Is Loop Calculation

KVÆRNER POWERGAS I.S.LOOP CALCULATION

INTRINSIC SAFE

LOOP

CALCULATIONS

File name: I.S. Calculation 1 of 8


“INTRINSIC SAFE” LOOP CALCULATION

1. OBJECTIVE

When ever an I.S device is used in a hazardous atmosphere which is based on the principle of restricting the electrical energy available in a hazardous – area circuits such that any sparks or hot surfaces that may occur as a result of electrical faults are too weak to cause ignition. The primary purpose of this calculation is to show that NO potential problem of stored energy exists on any intrinsic safe (I.S) cable when installed in a project. The actual checking of all I.S laid cables has very little to comment on and not required, but it is desirable that a checking of a couple of the longest laid cables on I.S service with in the plant. This will be sufficient to indicate that all is well and the objective is met.

2. APPLICATION

2.1 General

An Intrinsic safe system apparatus never stands by itself. Generally it is a part of a system in which only certified components must be used in a way to generate the safety of the system as well as compliance to the relevant local regulations. The simplified schematic of an I.S system is shown below.

Non Hazardous Location Hazardous Location

Connecting cable

In general, if the field Instruments are located in a hazardous atmosphere, then it shall be classified for use in that hazardous atmosphere which is achieved by using either an EExi or EExd instruments. As this is an I.S Loop calculation, hence, EExd instrument is not considered.

When an Intrinsic safe execution is adopted for a project, it must also be noted that all “Associated apparatus” shall be EExi certified.


Associated

Apparatus

I.S.

Apparatus


2.2 Analysis of a typical I.S. Installation

Refer to figure 1 shown below, it is possible to completely analyse a particular I.S. system installation.


Connection cable

Figure 1

EEx ia IIC T3 Cable Interconnection EEx ia IIC T3

Vmax = * C = * VA = *

Isc = * L = * Ia = *

Ca = * R = * Ceq = *

La = * Leq = *

L/R = *

Note: * Applicable values of the Apparatus selected for the project to be used which will depend upon cable vendor, barrier vendor and the field apparatus vendor selected.

In order to analyze an I.S. loop the following shall be taken care off:

The safety apparatus must be certified as intrinsically safe for installation in the relevant zone, gas group and gas ignition temperature.

It must be used within the ambient temperature limits for which it is designed and certified.

The associated apparatus (passive or active) must be certified as suitable for connection into the relevant zone and gas group.

The Electrical voltage and current parameters of the two apparatus must be matched and respected according to the relations mentioned below:


Associated

Apparatus

Safety

Apparatus


Step 1 Vmax < Va

Step 2 Isc < Ia

NOTE: The safety cannot be guaranteed if one of the two conditions is not respected.

Verify that the reactive part of the system i.e. capacitance and inductance, is within the safety limits. In practice this equation is as mentioned below:

Step 3 Ceq + C (cable) < Ca

Step 4 Leq + L (cable) < La

The associated apparatus “Ca” and “La” parameters must not exceeded by connected circuit considering, not only the field device, but also the connecting cable. Hence the equation becomes:

C (cable) = Ca – Ceq

L (cable) = La – Leq

The maximum length of the connecting cable, referred to its capacitance and inductance, is the lesser of the two relations. It may be noted that the worst data value is due to the inductance, but the effect of the increase of cable resistance with the increase of its length is not considered.

Note: The L/R ratio of the cable can be used as an alternative to the maximum value of L/R allowed by the associated apparatus. If this value is lower than that of the associated apparatus then the inductive effect of the cable can be ignored and the cable distance can be equivalent to the capacitance value.

3. ACTUAL CALCULATION OF I.S. LOOP WITH LONGEST CABLE RUN

Similar procedure / method shall be used as explained in the analysis with using actual parameters on some of the longest run I.S. cable for a project.

Calculation sheet format shall be as attached / enclosed along with an example calculation sheet for reference.



I.S. Loop Calculation ---- Format

Client: Loop number:

Project: Cable Number:

Job Number: Tag Number:


Associated Apparatus

Length = Length =



Vmax = C = C = VA =

Isc = L = L = Ia =

Ca = R = R = Ceq =

La = Leq =

L/R =

Barrier / Isolator

Field Instrument

Junction box Number


I.S. Loop Calculation ---- Format



EXAMPLE SHEET

I.S. Loop Calculation

Client: Borealis Loop number: TIC-422142-1

Project: Polyethylene Cable Number: MI-422142

Job Number: 040022 Tag Number: TT-422142-1


Associated Apparatus

Length = 310 Mts. (16x2x0.5mm2) Length=30 Mts.(1x2x0.5mm2)

Cable ABB make (FKAR-PG) Cable ABB make (FKAR-PG)


Vmax = 26.25V C = 90F/Km C = 90F/Km VA = 30 V

Isc = 93mA L = 0.68 mH/Km L = 0.68 mH/Km Ia = 300mA

Ca = 0.097 F R = R = Ceq = 0.005 F

La = 4.1mH Leq = 0.02 mH

L/R = 46H/

Follow the step as indicated in Sheet 3 and 4 of this document

Step 1 Vmax < Va

26.25 < 30 O.K.

Step 2 Isc < Ia

93 < 300 O.K.


Elcon isolator2026 model

644 series FR instrument

7422-AKL-004


Step 3 Ceq + C (cable) < Ca

0.005 + 0.0027 + 0.027 < 0.097

0.034 < 0.097 O.K.

Step 4 Leq + L (cable) < La

0.02 + 0.0204 + 0.2108 < 4.1

0.2512 < 4.1 O.K.

Hence, the selected cable is suitable for the given length.

Cable value is based on the vendor catalogue information.


Is Loop Calculation

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