is
exceeded. The consequence of this is that the leakage
current will be greater than 10 µA. In this case a barrier with
a higher nominal voltage should be used. The leakage
current is determined for an open intrinsically safe circuit
and this then represents the maximum value at the given
voltage.
4. Take account of the maximum series resistance of the
Zener barrier and its effect on the intrinsically safe circuit.
Make sure that this resistance does not cause an
inadmissibly high loss of voltage. In circuits having high
resistance - usually when voltage signals are being
transferred - this resistance is not relevant. If for example a
barrier has a max. series resistance of 1 kOhm, then the
resulting error is 0.1 %, if the input resistance of the
connected device is 1 MOhm.
5. Check whether or not the field device must be certificated for
use in the hazardous area. If certification is necessary,
check what the prerequisites are for permitting the field
device to be used in connection with a Zener barrier.
6. What is the overall length of the cabling between the voltage
supply and the field device? Note the number of conductors
in the system!