Principles of Zener Barriers



A Zener barrier is an associated equipment that is installed in the safe area. It is designed to limit the amount of energy that could appear in an electrical circuit passes through the hazardous area despite the connection before the barrier. A barrier consists of:

Resistors to limit the current

Zener diodes to limit the voltage

Fuses to protect the components

As any intrinsic safety equipment, the Zener barrier allows cables to short circuit to each other or to metallic parts connected to ground without danger.

The Zener barrier interfacing mode differs from others as there is no galvanic isolation. Cables that pass through the hazardous area thus share common features with those of the safe area. This implies equipotential grounding.

Figure (1) illustrates an intrinsic safety equipment (A) connected to a circuit (C) through a Zener barrier (B) that limits the current, the voltage and the power.

If a fault voltage occurs between the terminals (m) and (n), the Zener diode (protected by a fuse) limits the voltage that risks appearing in the hazardous area and the resistor limits the current to an acceptable value.

If a fault voltage occurs between the terminal (m) or (n) and the ground, the voltage of the wires (e) and (f) relative to ground will not exceed Vz provided that the Zener barrier is correctly grounded at (T1).



Current path in normal operation, U <= 9V
Current path in the overvoltage case, U <= 9V
The Zener diode becomes conducting
The fuse protects the Zener diode from destruction

The Zener barrier permits wires (e) and (f) to be short circuited without danger. However, if point (n) accidentally reaches a high potential relative to ground, a ground fault at (f) risks causing a dangerous spark.

To ensure the safety of such a wiring, point T1 must be connected to ground as illustrated in Fig.(2). So, in the event of a fault between (m) and (n), the voltage between (e) and (f) will not exceed Vz, the short circuit current between (e) and (f) will not exceed Vz/R, and the ground fault current will be zero for point (f), and equal to Vz/R for point (e).

Notice:
To validate the following statement, grounds T1 and T2 must be at the same potential.
Actually, a Vt difference of potential causes a loop current only limited by line and ground resistances.

Conclusion: Only an equipotential ground network can ensure the safety of a reference ground system.



A wide range of barriers has been developed to fit all type of installation. They differentiate by their electrical circuit diagrams, parameters and functions.
The electrical circuit diagram differs from a barrier to another.

There are two main types:
The 'single' barriers: The 'double' barriers:



Single barriers:
In this configuration, one of the two metrologic wires is directly connected to the ground (T1) at the barrier (Fig. (2)).

If there is a difference in the potential between T1 and T2, a ground fault (T2) can cause a loop current to occur.

Even if this current does not affect the system, it can impair the measurement of low level signals (e.g. Pt100, thermocouple). An important difference of potential can degrade the safety.

Double barriers:



Vt = Difference of potential between grounds T1 and T2

With a barrier of this type (Fig.(3)), a ground fault at (f) causes a loop current to occur:



The current It is lower than with a single barrier where it can take the value:

Another advantage is that the double barrier - unlike the single - ensures isolation of the metrologic wires relative to the ground corresponding to the Zener diode threshold.

The barriers and their functions

Three main functions:
A signal current transmission function
A signal voltage transmission function
A power supply function

In the current signal transmission function, the measured value is the current. The barrier is integrated in a loop connected to a current source. The barrier's protective diodes must not conduct.

The barrier brings an additional resistance which must not cause the acceptable loop resistance to be exceeded. The barrier is determined as follows:



Ue is the voltage for which a leakage current lower than or equal to I(t) is ensured.
If I is the current and V is the voltage needed in the hazardous area (fig. 4): The following relations must be checked for the equipment operates correctly:

V + I × (Rs + Rc) < Ue
U < Ue

If these relations are checked, the maximum ohmic value of the cable can be determined as follow:

Rc max. = [Ue - V - (Rs × I)] / I

The fuse resistance R Fu is so negligible that calculation can be made with the value RL of the bzg parameters instead of Rs.

Influence of the Zener barrier internal resistance:

Because of V - the voltage needed for the equipment operates in hazardous area-, the loop resistance Rc + RL must be compatible with the supply voltage at the barrier input.
An associated equipment has always an internal resistance (RL) in series with the terminals connected to the intrinsic safety equipment.

The intrinsic safety parameters Po and Io of the associated equipment (See chapter 1.3) are determined by this resistance.

This resistance RL can affect the operation of the connected intrinsic safety equipment by generating a voltage drop at its terminals:

^u = (RL + Rc) x I consumed by the transmitter

Example :
A 24V supplied transmitter through a Zener barrier with an internal resistor of 200O. The cable resistance is negligible.

Maximum current consumed by the transmitter: 21 mA
Voltage drop ?u due to the internal resistance: 0.021 x 200 =4.2V
Effective supply voltage V seen by the transmitter: 24-4.2= 19.8
V
In this example, RL is included in the calculation of the maximum load resistance specified by the transmitter manufacturer.

The effective supply voltage V must never be lower than the minimum supply voltage specified.

The voltage, current and power values limited by the barrier must also be considered.

The equipment operating voltage in the hazardous area must be lower than the Uo of the associated equipment.

The equipment operating current in the hazardous area must be lower than the Io of the associated equipment.

The operating power consumed by the equipment in the hazardous area must be lower than the Po of the associated equipment.

The voltage signal transmission function implies to check that the barrier resistance RL does not badly weaken the signal when the receiver input impedance Z is not illimited (fig. 5).



The value of the pulse V must be lower than Ue, otherwise the generator risks short-circuiting through the resistor Rs. The pulse frequency must be checked.

The power supply function is as follow (Fig. 6):



For this type of barrier the following relations must be checked:
If I is the current needed in the hazardous area.

I <= Ue / (Rs + R)

The fuse resistance R Fu is so negligible that calculation can be made with the value RL of the bzg parameters instead of Rs.


This information kindly issued by :

REGULATEURS GEORGIN UK
Newbold Road Rugby CV21 2NH
Email :uk@georgin.com
Tel : 0044(0)1.788.557.413
Web :
www.georgin.com

November 2012


   
   




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