VEGA Z728 User Manual

Zener barriers - operating instructions

Operating principle

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.

Hazardous area Safe area

Resistor

R1

Zener diodes

Figure 1.1 Circuit diagram

ZD1

ZD2 ZD3

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 non­hazardous 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

253 V DC.

Fuse

F1

AC or 250/

eff

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

and

z

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

) and Ik(Io) determined during certification.

z(Uo

I

is obtained by dividing Uz by the resistance R1. It should be

k

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.

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

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Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

Zener barriers - operating instructions

Operating principle

The operating range of a Zener barrier must therefore be such
that it is below the Zener voltage, so that the leakage current is
restricted to a minimum. Zener barriers are normally tested to
ensure that at the prescribed voltage the leakage current is
smaller than 10 µA.

Hazardous area Safe area

50 mA

Load

Figure 1.2 This figure shows a selection of leakage currents through

the Zener barriers under normal circumstances. The
Zener barriers conduct a maximum of 10 (1) µA leakage
current so long as the supply voltage is less than 25.5 V.
This is normal and has very little effect on the load. If the
voltage exceeds 25.5 V, the Zener diodes start to
conduct more current. This can have an effect on the
operating current and the accuracy. It is therefore
recommended that a controlled voltage source be used,
which maintains the voltage under the value at which the
diodes will start to conduct.
(A 24 V, 300 Ohm barrier is represented here as an
example)

10 µA

24 V

Power
supply

(+)

25.5 V

≤

(-)

These voltages are stated in the data sheet for a given barrier,
together with the leakage current. If the leakage current for a
given voltage differs from 10 µA, this is specifically stated.

Hazardous area Safe area

50 mA

Load

Figure 1.3 This figure shows that if the maximum permissible input

(supply) voltage is exceeded, the total current drains
through the Zener diodes, without reaching the explosive
surroundings.

24 V

Power
supply

(+)

25.5 V

>

(-)

Pepperl+Fuchs Zener barriers have a low series resistance,
given by the sum of the resistance R1 and the resistance value
of the fuse F1. Due to the low series resistance, an inadvertent
short-circuiting of terminals 1 and 2 can cause the fuse to blow.
In order to avoid this, some barriers are available with
electronic current limitation (CL-version).

If the Zener barriers are provided with a resistance, this limits
the short-circuit current to a safe value in the event of a short­circuit of the connecting wiring in the hazardous area or a
connection to earth of the wiring attached to terminal 1, as the
fuse blows.

Many barriers are available with a resistance connected
between the output terminals. These are used in
4 mA … 20 mA transmitter circuits. The resistance converts
the current in the intrinsically safe circuit into a voltage that can
be measured in the safe area.

Pepperl+Fuchs Zener barriers can be used in many
applications. In the simplest case, a single channel barrier with
a ground connection is used. But in many applications it is not
desirable that the intrinsically safe circuit is connected directly
to ground. If the circuit in the safe area is grounded, under
some circumstances grounding of the intrinsically safe circuit
can lead to faults within the system. In this case, quasi ground­free intrinsically safe circuits can be constructed with two or
more barriers. This floating circuitry can be simply achieved
with 2- or 3-channel barriers.

Double grounding of intrinsically safe circuits is not permitted.
The insulation voltage of the wiring and field devices,
measured with respect to ground, must be greater than
500 V AC. The permissible ambient temperature of the Zener
barriers is between -20 °C … 60 °C.

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

2

Zener barriers - operating instructions

y

Multi-channel barriers

1.2 Multi-channel barriers

Analogue circuits are often connected to two-channel barriers
(see Figure 1.5). Since there is no grounding on this type of
circuit, the system is a quasi floating one. It is termed "quasi
floating", because it is "one Zener voltage" above the ground
potential. Although it does not actually float, the signal-to-noise
ratio is improved.

A further advantage of multi-channel Zener barriers is that a
higher packing density can be achieved.

.

Hazardous area

4 mA ... 20mA

transmitter

Safe area

(+)

24 V

R

M

Power supply can
not be grounded.

Figure 1.4 Single-channel Zener barrier

1.3 Grounding of Zener barriers

Intrinsically safe circuits with Zener barriers without galvanic
isolation must be grounded. The cross-section of the ground

connection, using a copper conductor, must be at least 4 mm
(for further details see EN 60079-14, section 12.2.4). The
maintenance of these requirements prevents the occurrence of
a dangerous potential with respect to ground.

A fault of the type illustrated in figure 8.6 can cause a
dangerous spark if the Zener barrier is not grounded, but
grounding is provided via the field device in the intrinsically
safe circuit (Figure 1.5). If a potential occurs in the fault case,
which is higher than permitted (see Figure 1.6) the Zener
diodes become conducting and the current is conducted away
via the ground. The fuse "blows".

.

