IS Isolators vs. Zener Barriers

1. Introduction & Context

In hazardous classified areas (such as oil refineries or underground coal mines), Intrinsically Safe (IS) circuits are the gold standard for prevention of explosions. Intrinsic safety works by limiting the thermal and electrical energy available to a circuit to levels below what is required to ignite the surrounding explosive atmosphere—even under double-fault conditions.

To achieve this, IS barriers are placed in the safe area (control cabinet) between the PLC and the hazardous area field instruments. The two primary technologies used to limit this energy are Zener barriers and galvanic isolators.

2. The Core Issue

While both Zener barriers and galvanic isolators successfully limit voltage and current entering the hazardous area, they rely on fundamentally different electrical principles:

• Zener Barriers (Shunt Diode): These are simple, passive devices consisting of Zener diodes, resistors, and fuses. If a fault occurs in the safe area (such as a 120V mains line shorting to a 24V PLC loop), the Zener diodes conduct, shunting the excess fault energy directly to ground.
  • The Fatal Flaw: For a Zener barrier to work, it must have a direct, low-impedance connection (under 1 ohm) to the main service ground bus—referred to as the dedicated IS ground. If this grounding connection is loose, corroded, or disconnected, the Zener barrier cannot shunt the energy, allowing high voltage to pass directly into the hazardous area and ignite an explosion. Furthermore, a single ground fault on the field sensor can disable the protection loop.
• Galvanic Isolators: These are active electronic devices that use optocouplers, transformers, or capacitors to physically isolate the safe-area and hazardous-area circuits. There is no direct copper connection between the control panel and the field instrument.
  • The Advantage: Because the field circuit is completely floating and isolated from the safe-area power source, a galvanic isolator does not require a dedicated IS ground. This eliminates the single point of failure inherent to Zener barriers and makes the installation significantly cheaper, easier to maintain, and far more robust against ground loops and noise.

Because of this, modern industrial facilities have almost completely standardized on galvanic isolators for intrinsically safe instrument loops.

3. Actionable Takeaways

• Prefer Galvanic Isolators: Specify galvanic isolators rather than Zener barriers for all new intrinsically safe instrument loop designs. This eliminates the high maintenance costs and risks associated with maintaining a dedicated IS ground.
• Maintain Separations: Ensure that the safe-area wiring (black/standard) and intrinsically safe wiring (light blue) inside the control panel are kept physically separated by at least 2 inches (50 mm) or separated by a partition barrier to prevent electromagnetic coupling.
• Test Zener Ground Resistance: If your facility still utilizes Zener barriers, establish a strict preventative maintenance schedule to test the resistance of the dedicated IS grounding conductor back to the main ground bus. It must remain below 1 ohm.
• Use Light Blue Cables: Always use light blue jacketed cabling for all intrinsically safe wiring runs in the field, making it immediately clear to technicians that the circuit is intrinsically safe and must not be connected to standard power sources.