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Safety Instrumented System (SIS)

Intrinsic Safe Calculation

The intrinsic safe calculation is an activity done by Instrumentation Engineers in EPC or EPCM Industries. In this article, we shall see why is it done and How to do it.

Purpose of Intrinsic Safe

When Instrument is used in areas where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers, then special precautions to be taken in equipment construction and installation to ensure safe performance. There are several protection techniques in common use, each of which has its own set of advantages and disadvantages.

Intrinsic safety” is the safest and most easily implemented protection technique.

Intrinsic safety technique required the use of equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions.

Which is achieved by limiting the energy available to, and stored or generated by, the electrical equipment in the hazardous location to a level below that which can ignite the hazardous atmosphere.

Article 504 of the NEC defines an intrinsically safe system as “an assembly of interconnected intrinsically safe apparatus, associated apparatus, and interconnecting cables in which those parts of the system that may be used in hazardous locations are intrinsically safe circuits. “

The purpose of I.S calculation is to ensure that a selected combination of intrinsically safe apparatus, associated apparatus, and interconnecting cables are forming an Intrinsically safe loop that can be used in the hazardous area.

Components of IS Loop

Intrinsically safe apparatus

  • Non-simple device to be installed in a hazardous area location.
  • The non-simple device is one that stores or generates energy. Hence, non-simple devices have the entity parameters and must be certified as “Intrinsically safe” for use in the hazardous area. e.g. Transmitter

Associated apparatus

  • A device that is installed in a safe area and works as an interface between a hazardous location and a safe location. e.g. Intrinsic safety barrier

Interconnecting cables

  • Copper conductor single pair, Multipair cables.

Before moving on to IS Calculation, we must first understand the concept of Entity Parameters

Entity Parameters

Each apparatus is examined separately by Certification agencies and assigned a set of parameters, known as entity parameters.

The entity concept allows the user to identify acceptable combinations of intrinsically safe apparatus and associated apparatus.

Entity parameters categories as “Intrinsically safe apparatus” & “Associated apparatus”.

Entity parameters of Intrinsically safe apparatus (INSTRUMENT)

Vmax(Ui): Maximum voltage that may be applied safely to the intrinsic safe apparatus

Imax(Ii): Maximum current that may be applied safely to the intrinsic safe apparatus

Ci: Internal capacitance of intrinsic safe apparatus

Li: Internal inductance of intrinsic safe apparatus

Entity parameters of Associated apparatus (SAFETY BARRIER)

Voc (Uo): Maximum open circuit voltage that can appear across the intrinsically safe connections of the associated apparatus under fault condition

Isc (Io): Maximum short circuit current that can be drawn from the intrinsically safe connections of the associated apparatus under fault condition

Ca: Maximum capacitance that can be connected safely to the associated apparatus

La: Maximum inductance that can be connected safely to the associated apparatus

Performing IS Calculation

Evaluating conditions for Intrinsically safe Loop:

  • Vmax must be greater than or equal to Voc
  • Imax must be greater than or equal to Isc
  • Ci + Ccable must be less than or equal to Ca
  • Li + Lcable must be less than or equal to La

Actual value (capacitance and inductance) of cable to be referred from the cable manufacture catalog or if details are not available then as recommends in ANSI/ISA-RP 12.6 below-mentioned default values to be considered.

  • Cable capacitance: 60 pF/foot
  • Cable inductance: 0.20 μH/foot

In general practice, using entity parameters of Instrument and Barrier we actually find the max length of the cable we can lay for a particular IS loop.

Author: Kalpit Patel

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Intrinsic Safe Calculation

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