In the world of industrial automation, preventing explosions in hazardous areas is paramount. Whether you are working in oil and gas, pharmaceuticals, or chemical processing, Intrinsic Safety (IS) is the gold standard for protection. However, when designing these systems, engineers often hit a wall of confusion regarding two specific methodologies: the Entity Concept and FISCO.
While both methods aim to limit energy to prevent ignition, they apply to different technologies and require different verification processes. If you are struggling to understand how to match barriers with field devices or why Fieldbus systems seem to have their own set of rules, you are in the right place.
This guide breaks down the technical jargon into clear, actionable definitions to help you master Intrinsic Safety system design.
What is the “Entity Concept”?
The Entity Concept is the traditional, universal method used to determine if an Intrinsically Safe system is safe to operate. It is essentially a mathematical matching game between the energy source and the field device.
In an IS loop, you have two main components:
- Associated Apparatus: The energy source (e.g., an IS Barrier or Isolator located in the safe area).
- Intrinsically Safe Apparatus: The field device (e.g., a transmitter or sensor located in the hazardous area).
How It Works (The Verification Process)
Under the Entity Concept, you must compare the safety parameters of the source against the device, while also accounting for the interconnecting cable. For the system to be safe, the following logic must be met:
- Voltage (Uo≤UiUo≤Ui): The maximum output voltage of the barrier must be less than or equal to the maximum input voltage of the device.
- Current (Io≤IiIo≤Ii): The maximum output current must be less than the device’s limit.
- Power (Po≤PiPo≤Pi): The power output must not exceed the device’s power rating.
The “Cable” Complication
The Entity Concept gets tricky when calculating Capacitance (CC) and Inductance (LL). Cables store energy. Therefore, the calculation must ensure that the capacitance and inductance of the device PLUS the cable do not exceed what the barrier can safely handle.
Ccable+Cdevice<CbarrierCcable+Cdevice<Cbarrier
Lcable+Ldevice<LbarrierLcable+Ldevice<Lbarrier
Key Takeaway: The Entity Concept allows for a mix-and-match of different brands and devices, provided you perform the specific calculations for every single loop.
What is “FISCO”?
FISCO stands for Fieldbus Intrinsically Safe Concept.
As industrial networks evolved from simple 4-20mA loops to digital networks like Foundation Fieldbus and PROFIBUS PA, the Entity Concept became too cumbersome. Calculating cable parameters for a network with multiple devices on a single trunk was a nightmare.
Developed by the PTB in Germany and standardized in IEC 60079-11 (and IEC 60079-25/27), FISCO simplifies the certification process for Fieldbus systems.
How It Works (The “Plug-and-Play” Approach)
FISCO creates a standard physical layer. Instead of calculating voltage and current for every device, FISCO assumes a “mask” of safety parameters.
If a power supply, a field device, and the cables are all FISCO Certified, they are automatically compatible without complex calculations. You do not need to calculate inductance and capacitance for the individual devices.
The Rules of FISCO:
- One Source: Only one active power supply per segment.
- Fixed Limits: The system operates within defined voltage and current limits (usually Uo=17.5VUo=17.5V, Io=380mAIo=380mA for IIC gas groups).
- Cable Specs: The cable must meet specific resistance, inductance, and capacitance per kilometer ratings.
- Cable Length: The total length of the trunk and spurs is limited (typically 1000m for IIC Gas Groups).
Key Takeaway: FISCO eliminates the math. If the device has a “FISCO” label, you just need to ensure your cable length is within the standard limit.
Entity vs. FISCO: The Comparison Table
To summarize the differences, here is a quick comparison:
| Feature | Entity Concept | FISCO |
|---|---|---|
| Primary Use | Analog 4-20mA, HART, Discrete I/O | Foundation Fieldbus, PROFIBUS PA |
| Flexibility | High (Mix any rated device) | Medium (Must be FISCO certified) |
| Calculations | Complex (Voltage, Current, Power, L, C) | Minimal (Cable length only) |
| Cable Length | Limited by L/C calculations | Fixed limit (e.g., 1000m for IIC) |
| Device Quantity | Point-to-Point (usually 1 device) | Multi-drop (Many devices per segment) |
Which One Should You Choose?
The choice between Entity and FISCO is usually dictated by the technology you are deploying.
- Choose Entity Concept if: You are wiring traditional 4-20mA transmitters, switches, or solenoids. It is the standard for point-to-point wiring.
- Choose FISCO if: You are designing a modern Fieldbus network. It saves countless engineering hours by removing the need to calculate safety parameters for every node on the network.
Conclusion
Understanding the difference between the Entity Concept and FISCO is crucial for designing safe and compliant hazardous area systems. While the Entity Concept provides the fundamental math for ensuring energy is limited, FISCO offers a streamlined, standardized approach for digital fieldbus networks.
By correctly applying these concepts, you ensure not only compliance with IEC standards but also the safety of personnel and plant assets.
