Intrinsic Safe Calculation for Instrumentation Design Engineers

Intrinsic safety (IS) is an important concept in instrumentation and automation, particularly for engineers working in hazardous situations with fire or explosion hazards. This article will provide a thorough explanation of intrinsic safe calculations, their purpose, components, and how to conduct them successfully. By the conclusion, you’ll see examples of common instruments such as transmitters and switches.

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.”

Verifying that the selected arrangement of intrinsically safe equipment, related equipment, and connecting cables creates a safe loop appropriate for hazardous regions is the main goal of IS calculations.

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

Independent certification authorities must evaluate and approve intrinsically safe equipment before it can be used in hazardous environments. The customer should specify what type of approval is necessary for a certain application. 

The most prevalent certifying agencies are:

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

Specifies the requirements that any authorized intrinsic safety barrier must meet. This approach allows customers to choose barriers from various approved manufacturers as long as certain criteria are met. According to the ENTITY approval concept, two things (an instrument and a barrier) can be connected if the following conditions are satisfied:

Conditions for Interconnection of Intrinsically Safe Equipment (ENTITY Concept)

Where:

• Vmax = Maximum Open Circuit Voltage of the apparatus
• Imax = Maximum Short Circuit Current of the apparatus
• Voc = Maximum Open Circuit Voltage of the safety barrier
• Isc = Maximum Short Circuit Current of the safety barrier
• La = Maximum Allowed Inductance of the safety barrier
• Ca = Maximum Allowed Capacitance of the safety barrier
• Lw = Inductance of the interconnecting wiring
• Cw = Capacitance of the interconnecting wiring
• Ci: Internal capacitance of the instrument.
• Li: Internal inductance of the instrument.

Refer the below link for for the Installation Checklist for Intrinsically Safe Instrument (Apparatus)

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Step-by-Step Procedure for Performing IS Calculations

Intrinsic safety (IS) calculations are critical for providing the safe operation of instrumentation in hazardous environments by determining the compatibility of system components. Engineers may design dependable and safe IS loops for a wide range of applications by evaluating entity specifications and cable properties. These calculations contribute to the integration of intrinsically safe equipment and barriers, thereby preventing risks in explosive circumstances. Practical examples include pressure transmitters and temperature switches, which show how these calculations are used. Engineers in the EPC and EPCM industries must understand IS calculations in order to assure safety and compliance in complicated industrial environments.

To determine the suitability of an intrinsically safe loop, follow these steps:

Step 1: Voltage Condition

Ensure that the voltage rating of the instrument is compatible with the safety barrier:

• Vmax ≥ Voc.

Step 2: Current Condition 

Ensure that the current rating of the instrument is greater than the safety barrier by performing the following checks:

• Imax ≥ Isc.

Step 3: Capacitance Condition 

The system’s overall capacitance, including that of the instrument and cable, cannot be greater than the permissible capacitance of the safety barrier:

• Ci + Ccable ≤ Ca.

Step 4: Inductance Condition 

The total inductance of the system, including the instrument and cable, must not exceed the safety barrier’s allowable inductance:

• Li + Lcable ≤ La.

Default Cable Parameter Values

When the actual capacitance and inductance values of the cable are unavailable, default values as recommended by ANSI/ISA-RP 12.6 can be used:

• Cable Capacitance: 60 pF/foot.
• Cable Inductance: 0.20 μH/foot.

Practical Examples of Intrinsically Safe Instruments

Example 1: Pressure Transmitter

Instrument Parameters:

• Vmax = 28V
• Imax = 110mA
•  Ci = 0.05μF
• Li = 0.1mH

Barrier Parameters:

• Voc = 24V
• Isc = 100mA, 
• Ca = 0.083μF
• La = 0.2mH

Cable Capacitance and Inductance (Default):

• Capacitance: 60 pF/foot
• Inductance: 0.2 μH/foot

Calculation:

Voltage condition: 

28V ≥ 24V (Pass)

Current condition: 

110mA ≥ 100mA (Pass)

Capacitance condition: 

0.05μF + (Cable length × 60pF/foot) ≤ 0.083μF
Max cable length = (0.083μF – 0.05μF) / 60pF/foot = 550 feet

Inductance condition: 

0.1mH + (Cable length × 0.2μH/foot) ≤ 0.2mH
Max cable length = (0.2mH – 0.1mH) / 0.2μH/foot = 500 feet

Result: 

The maximum cable length for this loop is limited to 500 feet.

Example 2: Temperature Switch

Instrument Parameters:

• Vmax = 20V
• Imax = 50mA
• Ci = 0.02μF
• Li = 0.05mH

Barrier Parameters:

• Voc = 18V
• Isc = 40mA
• Ca = 0.1μF
• La = 0.15mH

Calculation:

Voltage condition

20V ≥ 18V (Pass)

Current condition

50mA ≥ 40mA (Pass)

Capacitance condition

0.02μF + (Cable length × 60pF/foot) ≤ 0.1μF
Max cable length = (0.1μF – 0.02μF) / 60pF/foot = 1,333 feet

Inductance condition

0.05mH + (Cable length × 0.2μH/foot) ≤ 0.15mH
Max cable length = (0.15mH – 0.05mH) / 0.2μH/foot = 500 feet

Result

The maximum cable length for this loop is limited to 500 feet.

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