Hazardous area

4 mA ... 20mA

transmitter

Figure 1.5 Two-channel Zener barrier

2

Safe area

(+)

24 V

R

M

Hazardous

Safe area

area

Hazardous potential

Hazardous potential

Figure 1.6 Non-grounded Zener barrier

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

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Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

AC/DC
supply
voltage

Fault

Power suppl

Transformer

Hazardous
area

Safe area

Fault

Power supply

Transformer

Fault current

Intrinsically
safe ground

Figure 1.7 Grounded Zener barriers

The system must have its own independent ground
conductor, through which no supply system current
flows.

Date of issue 05/23/03

1.4 Installation notes

Pepperl+Fuchs Zener barriers in the Z7, Z8 and Z9 series can
be mounted on a standard rail to EN 50022 in 3 different
arrangements.

• Equipotential bonding via the standard rail (grounding of all
snapped-on Zener barriers)

Insertion strip
ZH-ES/LB

Label carrier
ZH-Z.BT

DIN rail NS 35/7.5
35 mm standard rail
to DIN EN 50 022

Clamp
ZH-Z.USLKG5

Figure 1.8 Equipotential bonding via the standard rail

• Group grounding through insulated mounting

Mounting block
ZH-Z.AB / NS

Figure 1.9 Insulated mounting (Individual grounding)

• Individual grounding through insulated mounting

Mounting block
ZH-Z.AB/SS

Single socket
ZH-Z.ES

N-Combined rail
ZH-Z.NLS-Cu3/10

Zener barriers - operating instructions

Installation notes

Pepperl+Fuchs Zener barriers also feature a space-saving

12.5 mm housing which incorporates up to 3 channels.

11012.5

115

Figure 1.11 Mechanical features

Construction: Modular terminal housing in Makrolon,
flammability classification UL 94: V -0

Fixing: Snaps onto 35 mm standard rail to DIN EN 50022
Connection options: Self-opening terminals, max. core cross-

section 2 x 2.5 mm²
The barriers are usually installed in racks or control cabinets.

They can be built into housings under production conditions,
with the proviso that the housing must afford adequate
protection. They can also be employed in hazardous areas,
when it has been ascertained that the housing has been
certificated for this purpose.

The installation must be carried out in such a way that the
intrinsic safety is not compromised by the following factors:

• Danger of mechanical damage

• Non-authorised changes or influence exerted by external
personnel

• Humidity, dust or foreign bodies

• Ambient temperature exceeding the permissible level

• The connection of non-intrinsically safe circuits to
intrinsically safe circuits

Grounding of the mounting rail is of the normal type, i. e. both
ends are connected to the intrinsically safe ground. This also
simplifies checking the grounding.

Many installations provide the option of subsequent expansion.
Replacement cable for this purpose can be connected to the
Z799 dummy barrier and unused cable can be connected to
the intrinsically safe ground.

Connector for
ground lead
ZH-Z.AK4

Connector
ZH-Z.AK16

Ground rail feed
ZH-Z.LL

Spacing roller
ZH-Z.AR.125

Figure 1.10 Insulated mounting (Individual grounding)

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

4

Zener barriers - operating instructions

Zener barrier specifications

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

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 (Ik)

)

z

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 (Uin 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, IK is now IO).

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 common­mode 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!

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

5

Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

7. The following points have to be clarified if special field
devices are used.

• If the field device is a 4 mA ... 20 mA transmitter: What load
in ohms can be connected to the transmitter so that it can
attain 20 mA as before?

• If the field device is a current/pressure converter: What load
can be connected to the controller card so that it can attain
20 mA as before?

• If the field device is a transmitter: How high is the load in the
safe area? (typically, resistances of up to 250 Ohm are used
in the controller)

Z ...

Zener barriers - operating instructions

How to select the correct barrier

Barrier with replaceable back-up fuse

The introduction of a replaceable back-up fuse ahead of the
integrated fuse provides protection against faults which could
occur during the commissioning of the system. It is always

Type

Channels

Z715.F 1 106 13 13.6 100 63 217,063

Z728.F 1 327 27 28 80 50 217,05

Z728.H.F 1 250 27 28 80 50 217,05

Z765.F 2 106

Z779.F 2 327

Z779.H.F 2 250

Z787.F 2 327

Z787.H.F 2 250

Z960.F 2 64

Z961.F 2 106

Z966.F 2 166

Max. series

resistance

Ohm V V mA mA

106

327

250

36 + 0.9V

25 + 0.9V

64

106

166

Uin

at 10 µA

13

13
27
27

27
27
27

27
27
27

6.5

6.5

6.5

6.5
10
10

arranged that the outer fuse will respond before the inner,
innaccessible fuse. The fuses used are specially intended for
use on barriers.

Uin

max

13.6

13.6
28
28

28
28
28

28
28
28

9.5

9.5

8.1

8.1

11.7

11.7

Fuse rating External fuse Fuse supplied

100

100

80
80

80
80
80

80
80
80

80

80
160

160
100
100

63

63
50
50

50
50
50

50
50
50

50

50
100

100

63

63

by LITTLEFUSE

217,063

217,05

217,05

217,05

217,05

217.05

217.1

217.063

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

Pepperl+Fuchs Group • Tel.: Germany [phone] • USA [phone] • Singapore [phone] • Internet http://www.pepperl-fuchs.com

6

Zener barriers - operating instructions

How to select the correct barrier

Type Nominal data Ex-characteristics for [EEx ia] IIC Certification no.

