Zener barrier

A Zener barrier is electrical equipment that is used as a safety barrier in explosion protection. Their task is to prevent the ignitability of circuits which are laid in an explosive atmosphere . It is intended to prevent ignitable energy , voltage or current from entering an explosive atmosphere. Typical applications are measurement and control technology in mining , petrochemicals and similar endangered industrial areas . The name of this special safety barrier is based on the use of a Zener diode (Z-Diode) as a central protective element.

application

Application example

In large areas of manufacturing companies, processes of all kinds are automated. The reliability of these complex procedures is to be ensured by special measuring and control circuits. In plants in the chemical industry, in which explosive atmospheres are often present, temperatures are monitored, pressures checked and other physical parameters observed, which in turn have an influence on certain actuators , such as B. have servo or valve drives. At the measuring and influencing points, which are almost always in the hazardous area, special sensors and actuators are used that are suitable for use in accordance with the requirements of explosion protection. The evaluation devices are usually not designed to be explosion-proof and are therefore installed in special control rooms in which no explosive atmosphere can occur. These rooms are also called the safe area . To prevent the evaluation electronics from enabling ignition in the potentially explosive area, a Zener barrier is switched in the sensor's connection circuit, with which the so-called intrinsically safe circuit can be routed separately from the safe area into the potentially explosive area.

Technical

The idea of the Zener barrier is based on the intrinsic safety type of protection . According to this, the Zener barrier is defined as associated equipment that is installed outside the potentially explosive environment in the safe area. The intrinsically safe circuit is laid in the potentially explosive environment. It is thus protected against the ingress of inadmissibly high energy, voltage and current and the associated risk of sparks. The essential component for the limitation is a Zener diode, as the Zener barrier, as a special case of the safety barrier, is not based on the principle of galvanic isolation . The resistance and the fuse are also regarded as safety-relevant components , since they limit the maximum expected current. Since conventional electrical and electronic circuits do not meet the requirements for intrinsic safety , a Zener barrier is connected between this and the intrinsically safe circuit. In contrast to stabilization circuits based on Zener diodes, there is no adjustment of the voltage here, but the Zener barrier is destroyed if an overvoltage occurs.

Potential errors must be taken into account when designing the circuit and the individual components . This means that the possibility of a component failure or an excessive connection voltage must be considered. The z. B. in that the Zener diode is installed two to three times in parallel. This redundancy reduces the risk that the circuit could ignite in the event of a failure. Furthermore, the components are overdimensioned several times with regard to their load-bearing capacity. The risk of an explosion has to be weighed, i.e. H. The risk of explosion is greater, the more likely there is an explosive atmosphere in the vicinity of the circuit. The devices must also not be repairable, so that unskilled service personnel cannot pose a risk. This can be achieved z. B. by housings that cannot be opened non-destructively or by potting the circuit.

Legal

When developing Zener barriers, the legal requirements for explosion protection must be observed. In the European Union, this is essentially Directive 2014/34 / EU (formerly 94/9 / EC) of the European Parliament and the Council, as well as its implementation in the national law of the member states. Furthermore, the harmonized standards EN 60079-0 (basic requirements) and 60079-11 (intrinsically safe devices) can be used for conformity assessment . For Zener barriers with which electrical devices are to be protected in zones 1 and 0 (categories 2 and 1), an EC type-examination certificate from a notified body is required. In countries that do not belong to the EU, other country-specific laws and regulations apply.

For applications with Zener barriers, all rules of engineering care must be observed when making the selection. On the one hand, too high an input voltage in the safe area leads to the destruction of the equipment, which, however, does not involve any safety-relevant risks. On the other hand, it must be checked on the intrinsically safe side whether the selected equipment is compatible with the selected Zener barrier, because ignitable values can be achieved for voltage, current or energy, especially when interconnecting various active equipment, which represents a safety-relevant risk. Special standards deal with the assessment of the interconnection of circuits and intrinsically safe systems. Reference is made to DIN EN 60079-14 (installation requirements, VDE 0165, part 1) and DIN EN 60079-25 (intrinsically safe systems, proof of intrinsic safety).

