- Purpose of Intrinsic Safety
- Components of an Intrinsically Safe Loop
- Understanding Intrinsic Safety (IS) Parameters and Certification Process
- Certification Agencies for Intrinsic Safety Equipment
- Types of Approval Concepts
- Intrinsic Safety Entity Parameter Compatibility Checks
- Intrinsic Safety Entity Parameters Explained
- Intrinsic Safety Cable Capacitance and Inductance Calculation
- Practical Intrinsic Safety Calculation Example for a Pressure Transmitter
- How to Calculate Maximum Cable Length for an Intrinsic Safety Loop
- Common Intrinsic Safety Calculation Mistakes
- Intrinsic Safety Design Considerations for Process Plants
- Frequently Asked Questions About Intrinsic Safety Calculation
Intrinsic safety calculation is an essential part of instrumentation design for hazardous areas where flammable gases, vapours or combustible dust may create an explosion risk. It verifies whether the field instrument, safety barrier or galvanic isolator, and connecting cable meet the applicable electrical compatibility requirements.
This guide explains how to perform intrinsic safety entity parameter calculations, verify voltage, current and power compatibility, calculate cable capacitance and inductance, and determine whether an instrument loop is suitable for the proposed installation.
The procedure is useful for instrumentation design engineers, EPC teams, hazardous area specialists, commissioning engineers and maintenance professionals working in refineries, oil and gas facilities, petrochemical plants and other process industries.
Purpose of Intrinsic Safety
Strict safety measures must be taken when instruments are utilized in areas with flammable gases, vapors, liquids, combustible dust, or ignitable fibers. There are several different protection strategies available, each with its own set of benefits and drawbacks.
Among these strategies, intrinsic safety is the most dependable and simple to implement. It ensures that equipment and wiring in hazardous areas do not discharge enough electrical or thermal energy to ignite the hazardous atmosphere, even under normal or extraordinary circumstances.
Intrinsic safety is achieved by limiting the energy available to electrical equipment in the potentially hazardous region. This limitation ensures that the energy remains below the environmental igniting threshold.
According to Article 504 of the National Electrical Code (NEC), an intrinsically safe system is “an assembly of interconnected intrinsically safe apparatus, associated apparatus, and interconnecting cables where the hazardous location circuits are intrinsically safe.”
Components of an Intrinsically Safe Loop
An intrinsically safe loop consists of the following components:
Intrinsically Safe Apparatus
- Non-simple devices installed in hazardous areas.
- These devices store or generate energy, and hence, must be certified as intrinsically safe.
- Example: Transmitters, flowmeters.
Associated Apparatus
- Those devices that are installed in safe regions and serve as an interface between safe and hazardous zones.
- Example: Intrinsic safety barriers, isolators.
Interconnecting Cables
- Copper conductor cables, either single-pair or multi-pair.
- These cables must be carefully selected to ensure compatibility with the other components.
Refer the below link to know What is intrinsically safe system and what is its importance?
Understanding Intrinsic Safety (IS) Parameters and Certification Process
Intrinsic safety (IS) is a crucial safety concept for preventing electrical equipment from igniting hazardous areas, such as those found in industries dealing with flammable gases, vapors, or dust. It involves designing equipment and wiring to limit energy levels (voltage, current, capacitance, and inductance) so that they are below the ignition thresholds of explosive atmospheres.
Key Entity Parameters
Entity parameters are assigned to both intrinsically safe apparatus (instruments) and associated apparatus (safety barriers). These parameters ensure compatibility and safety in an intrinsically safe loop.
Intrinsically Safe Apparatus (Instruments)
- Vmax (Ui): Maximum voltage that can be safely applied to the instrument.
- Imax (Ii): Maximum current that can be safely applied to the instrument.
- Ci: Internal capacitance of the instrument.
- Li: Internal inductance of the instrument.
Associated Apparatus (Safety Barriers)
- Voc (Uo): Maximum open-circuit voltage under fault conditions.
- Isc (Io): Maximum short-circuit current under fault conditions.
- Ca: Maximum allowable external capacitance.
- La: Maximum allowable external inductance.
Click here to know more about Difference Between Intrinsically Safe and Explosion-Proof
Certification Agencies for Intrinsic Safety Equipment
The most prevalent certifying agencies are:
| Country | Agency |
| USA | FM, UL |
| Canada | CSA |
| Great Britain | BASEEFA |
| France | LCIE |
| Germany | PTB |
| Italy | CESI |
| Belgium | INEX |
Note:It is important to note that approval from any of the European agencies listed above provides a CENELEC approval, which enables the units to be regarded approved in a number of European nations.
Types of Approval Concepts
There are two main certification concepts for equipment used in hazardous environments:
LOOP Concept
Specifies the specific part numbers and products that may be utilized in the loop. No variations from these units are permitted.
ENTITY Concept
Refer the below link for for the Installation Checklist for Intrinsically Safe Instrument (Apparatus)
Intrinsic Safety Entity Parameter Compatibility Checks
Before connecting a field instrument to an intrinsic safety barrier or galvanic isolator, compare the certified output parameters of the associated apparatus with the certified input parameters of the field instrument.
