- Need of an infallible equipotential ground system, that must endure in time
- Need of ground isolated hazardous location devices
- Voltage drop on the series resistor
- Unprotected against short circuits
- Possible measuring errors due to the leakage current of the Zener diodes
Passive Intrinsically Safe Barriers use Zener diodes to limit the voltage toward the hazardous location and fused "infallible" resistors to limit the current.
In order to use such barriers an equipotential ground system, separated by the plant ground and safely connected to a single ground point, must be present. The resistance of the Intrinsically Safe circuit ground connection must be lower than 1 ohm.
If a equipotential ground system is not available, Zener barriers cannot be used.
Hazardous location devices as thermocouples, resistance temperature detectors, transmitters, contacts, electropneumatic converters, electrovalves, etc., must be isolated from ground (at least 500 Vac) or cannot be used.
The Zener diodes leakage current can introduce an error on the low level of the signal.
The barrier series resistance reduces the field voltage, available to transmitters or converters, therefore limiting their use.
A short circuit, even if accidental, can provoke the blowing of the fuse.