Function description

Schematic diagram

The basic circuit diagram shows the function of the Zener barrier. The intrinsically safe circuit that leads into the hazardous area is connected to terminals 3 and 4. The evaluation electronics are connected to terminals 1 and 2 in the control cabinet. This emits a small, harmless voltage that is sufficient for the purpose of the circuit, in the example given this is a temperature measurement. In this normal state, the Zener barrier behaves passively. The Zener diode blocks because the voltage applied between terminals 1 and 2 is less than its breakdown voltage . The resistor R1 was chosen to be correspondingly low so that it does not generate a noticeable voltage drop that would have a negative impact on the function of the circuit.

If the voltage between terminals 1 and 2 rises above the breakdown voltage of the Zener diode due to a fault in the control cabinet, it becomes conductive. The current through the Zener diode blows the fuse F1. This separates the intrinsically safe circuit at terminal 3 from the impermissible overvoltage and avoids the risk of an explosion due to sparks.

Terminals 2 and 4 are grounded together so that they can never be subject to an impermissibly high voltage.

The Zener barrier in the design shown is only suitable for applications in which a positive voltage is always applied to terminal 1: The Zener diode must be operated in reverse direction during normal operation.

Problems

Zener barriers are inexpensive equipment for limiting the ignitability of intrinsically safe circuits in potentially explosive areas. If used correctly, inexpensive elements do not represent a security risk, but can help ensure that an acceptable level of security can be achieved even in projects with tight financial leeway.

When planning or using Zener barriers, the following points must be observed:

They do not have any galvanic separation that isolates both circuits from each other. Therefore it can happen that a potential builds up compared to the earth potential. In the event of a fault, this can result in a high-energy spark against ground, which ignites an explosive mixture. The solution to this problem is the mandatory connection of the circuit with the equipotential bonding, which, however, can cause problems for the functionality of the circuit.
The resistance in the series circuit of the circuit has a relatively high value to limit the current. This can have a detrimental effect on the function of the circuit. According to the rules for series connection of resistors, part of the voltage limited by the Zener diodes drops across this series resistance. Under certain conditions, this can mean that the actual function of the system is no longer guaranteed. Then other options have to be chosen.
According to the safety regulations, Zener barriers may not be repaired. If too high a voltage occurs in the non-intrinsically safe connection area, which causes an excessive current, the fuse will respond. This renders the device unusable and the installation of a possibly wrong fuse is not possible. Some manufacturers avoid this problem with a second, exchangeable fuse, which is installed in series with the explosion-relevant fuse and is designed in such a way that it responds earlier. After replacing this fuse, the Zener barrier can continue to operate.

standardization

*	EN IEC 60079-0	2018	Explosive atmosphere	Part 0:	Equipment - General Requirements	(Previous standard EN 50014)
*	EN 60079-11	2012	Explosive atmosphere	Part 11:	Device protection through intrinsic safety "i"	(Previous standard EN 50020)

As national standards, these standards are labeled DIN , ÖNORM or SN

literature

W.-D. Can: Explosion protection through intrinsic safety. Friedr. Vieweg & Sohn Verlagsgesellschaft, Braunschweig / Wiesbaden 1993, ISBN 3-528-06540-0 .
W. Gohm: Explosion protection in MSR technology - guidelines for the user. Hüthig, Heidelberg 2006, ISBN 3-7785-4005-X .
E. Lienenklaus, K. Wettingfeld: Electrical explosion protection according to DIN VDE 0165. 2nd edition. VDE-Verlag, Berlin / Offenbach 2001, ISBN 3-8007-2410-3 .
H. Olenik among others: electrical installation and equipment in potentially explosive areas. Hüthig & Pflaum, Munich / Heidelberg / Berlin 2000, ISBN 3-8101-0130-3 .