The following five checks form the basic entity compatibility assessment.
| Parameter | Required condition | Meaning |
| Voltage | Uo ≤ Ui | Barrier output voltage must not exceed the instrument input voltage rating |
| Current | Io ≤ Ii | Barrier output current must not exceed the instrument input current rating |
| Power | Po ≤ Pi | Barrier output power must not exceed the instrument input power rating |
| Capacitance | Ci + Ccable ≤ Co | Instrument and cable capacitance must remain within the permitted limit |
| Inductance | Li + Lcable ≤ Lo | Instrument and cable inductance must remain within the permitted limit |
Here, Uo, Io, Po, Co and Lo are the certified output parameters of the associated apparatus. Ui, Ii, Pi, Ci and Li are the certified input parameters of the intrinsically safe field instrument.
The loop can pass these basic entity checks only when all applicable conditions are satisfied. Additional restrictions in equipment certificates, control drawings, gas group requirements and applicable installation standards must also be verified before the design is approved.
Intrinsic Safety Entity Parameters Explained
Uo: Maximum Output Voltage
Uo is the maximum certified output voltage of the associated apparatus, such as an intrinsic safety barrier or galvanic isolator. It must not exceed the Ui rating of the connected field instrument.
Io: Maximum Output Current
Io is the maximum certified output current of the associated apparatus. It must be less than or equal to the Ii rating of the field instrument.
Po: Maximum Output Power
Po is the maximum certified output power available from the associated apparatus. It must not exceed the Pi rating of the field instrument.
Co: Maximum External Capacitance
Co specifies the permitted external capacitance for the associated apparatus under its certification conditions. The combined capacitance of the field instrument and connecting cable must remain within this limit.
Lo: Maximum External Inductance
Lo specifies the permitted external inductance for the associated apparatus under its certification conditions. The combined inductance of the field instrument and connecting cable must remain within this limit.
Ui: Maximum Input Voltage
Ui is the maximum certified input voltage of the intrinsically safe field instrument. It must be greater than or equal to the barrier Uo value.
Ii: Maximum Input Current
Ii is the maximum certified input current of the field instrument. It must be greater than or equal to the barrier Io value.
Pi: Maximum Input Power
Pi is the maximum certified input power of the field instrument. It must be greater than or equal to the barrier Po value.
Ci and Li: Internal Capacitance and Inductance
Ci is the internal capacitance and Li is the internal inductance specified for the field instrument. These values must be included when calculating the total capacitance and inductance of the loop.
Intrinsic Safety Cable Capacitance and Inductance Calculation
The connecting cable contributes capacitance and inductance to an intrinsically safe loop. These values must be included when checking the compatibility of the barrier and field instrument.
Cable Capacitance Formula
Cable capacitance = Cable capacitance per unit length × Cable length
Total loop capacitance = Instrument capacitance + Cable capacitance
The required condition is:
Ci + Ccable ≤ Co
Cable Inductance Formula
Cable inductance = Cable inductance per unit length × Cable length
Total loop inductance = Instrument inductance + Cable inductance
The required condition is:
Li + Lcable ≤ Lo
Practical Intrinsic Safety Calculation Example for a Pressure Transmitter
Consider a pressure transmitter installed in a hazardous area and connected to a certified intrinsic safety barrier in the safe area.
Field Instrument Parameters
| Parameter | Value |
| Ui | 28 V |
| Ii | 110 mA |
| Pi | 0.80 W |
| Ci | 0.05 µF |
| Li | 0.10 mH |
Barrier Parameters
| Parameter | Value |
| Uo | 24 V |
| Io | 100 mA |
| Po | 0.60 W |
| Co | 0.083 µF |
| Lo | 0.20 mH |
Cable Parameters
Cable length = 500 ft
Cable capacitance = 60 pF per ft
Cable inductance = 0.20 µH per ft
Step 1: Check Voltage Compatibility
Uo ≤ Ui
24 V ≤ 28 V
Result: Pass
Step 2: Check Current Compatibility
Io ≤ Ii
100 mA ≤ 110 mA
Result: Pass
Step 3: Check Power Compatibility
Po ≤ Pi
0.60 W ≤ 0.80 W
Result: Pass
Step 4: Calculate Total Capacitance
Cable capacitance = 500 × 60 pF
Cable capacitance = 30,000 pF = 0.030 µF
Total capacitance = 0.050 + 0.030
Total capacitance = 0.080 µF
Compare with Co:
0.080 µF ≤ 0.083 µF
Result: Pass
Step 5: Calculate Total Inductance
Cable inductance = 500 × 0.20 µH
Cable inductance = 100 µH = 0.10 mH
Total inductance = 0.10 + 0.10
Total inductance = 0.20 mH
Compare with Lo:
0.20 mH ≤ 0.20 mH
Result: Pass at the stated limit
Overall Result
All five basic entity parameter checks satisfy the stated limits for this illustrative example. However, the inductance result has no margin, and the complete loop must still be verified against the actual equipment certificates, cable data, applicable restrictions and approved control drawing before acceptance.
How to Calculate Maximum Cable Length for an Intrinsic Safety Loop
The available capacitance and inductance limits can be used to estimate the maximum cable length permitted by each parameter.
