How to calculate an intrinsically safe loop approval
By G Friend, Extech Safety Systems
Electricity+Control March 2014 (pages 32-34)
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For an explosion to happen, all three reagents need to be present - gas or dust, oxygen and source of ignition (spark or heat). Intrinsic safety works on the principle of removing the source of ignition. This can be achieved by using a zener barrier or a galvanic isolator.
A zener barrier is a device where the voltage is limited by a zener diode and the current by a resistor. The fuse is there to protect the zener diode. The key to safety is the intrinsically safe earth. Without it, there is no protection. If the input voltage increases above the zener diode voltage, the zener conducts and the fuse blows – and the zener barrier has to be replaced. In addition, the zener barrier has a volt drop across it under operating conditions so careful calculation must be done to ensure there is sufficient voltage at the field device.
A galvanic isolator is an active device that limits energy without depending on the intrinsically safe earth for safety. It also has the advantage of supplying higher voltage at the hazardous area terminals and allowing longer cable lengths.
The principle for loop approval is the same for zener barriers and galvanic isolators.
- For an explosion to happen, three reagents need to be present.
- Intrinsic safety works on the principle of removing the source of ignition.
- The key to safety is the intrinsically safe earth.