Okay, you've identified a common area of confusion when designing intrinsically safe circuits. Let's break down the issues you're encountering with the Entity Concept and those Pepperl+Fuchs control drawings.

Understanding the Entity Concept and Intrinsic Safety Parameters

The Entity Concept is a simplified method for interconnecting intrinsically safe apparatus. The core idea is ensuring the output parameters of the non-hazardous area device (like your KFD2-SRA-Ex4 switch amplifier) are always less than the input parameters of the hazardous area device (your NCN3-F25-N4-V1 sensor) with appropriate safety factors built-in.The parameters you are concerned about (Voc, Isc, Po, Co, Lo, Vi, Ii, Ci, Li) represent the maximum levels of voltage, current, power, capacitance, and inductance, respectively. Voc (Voltage open circuit)

The maximum open-circuit voltage the non-hazardous area device can produce under worst-case fault conditions.

Isc (Current short circuit)

The maximum short-circuit current the non-hazardous area device can produce under worst-case fault conditions.

Po (Power output)

The maximum power the non-hazardous area device can deliver to the hazardous area device under worst-case fault conditions.

Co (Capacitance output)

The maximum allowed total capacitance the non-hazardous area device can drive, including cable capacitance.

Lo (Inductance output)

The maximum allowed total inductance the non-hazardous area device can drive, including cable inductance.

Vi (Voltage input)

The maximum voltage the hazardous area device can withstand without becoming a source of ignition.

Ii (Current input)

The maximum current the hazardous area device can withstand without becoming a source of ignition.

Ci (Capacitance input)

The internal capacitance of the hazardous area device.

Li (Inductance input)

The internal inductance of the hazardous area device.

Why is `Po` Sometimes Missing?

You are correct to question the omission of `Po` on some control drawings. Here's why it might be missing, and why you should proceed with caution:1. Fault Calculation Based on Voc/Isc

Sometimes, `Po` isn't explicitly stated because the entity standards assume that it can be derived from Voc and Isc. Specifically, `Po` = (Voc Isc) / 4. This is a simplified calculation based on the maximum power transfer theorem.
2.

Standard Requirements The different standards will treat `Po` differently. You will need to check with the applicable standard that is relevant to your system.3. Oversight

While unlikely from a reputable manufacturer like Pepperl+Fuchs, there's a small chance it's an omission.

Therefore, DO NOT assume `Po` is unimportant, even if not explicitly stated. In absence of the parameter on the control drawing, you should calculate the output power and verify the device's documentation to ensure compliance.

Analyzing the Control Drawings KFD2-SRA-Ex4 (Switch Amplifier)

You've correctly identified the Voc, Isc, Co, and Lo values. You must calculate `Po`. Use the formula: Po = (Voc Isc) / 4 = (10.6V 0.0195A) / 4 = ~0.0516W or 51.6mW. These parameters represent the maximum output capabilities of the switch amplifier under fault conditions.

NCN3-F25-N4-V1 (Proximity Sensor)

This is where your biggest problem lies. You need to understand what "Type 1, 2, and 3" refer to. Do not proceed without clarification from the manufacturer or distributor!



Possible Interpretations of Type 1, 2, and 3 (Without Clarification, These are Just Educated Guesses)



ATEX Category

Could refer to different ATEX equipment categories (e.g., different levels of protection for Zone 0, Zone 1, or Zone 2). But you mentioned NEC in USA, and it is very unlikely this would be the case

Internal Resistors

Might be tied to how the sensor is internally protected or configured to meet intrinsic safety requirements, and is not being expressed clearly.

Fault Conditions Assumed During Testing

Could be tested under different fault conditions.

Crucially

Each "Type" will have a different set of Vi, Ii, Pi, Ci, and Li values. You must know which set of parameters applies to your specific application.

Recommendation

Contact Pepperl+Fuchs technical support and explicitly ask what "Type 1, 2, and 3" represent in the context of US/NEC Class I, Zone 1/2 certification. Get the full set of Vi, Ii, Pi, Ci, and Li values for the "Type" appropriate for your installation.

Is it Safe to Conclude You Can Use These Sensors with the Amplifier?

The answer is: NOT YET!You can only determine safety after you know the Vi, Ii, Pi, Ci, and Li values for the correct "Type" of sensor configuration and properly calculate the output power on the switch amplifier control drawing:1.

Calculate Po

You did this correctly: Po = (Voc Isc) / 4 (for the switch amplifier). Ensure this aligns with the control drawing.2.

Compare Parameters

Only after you know the sensor parameters can you check: Voc < Vi Isc < Ii Po < Pi Co > Ccable + Ci Lo > Lcable + Li3.

Cable Parameters

You are correct that for short cable runs, Ccable and Lcable are often negligible. However, document your assumption. If the cable run is longer, you must factor in the cable's capacitance and inductance per unit length. Consult the cable manufacturer's data sheet.

Addressing Your Specific Scenario (Wiring Two Sensors to One Amplifier)Wiring two sensors into one amplifier channel might be possible, but requires VERY careful analysis and likely isn't advisable. Here's why:

Combined Parameters

You'd need to determine the combined Ci and Li values of both sensors connected in parallel. The calculations depend on whether they're truly in parallel or if internal resistors within the sensors influence the equivalent inductance and capacitance.

Fault Propagation

Consider what happens if one sensor fails in a specific way (e.g., a short circuit). Could that failure propagate to the other sensor or the amplifier channel and compromise intrinsic safety?

Certification Implications

This configuration might invalidate the original certifications of the devices. It's crucial to confirm that wiring sensors in this way is explicitly supported by the manufacturers of both the sensor and the switch amplifier, and that the entire circuit retains its intrinsic safety rating.

In summary, while it might be appealing to reduce the number of amplifier channels, combining sensor signals on one channel adds significant complexity and risk and is typically avoided in IS design.

Recommendations1.

Contact Pepperl+Fuchs

Get clarification on the "Type 1, 2, and 3" designations for the sensor and obtain the full set of Vi, Ii, Pi, Ci, and Li values for the appropriate "Type" applicable to your installation conditions.2.

Calculate Output Power

Perform a precise calculation of `Po` from the switch amplifier parameters, ensuring it agrees with the control drawing.3.

Verify Entity Parameters

Compare Voc, Isc, Po, Co, and Lo from the switch amplifier to the Vi, Ii, Pi, Ci, and Li values of the sensor (after you have the correct values).4.

Avoid Combining Sensor Signals

Seriously reconsider wiring two sensors into one amplifier channel unless it is explicitly permitted and supported by the manufacturers. The potential risks and complexity outweigh the benefits.5.

Document Everything

Thoroughly document all your assumptions, calculations, and the reasoning behind your design decisions. This is essential for safety reviews and future maintenance.6.

Consult with an Intrinsic Safety Expert

If you are not highly experienced in designing intrinsically safe circuits, it is strongly recommended to consult with a qualified expert to review your design. Intrinsic safety is complex, and errors can have serious consequences.

Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional engineering advice. Always consult with qualified professionals and refer to the relevant standards and regulations for your specific application. Flag for review