+ ve - ve a.c. V W Uz (V) R

Z705

–

Z710

–
–

Z713 Z813 – 15.75 22 15.75 21.8 723 2.84 0.48 0.076 12.5 BAS 01 ATEX 7005

Z715

Z715.F

–

Z715.1K

–

Z722

Z728
Z728.H

Z728.F

Z728.H.F

Z728.CL

–

Z755 Z855 – 5

Z805

–

Z810

–

Z810.CL

Z815

Z815.F

–
–
–

Z822

Z828
Z828.H

Z828.F

Z828.H.F

Z828.CL

–

–

Z905

–

Z910

–

–
–

Z915

–

Z915.1K

–
–
–

–
–
–

Z928

5

5
10

10
10

15
15

15
15
15

22
28
28

28
28
28

28

5

10

4.94

10

4.98

50

9.56

50

9.94

50

9.56

100

14.7

100

14.7

100

15.0

1k

14.7

1k

150
300
240

300
240
300

300

10104.94

4.94

4.94

15

22
28
28

28
28
28

28

(W) IK(mA) P

min

9.8

9.8
49

49
49

98
98

98

980
980

147
301
235

301
235
301

301

9.8

9.8

4.9

504
499
195

203
195

150
150

153

15
15

150

93

119

93

119

93

93

504

504

1008

max

0.62

0.61

0.47

0.50

0.47

0.55

0.55

0.57

0.06

0.06

0.82

0.65

0.83

0.65

0.83

0.65

0.65

0.62

0.62

1.25

(W) C

max

100
100

3

3
3

0.58

0.62

0.58

0.58

0.58

0.17

0.083

0.083

0.083

0.083

0.083

0.083

100

100
100

(µF) L

(mH) L/R Ratio

max

0.14

0.14

0.86

0.86

0.86

1.3

1.45

1.3
144
144

1.45

3.05

1.82

4.21

2.59

3.05

3.05

0.14

0.14

0.03

57

BAS 01 ATEX 7005

57

BAS 01 ATEX 7005

73

BAS 01 ATEX 7005

73

BAS 01 ATEX 7005

73

BAS 01 ATEX 7005

64

BAS 01 ATEX 7005

67

BAS 01 ATEX 7096

64

BAS 01 ATEX 7005

570

BAS 01 ATEX 7005

570

BAS 01 ATEX 7005

45

BAS 01 ATEX 7005

56

BAS 01 ATEX 7005

44

BAS 01 ATEX 7005

55

BAS 01 ATEX 7096

44

BAS 00 ATEX 7096

56

BAS 01 ATEX 7005

56

BAS 01 ATEX 7005

57

BAS 01 ATEX 7005

57
22

– – Z955 5

Z757

–

–

–

Z764 Z864 – 12121k1k11.6

– – Z964 12

Z765

Z765.F

Z857

–

–

–

Z865

Z865.F

–

Z961

Z961.F

Z961.H

–

–

10104.89

5

7

10

7

10

9

100

9

100

9

100

9

100

9

360

9

360

121k1k

15

100

15

100

15

100

15

100

4.89

9.78

7.14

7.14

7.14

17.4

17.4

17.4

11.6

11.6

14.7

14.7

14.7

14.7

14.7

14.7

8.7

8.7

8.7

8.7

8.7

8.7

12
12

24

9.8

9.8

4.9

9.8

9.8

4.9

98
98
98

98
98
98

352.8

352.8
355

980
980

490

980
980

490

98

98
49
98

98
49

499

499
998

729
729

1457

89
89

178

89
89

178

25
25
49

12
12

24

12
12

24

150

150
300
150

150
300

0..61

0.61

1.22

1.3

1.3

2.6

0.19

0.19

0.39

0.192

0.192

0.384

0.05

0.05

0.11

0.03

0.03

0.06

0.04

0.04

0.08

0.55

0.55

1.1

0.55

0.55

1.1

100

100

3.3

13.5

13.5

13.5

4.9

4.9

0.346

4.9

4.9

0.31

4.9

4.9

0.346

1.41

1.41

1.41

1.41

1.41

0.125

0.58

0.58

0.58

0.62

0.62

0.62

0.14

0.14

0.03

0.07

0.07

0.02

4.69

4.69

1.14

4.39

4.39

1.07

57
57

15.2

240
240

61

240
240

61

1.3

1.3

0.32

1.45

1.45

0.32

57

BAS 01 ATEX 7005

57
22

28

BAS 01 ATEX 7005
28
11

182

BAS 01 ATEX 7005

182

72

176

BAS 01 ATEX 7096

176

67

BAS 01 ATEX 7005

613
613
249

1.0

BAS 01 ATEX 7005

1.0

360

1.0

BAS 01 ATEX 7005

1.0

360

64

BAS 01 ATEX 7005

64
22
67

BAS 01 ATEX 7096

67
22

Date of issue 05/23/03

Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany

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