What are the difference between Intrinsic Safe and Non-Intrinsic Safe Cables ?

Difference between IS and Non-IS cables

    The IS and Non-IS cables are similar in construction like 1 pair, 2 pair or multi-pair cables with shield, insulation, armor etc.
    The entity parameter of the cables needs to be verified in IS calculations and if these values fulfill the requirements then they can be used in IS loops.
    The entity parameters of cables needs to be checked prior to ordering or selection of cables, specially for use in Gas Group IIC.
    The IS cable outer sheath insulation color is BLUE (most of cases) to identify in the plant which cables are IS cables while non IS cable outer sheath color is either Black or Grey.
    The terminals used in JB and in control cabinets is Blue color for IS circuit where as for Non IS it is generally Grey color.
    Generally IS and Non IS cables are segregated in cable trays/ duct while laying in the field (depending on project specifications).


IS (Intrinsic Safe) Loop calculation

Fundamentally the purpose of the IS circuits is to limit the energy going into the field so as to minimize the risk of ignition in hazardous atmosphere










Article by :
Nilima singh
Instrumentation and control Engineer
13 year experience in Detail engineering
Worked on various Refinery, Oil & Gas, Petrochemical and chemical projects

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For calculation tool

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Are you looking for an intrinsic safety barrier for your IS product?

Intrinsic safety is a system concept, and it is necessary to consider the safety of each component of the system in the loop.

An Intrinsic safety system involves three main components:

    An intrinsically safe field device
    A safety barrier, the energy limiting device
    Intrinsically safe field wiring which connects the barrier to the field device

The challenge is to correctly  select and match the safety barrier with the IS device located in a hazardous area for a safe connection.

The answer involves using the entity concept.

"Entity concept" is a globally recognized method that specifies the maximum energy a given safety barrier can ever deliver. The same method specifies the maximum energy a given field device can ever receive and still be safe.

In this way, the Entity Concept allows for the interconnection of intrinsically safe field devices with safety barriers that are not specifically tested in combination.

The entity concept is based on what are called "entity parameters" - allowable values on which approval was based.

For the intrinsically safe field device, the entity parameters relate to the maximum allowable amount of voltage, current and power which may be received as an input, as well as the maximum allowable equivalent internal capacitance and inductance.

The entity parameters of an IS field device are:

    Ui     the highest acceptable input voltage
    Ii     the highest acceptable input current
    Pi    the highest acceptable input power
    Ci     the maximum equivalent internal capacitance
    Li     the maximum equivalent internal inductance

For the safety barrier, the maximum allowable the entity parameters are  the maximum amount of voltage, current and power the safety barrier can deliver to a hazardous area, as well as the maximum permitted capacitance and inductance that may be safely connected to the output of the safety barrier.  The entity parameters for a safety barrier are listed on its certificate, as well as on its nameplate and installation drawing.

The entity parameters of a safety barrier are:

    Uo    the maximum voltage a barrier can deliver to the hazardous area in the case of a fault
    Io    the maximum current a barrier can deliver to the hazardous area in a fault situation
    Po    the maximum power a barrier can deliver to the hazardous area
    Co    the maximum permitted capacitance that may be safely connected to the output of the safety barrier
    Lo    The maximum permitted inductance that may be safely connected to the output of the safety barrier

In order to have an accurate and safe connection between the safety barrier and the IS field device, the input voltage of the IS field device must be greater  than or equal to the output voltage of the safety barrier. (Uo ≤ Ui)

The same relationship is true for current and power. (Io ≤ Ii and Po ≤ Pi )

The maximum equivalent internal capacitance of the intrinsically safe device must be  less than or equal to the maximum allowed capacitance of the connected safety barrier (Co ≥ Ci). The same is true of the inductance value (Lo ≥ Li).

Because the field wiring cables have capacitance and inductance values, these values must also be taken into account. The total capacitance of the intrinsic safety system is therefore the capacitance of the field device PLUS the capacitance of the cables. This total must be less than or equal to the maximum permitted value of the safety barrier. The same is true for inductance.  (Co ≥ Ci + Ccable and Lo ≥ Li + Lcable )

Summary:

    Uo ≤ Ui
    Io ≤ Ii
    Po ≤ Pi
    Co ≥ Ci + Ccable
    Lo ≥ Li + Lcable



We have created an easy-to-use Intrinsic Safety Wiki to help you find the safety barrier that is safe to provide energy for your intrinsically safe device. Simply fill in the following fields and input the entity parameters of your IS device. Please note that all the fields must be filled. Pay attention to the units for each field. You can also add the maximum length in meters of the connection cable and our IS Wiki will calculate it into the equations. We consider that cables have 200pF/m capacitance value and 1µH/m inductance value.

Notes:

1. You need to check the compatibility of the selected safety barrier with your IS field device regarding the gas group/ dust groups and the equipment protection level (EPL).

2. For installations in which both the Ci and Li of the intrinsically safe apparatus exceed 1% of the Co and Lo parameters of the associated apparatus (excluding the cable), only 50% of Co and Lo parameters are applicable and shall not be exceeded.

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