1 What is a Zener barrier?
A Zener barrier is a passive safety element that limits electrical signals or circuits between a safe and a potentially explosive area in such a way that no ignitable energy can enter the hazardous area.
It usually consists of:
Resistors for current limitation,
Zener diodes for voltage limitation,
and a fuse that interrupts the circuit in the event of a fault.
This ensures that the voltage, current and energy in the circuit remain low enough to prevent the ignition of an explosive atmosphere even in the event of a fault.
2 Why are Zener barriers used?
In ATEX zones, electrical energy is always a potential ignition risk. Sensors, switches or probes installed in zone 0, 1 or 2 (gas) or zone 20, 21 or 22 (dust) therefore require safe energy limitation.
Zener barriers fulfill this purpose:
They protect intrinsically safe circuits (Ex i).
They enable the use of normal control or evaluation devices outside the zone.
They are a cost-effective and proven solution for analogue or binary signals.
Typical applications:
Temperature, pressure and level measurement
Sensors on pipes and containers
Signal lines between the control system and field devices
3. planning and use - what needs to be considered?
🔹 a) Certification and protection class
Zener barriers must have valid ATEX or IECEx approval. The type plate contains markings such as:
II (1) G [Ex ia Ga] IIC II (1) D [Ex ia Da] IIIC
These indicate the zone and substance group for which the appliance is approved.
🔹 b) Installation location
Zener barriers are installed in the safe area, usually on a DIN rail in the control cabinet.
They must not be installed in the Ex zone themselves.
🔹 c) Earthing
A secure earth connection is essential.
This is the only way to safely dissipate excess energy that is dissipated via the Zener diodes.
Missing or loose earthing is a common and dangerous installation error.
🔹 d) Loop and cable design
Limit values apply to each Zener barrier for:
Maximum voltage and current
maximum inductance (L) and capacitance (C) of the line
These values must be taken into account in the loop calculation.
Exceeding the limits can invalidate the intrinsic safety - even if all individual components are certified.
🔹 e) Documentation
Every installation of intrinsically safe circuits must be documented:
Circuit diagram with assignment of barrier ↔ sensor
Earthing point and loop parameters
Certificates and proof of conformity
4. What a Zener barrier cannot do
Zener barriers are simple, reliable and inexpensive – but not universally applicable.
The following points must be clear:
They do not provide galvanic isolation. Potential differences between the measuring circuit and the control system can therefore lead to measurement errors.
They are not suitable for power circuits or actuators with higher energy requirements.
They will not function if the grounding is inadequate.
They are not maintenance-free – the earth connection and fuses in particular must be checked regularly.
In many modern systems, galvanically isolated intrinsically safe isolation amplifiers are therefore increasingly being used when measurement accuracy or EMC requirements increase.
5. labeling and testing
Zener barriers are clearly marked to indicate their area of application.
Exemplary labeling:
Ex ia IIC Ga Uo = 28 V DC Io = 93 mA Po = 660 mW Lo = 10 mH Co = 83 nF
These values indicate the maximum energy that the output circuit can deliver - and therefore whether the connected sensor circuit remains within the permissible limits.
Checklist for testing:
Is the labeling complete and legible?
Do the voltage, current and power match the planned application?
Is the earthing documented and measured?
Is the cable length within the calculated limits?
6. Do
Zener barriers are a proven and simple way of transmitting electrical signals intrinsically safely into potentially explosive atmospheres.
They offer protection through energy limitation, are robust and inexpensive – but require careful planning, earthing and documentation.
Errors in earthing or loop calculation make even the best barrier ineffective.
Therefore, Zener barriers are not installation components, but safety components.
7. in short
| Advantage | Border |
| Simple, cost-effective solution | No galvanic isolation |
| Proven technology | Only for signal/sensor circuits |
| Broad approval for ATEX zones | Strict earthing obligation |
| Low maintenance with correct installation | Pure energy limitation - no EMC protection |