Interface selection: Zener barriers or galvanic isolation interfaces
The selection between Zener barriers and galvanic isolation interfaces is typically driven by site standards, engineering preference, and installation practices. Both interface types are widely accepted for intrinsic safety applications and each offers distinct advantages and limitations.
Zener barriers
Zener barriers are relatively simple devices and are generally more flexible in application. In many cases, a single Zener barrier type can be used across multiple circuit configurations. They are cost-effective and widely supported within intrinsic safety systems.
A common concern with Zener barriers is their strict earthing requirement. While maintaining a dedicated intrinsic safety ground is often perceived as complex, it is typically straightforward in practice. However, when only a small number of Zener barriers are installed, the requirement for a high-integrity earth connection can introduce additional installation effort and cost.
Galvanic isolation interfaces
Galvanic isolation interfaces (also referred to as isolators) are typically designed for specific signal types or applications and offer less flexibility than Zener barriers. Their primary advantage is that they do not rely on a high-integrity intrinsic safety earth to maintain safety.
To prevent cables from charging to uncontrolled potentials and accumulating stored capacitive energy that could become incendive, a discharge path to earth should still be provided. This is normally achieved using a resistance in the range of 200 kΩ to 1 MΩ. This discharge connection is not considered an earth reference for the instrumentation loop.
Selecting an intrinsic safety interface
The intrinsic safety interface, classified as associated apparatus, ensures the protection of the field device such as an alarm horn or strobe by limiting electrical energy. Safety is maintained only when voltage, current, and power delivered to the field device remain below the maximum permitted values.
These limits are defined as entity parameters (also known as safety parameters) and typically include:
- Voltage
- Current
- Power
- Capacitance
- Inductance
Not all parameters are always specified if they are not applicable or can be derived from other values.
E2S alarm horns and strobes entity parameters
With an example from our partners, E2S, whose range of intrinsically safe sounders and beacons share identical entity parameters. The suffix “i” denotes input parameters of the field device.
Entity Parameters:
- Ui: 28 V
- Ii: 93 mA
- Pi: 660 mW
The integrity of the apparatus is maintained and the device remains intrinsically safe provided these values are not exceeded. Accordingly, the selected associated apparatus (Zener barrier or isolator) must have output parameters less than or equal to these values.
⚠️ Note: The power rating is not a direct calculation of voltage multiplied by current. Entity parameters represent safety limits, not normal operating values.
Capacitance and inductance considerations
The capacitance and inductance values represent the contribution of the field device to the intrinsically safe circuit.
- Ci: 0 µF
- Li: 0 mH
For E2S alarm horns and strobes, both capacitance and inductance contributions are zero, which significantly simplifies the intrinsic safety assessment and loop calculations.
Barrier selection criteria
A suitable intrinsic safety barrier or isolator must meet the following output limits:
- Uo ≤ 28 V
- Io ≤ 93 mA
- Po ≤ 660 mW
When these criteria are satisfied, the integrity of the intrinsically safe circuit is maintained.
Explore intrinsically safe alarm horns, strobes, and manual call points here: