What are Intrinsic Safety Barriers?
Safety barriers are protective devices that work to limit energy to the field within the minimum ignition level of the hazardous mixture.
Barriers must be used to interconnect electrical appliances located in a hazardous location with electrical appliances located in a non-hazardous location (associated appliances).
There are two types of barriers,
- Diode safety barriers, or “passive” barriers,
- Galvanically isolated safety barriers or “active” barriers
Circuit diagram for passive barriers is shown below:
Working of Passive barriers:
If there is a dangerous voltage coming from the safe area (250 Vac max.), The Zener barrier diverts the fault current to the ground until the fuse breaks, thus maintaining a “safe” (Voc) open circuit voltage to the hazardous location, while the maximum short-circuit current in the field is defined by
Isc = Voc / Rlim.
Advantages of Passive barriers:
- Lower component costs
- Simple and reliable operation.
- The possibility of transforming a standard device into an intrinsically safe system.
- More flexibility
Limitations of Passive barriers:
- The requirement of an equipotential ground system
- The existence of problems with the current return caused by the absence of input / output isolation.
- The reduction of the available voltage for the transmitter caused by the limiting resistance, and the introduction of errors when the limiting resistor is connected to resistance temperature detectors.
- Creates errors by the limiting zener due to the leakage of current toward ground.
- The possibility of permanent damage to the barrier in case of a fault or an incorrect connection.
Working of Active barriers:
This configuration does not allow the dangerous voltage (250 Vac max) that is present in the terminal blocks, which are in a non-hazardous location, to be transferred to the power limiting circuit that it must be able to tolerate, during a fault condition. , the maximum voltage of the secondary side.
Since the entire circuit is floating with respect to earth, it is not possible for the fault current, due to the 250 Vac, to pass through the power limiting circuit; therefore, it is not necessary to ground the power limiting circuit.
Advantages of Active barriers:
- A grounded system is not required
- Grounded sensors can be used
- Avoids the problems of the return currents and allows a high common-mode rejection.
- Output signals can be directly used
Limitations of Active barriers:
- Higher component costs, although installation costs are more comparable.
- Designed for specific applications, so they are less flexible.