Maximum Cable Length Based on Capacitance
Maximum cable length = (Co − Ci) ÷ Cable capacitance per unit length
Maximum Cable Length Based on Inductance
Maximum cable length = (Lo − Li) ÷ Cable inductance per unit length
The permitted length must satisfy both applicable limits. Use consistent units throughout the calculation.
For example, if the capacitance calculation permits 400 metres and the inductance calculation permits 300 metres, the inductance limit is more restrictive. The maximum permitted length cannot exceed 300 metres on the basis of these two calculations alone.
The final design must also account for the cable manufacturer’s data, equipment certification conditions and any additional circuit restrictions. A mathematical cable length result does not by itself establish intrinsic safety compliance.
Common Intrinsic Safety Calculation Mistakes
| Mistake | Why It Matters |
| Reversing Uo and Ui comparison | Can lead to an incorrect voltage compatibility decision |
| Comparing normal operating current instead of certified Io | Uses the wrong current parameter |
| Ignoring Po and Pi | Leaves power compatibility unverified |
| Omitting cable capacitance | Underestimates total loop capacitance |
| Omitting cable inductance | Underestimates total loop inductance |
| Mixing nF and µF | Can create a significant calculation error |
| Mixing µH and mH | Can produce an incorrect inductance result |
| Using assumed cable data for final approval | May misrepresent the actual cable characteristics |
| Ignoring certificate restrictions | Can invalidate an otherwise acceptable basic comparison |
| Checking only the barrier and transmitter | Leaves other installation requirements unverified |
| Ignoring signal compatibility | May result in an unsuitable interface for the instrument |
| Failing to update loop documentation | Leaves the approved design inconsistent with the installation |
Always use certified parameter values and verify the complete loop before approving the installation.
Intrinsic Safety Design Considerations for Process Plants
Entity parameter compatibility is an important part of intrinsic safety design, but it is not the only requirement.
Instrumentation engineers should also verify:
- The hazardous area classification and applicable equipment protection level.
- The gas group and temperature classification.
- The certification of the field instrument and associated apparatus.
- Cable segregation, routing and installation requirements.
- Grounding requirements for the selected barrier type.
- Compatibility with the required 4 to 20 mA, HART or other signal function.
- Any additional capacitance, inductance or resistance restrictions stated in the certificates.
- The approved control drawing and system documentation.
- Compliance with the applicable hazardous area design and installation standards.
A loop that passes the five basic entity checks must not automatically be considered approved. The complete installation must meet the requirements of the applicable certificates, standards and project documentation.
Click here for Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation?
Frequently Asked Questions About Intrinsic Safety Calculation
What is intrinsic safety calculation?
Intrinsic safety calculation verifies the compatibility of a field instrument, associated apparatus and connecting cable for a hazardous area loop. It includes entity parameter checks and other applicable installation requirements.
What are the five intrinsic safety entity parameter checks?
The five basic checks are voltage, current, power, capacitance and inductance. The conditions are Uo ≤ Ui, Io ≤ Ii, Po ≤ Pi, Ci + Ccable ≤ Co and Li + Lcable ≤ Lo.
What is the difference between Ui and Uo?
Ui is the maximum certified input voltage of the field instrument, while Uo is the maximum certified output voltage of the associated apparatus. The required condition is Uo ≤ Ui.
What is the difference between Ii and Io?
Ii is the maximum certified input current of the field instrument, while Io is the maximum certified output current of the associated apparatus. The required condition is Io ≤ Ii.
What is the difference between Ci and Co?
Ci is the internal capacitance of the field instrument, while Co is the maximum permitted external capacitance of the associated apparatus. Instrument and cable capacitance together must not exceed Co.
What is the difference between Li and Lo?
Li is the internal inductance of the field instrument, while Lo is the maximum permitted external inductance of the associated apparatus. Instrument and cable inductance together must not exceed Lo.
How do I calculate intrinsic safety cable capacitance?
Multiply the cable capacitance per unit length by the installed cable length. Add the result to the field instrument capacitance and compare the total with Co.
How do I calculate intrinsic safety cable inductance?
Multiply the cable inductance per unit length by the installed cable length. Add the result to the field instrument inductance and compare the total with Lo.
Can a 4 to 20 mA transmitter be intrinsically safe?
Yes, if the transmitter, associated apparatus, cable and complete loop meet the applicable intrinsic safety requirements. A 4 to 20 mA signal alone does not establish intrinsic safety.
Is an intrinsic safety barrier the same as a galvanic isolator?
Both can serve as associated apparatus in suitable intrinsically safe circuits, but their electrical isolation and grounding arrangements differ. Selection must follow the equipment certification and application requirements.
Can an intrinsic safety loop pass voltage checks but fail capacitance checks?
Yes. A loop may satisfy voltage, current and power limits but exceed the permitted capacitance or inductance limits. Every applicable check must be satisfied.
Which standards are relevant to intrinsic safety calculations?
IEC 60079 series standards address equipment protection and hazardous area installation requirements, including intrinsic safety equipment and installations. Confirm the applicable editions and project requirements before approving a design.
