What is a safety barrier?
- Safety barriers are intrinsically safe Shunt diodes.
- These barriers are passive network devices that use Zener diodes, resistors, and fuses to deflect surplus or extra electrical energy to the ground via earthing systems.
- To prevent sparking or overheating of plant equipment located in hazardous areas.
- Safety barrier is a circuit between the control room and field equipment to restrict energy.
- These intrinsically safe barriers are used to link or connect between two electrical devices located in a hazardous area and safe area.
What are the types of intrinsic safe barriers used?
The two types of intrinsic safe barriers are
- Passive Barriers
- Galvanically Isolated Barriers
Passive Barriers:
- Passive barriers don’t have any operational parts.
- These barriers have the ability to stop or block risks on their own.
- Passive barriers consume or arrest energy and transfer the same energy to their foundation.
- These can be individual barriers or part of a large-scale passive barrier system.
Galvanically Isolated Barriers:
- Galvanically Isolated Barrier deals with any kind of difficult issues that are not resolved by the Zener Barrier.
- In fact, these barriers restrict potentially dangerous energy from being transmitted to intrinsically safe circuits instead of dissipating it.
What are active and passive barriers?
Active Barriers:
- Active barriers consist of moving elements.
- These barriers restrict the hazardous AC voltage source of 250 Vmax present in the terminal blocks which are located in a non-hazardous location, to be transferred to the power limiting circuit that must have the ability to tolerate the case of a fault condition.
- Since the entire circuit is floating with respect to the earth, it is not possible for the fault current, due to the 250 Vac, to pass through the power limiting circuit therefore, grounding of the power limiting circuit is not required.
Passive Barriers:
- Passive barriers do not have moving elements
- If there are any dangerous AC voltages of 250 V max coming from the safe area.
- The Zener barrier diverts the fault current to the ground until the fuse breaks, thus maintaining a safe open circuit voltage to the hazardous location.
- The maximum short-circuit current in the field is given by
How does a safety barrier physically work?
- The barrier consists of a series resistor and a fuse with a Zener diode with earthed or grounded.
- These series resistors restrict the current to around 100 mA from a 28 V supply when the dangerous terminals are shorted.
- The Zener diode functions at the 30 V regions and the fuse has a current of around 30 mA.
- These barriers certify that more current and voltage are blown, and restrict dangerous energy levels away from the hazardous region.
Why do we use safety barriers in PLC systems?
- An isolated safety barrier minimizes the probability of a ground loop by separating the field device from the PLC.
- Safety barriers are best selected to isolate field signals because it is quite hard to predict all possible ground loops in the design before commissioning the system.
Where are intrinsically safe barriers used?
Intrinsic safety barriers are used in a variety of applications such as.
- Switches.
- Solenoids.
- Led Indicators.
- Load Cells.
- Potentiometers.
- Resistance Temperature Detectors (RTDs).
- Strain Gauges.
What do you mean by one-way and two-way barriers?
One Way Barrier:
- The security barrier secures failure in the safe region such as the control room.
- Because it will not provide sufficient energy to flame up the gaseous atmosphere in the hazardous area.
- However, this system is said to be intrinsically unsafe, if each and every piece of electrical equipment in the circuit is not intrinsically safe.
Two-Way Barrier:
- The one-way barrier is not convenient for 4-20mA current loops that are floating on either side of the ground. Two-way barriers are adopted with a drifting supply.
- This barrier has additional Zener and resistances.
- The resistance block consists of three diodes connected in series.
- This block functions as a
- Resistance to allow reverse current to pass during normal operating conditions.
- Diode to block or halt the flow of hazardous reverse current during faulty operating conditions.
- Grounding of resistance less than 1ohm is good and important.
- Each and every earth cable should be grounded at the same point.
- The loop current circulates on the landline if grounding is not done at the same point.
- If this happens there is a possibility of damage to other equipment in the system.
The below schematic diagram represents a typical Zener barrier earthing system.
Explain the difference between Barrier and Isolator:
Let us consider an example of Zener barriers and galvanic isolators to distinguish between them.
- These Barriers and Isolators are used in instruments and circuits that are intrinsically safe.
- The loop current is minimized by the barrier to a safe level to avoid spark and ignition.
- The galvanic isolator is used to interface signals of varying common mode voltages.
The following table mentions the differences between Zener barriers and Galvanic isolators.
Zener Barrier Vs Galvanic Isolator:
| Zener Barrier | Galvanic Isolator |
|---|---|
| Simple with a good mean time between failures. | Complex with a low mean time between failures value. |
| Used when the front-end engineering demand is highly adaptable. | It is used in application-specific requirements. |
| It has low dissipation. | It has a high power dissipation of about 2 VA. |
| It is loop powered. | It has a separate power supply |
| Used where a constricted voltage source is available in a dangerous region. | These are used where a maximum voltage source is available in hazardous and safe areas. |
| It has a higher packing density. | It has a lower packing density. |
| It has a lower cost. | Its cost is high. |
| The frequency response is good. | It has a limited frequency response. |
| It has higher accuracy and linearity. | It has lower accuracy and linearity. |
| It will force zero volt levels on the system. | It provides complete Isolation between signals. |
| It is more exposed to lightning and other surges | It is less exposed to lightning and other surges |
What are the Advantages and Disadvantages of active barriers?
Advantages of Active barriers:
- A grounded system is not required.
- Grounded sensors can be used.
- Escapes from the trouble of the return currents and allow a high common-mode rejection.
- Output signals can be directly used.
Disadvantages of Active barriers:
- Higher component costs, although installation costs are more comparable.
- These are less flexible because they are designed for particular applications, so they are less flexible.
What are the Advantages and Disadvantages of passive barriers?
Advantages of Passive barriers:
- Lower component costs.
- Simple and reliable operation.
- The chance of modifying a standard device into an intrinsically safe system.
- More flexibility.
Disadvantages of Passive barriers:
- An equipotential ground system is required.
- The existence of trouble or error with the return current is caused by the absence of input and output isolation.
- Limiting resistance causes trimming of available voltage from the transmitter.
- Errors are introduced when RTDs are connected to limiting resistors.
- Creates errors by limiting Zener due to the leakage of current toward the ground.
- Due to an incorrect connection or fault, there is a chance of permanent damage to the barrier system.
What are the applications of safety barriers?
Safety barriers are used in intrinsic safety systems (explosion-proof technology) to safeguard the production pieces of equipment or production systems from accidents by miss operation.
