The Zener barrier limits the power to the field and preserve the integrity of the field equipment.
Key components are:
Zener Diode - Limits the voltage within the intrinsically safe circuit.
Resistor1 - Limits the current going into the intrinsically safe circuit.
Fuse - limits the current to the Zener diode so in order to size it's power rating
The schematic for simplicity is an Ex ic Zener diodes would be duplicated for Ex ib triplicated for Ex ia to achieve the appropriate EPL.
Only a few Zener barriers types are required to service most application loops and their simplicity makes them versatile in use.
The Earth is critical for a Zener barrier, and there exists a lot of confusion and mis-information regarding Earthing, in most cases an 'intrinsic safety' earth is easy to achieve but may be an inconvenience.
Isolation interface barriers seem to be the preferred choice for most applications other than some specific complex circuits.
In simple terms and isolation interface barrier is just a Zener barrier with electrical isolation between the hazardous and non‑hazardous area, consequently an intrinsic safety earth is not required as there is no direct electrical connection between the hazardous and non‑hazardous areas.
The above is a 2-port isolator i.e. loop powered. More common, due to power limitations of the circuit a 3 port isolator is used for an additional power supply.
As Isolation interfaces are active rather than passive like Zener barriers, due to the complex nature of the circuitry they are generally designed for a specific purpose.
In addition, Diode return barriers exist, they replace the Resistor with a Diode (or 3). Used for a return signal they allow a one-way signal input and do not contribute any current to the intrinsically safe circuit. ↩