Wires

Intrinsically Safe Parameters: Removing the Confusion

Intrinsic Safety is the one protection type that is the most misunderstood. 

Author: Brad Guy 

It is often heard throughout the industry “Everything you take in to a hazardous area must be intrinsically safe”. 

Intrinsic Safety (IS) is the generic phrase that tends to be used as a generalisation for electrical equipment that can be taken into a hazardous area This is far from the truth. 

It is the one protection type that is the most misunderstood. 

Intrinsic Safety is merely one protection type of many that are available and accepted to be used within a hazardous area. 

In this article, we’ll go about removing the confusion that exists around IS. 

Introduction 

The information in this article has been formulated from the following standards; 

  • AS/NZS 60079.11 (IEC 60079.11) 
  • AS/NZS 60079.14 (IEC 60079.14) 
  • AS/NZS 60079.25 (IEC 60079.25) 

Barriers are protective devices that have no protection of their own and are to be installed in a non-hazardous area, they provide “Intrinsic Safety” for all cable and equipment certified as Intrinsically Safe (or simple apparatus, explained later) knows as the intrinsically safe loop. 

Some example of Intrinsically Safe Barriers are shown below (Figure 1): 

Figure 1

Figure 1 

 

Intrinsically Safe barriers need to be certified by a suitable certification scheme (ANZEx or IECEx) to be used in an Australian installation.  

The cabling and devices within the loop need to satisfy certain parameters to allow the barrier to be able to provide Intrinsic Safety for the circuit. 

Figure 2

Figure 2 

Systems 

Parameters of the IS barrier(s) cable parameters, device parameters all need to be considered in the form of basic calculations.  

When an entire Intrinsically Safe system is vendor supplied as a Certified System, the calculations will have already been carried out by the vendor and the system designed accordingly. Certified IS systems can be installed as a complete Intrinsically Safe System without the need to carry out further calculations. 

Entity Concept 

If the Intrinsically Safe equipment has not been provided as a “Certified IS System,” it is known as the “entity concept, and basic calculations are required to be carried out to ensure that the barrier is providing the protection required to maintain the Intrinsic Safety of the IS loop. 

Spread sheets have been developed in many different shapes and forms that automatically carry out these calculations once the parameters have been entered – however these may not all be accurate. 

There are variations in how intrinsically safe calculations are completed depending on values of certain parameters; these calculations ensure that the barrier can provide the required protection for all equipment connected to the Intrinsically Safe loop (including cables and simple apparatus). 

Simple Apparatus 

Simple apparatus is equipment that does not need to be certified to be used in an intrinsically safe loop. However, it still needs to satisfy certain requirements: 

  • Simple apparatus needs to satisfy requirements shown below (Figure 3) according to AS/NZS60079.14 
  • Simple apparatus still needs to be protected by a barrier (even if the Simple Apparatus is the only equipment in the IS loop) 
  • Calculations still need to be carried out and the outcome be within acceptable limits 

Figure 3

Figure 3 

Simple apparatus may need extra consideration when carrying out Intrinsically Safe calculations if the equipment can create capacitance or inductance (see below): 

The total inductance and capacitance of all the connected apparatus included in the system and any cable inductance and capacitance shall be less than or equal to Lo and Co for the source of power. 

Cables 

The electrical capacitance and inductance parameters (Cc and Lc) or (Cc and Lc/Rc) for all cables used shall be determined according to a), b) or c): 

           a) the most onerous electrical parameters provided by the cable manufacturer;

           b) electrical parameters determined by measurement of a sample;

           c) 200 pF/m and either 1μH/m or 30μH/Ω where the interconnection comprises two or three cores of a conventionally constructed cable (with or without screen). 

Associated Apparatus (IS Barriers) 

Equipment

This means that the primary side of the barrier cannot be supplied by a voltage (Um) greater than what is documented on the label on the equipment. 

As an example, the in the top left of Figure 1 above we can see that Ufor that particular barrier equals 20 – 250 VDC/AC. 

The barrier also needs to withstand a short circuit current of 1500 A preventing extra current flow from entering the hazardous area and causing an ignition source. 

 

Intrinsic Safety levels 

Intrinsic safety comes in different levels of protection: – 

  • Ex ia has the highest level of IS protection and is suitable for Zone 0 (EPL Ga) which is where the flammable atmosphere is present continuously or for long periods. Ex ia is also suitable for areas classified as a Zone 1 or Zone 2. 
  • Ex ib is suitable for Zone 1 (EPL Gb) which is where the flammable atmosphere can be present in normal operation. Ex ib is also suitable for areas classified as a Zone 2 (EPL Gc) 
  • Ex ic is the lowest level of IS protection and is only suitable for Zone 2 (EPL Gc) which is where a flammable mixture is not expected to be present in normal operation and if it is it will be for a short time only. 

Gas Grouping 

Group II is the gas group that is used for all gas industries that is not underground coal 

There are three different Gas sub Groups in Gas Group II when it comes to hazardous areas and all gases or mixtures of gases will belong to one of the following Gas sub groups as shown below determined by the ignition energy that it takes to ignite the gas mixture when within the explosive range. 

Gas Grouping

Looking at the table above it shows the following: 

  • Gas group IIC is the most easily ignitable group with a Minimum Ignition Energy (MIE) of 20 micro Joules 
  • Gas group IIB has a Minimum Ignition Energy (MIE) of 60 micro Joules 
  • Gas group IIC has a Minimum Ignition Energy (MIE) of 180 micro Joules and takes the most energy to ignite (most difficult of the three gas groups to ignite) 

Barrier Suitability 

Apart from calculation, barriers require to be matched to the certified IS equipment and the hazardous areas they are intended to be used for according to their Certification details and markings. 

Determine the lowest default value for both protection type and gas group as follows: 

Barrier Suitability

Parameters 

All intrinsically safe equipment is tested and certified and provided with acceptable parameters that the equipment can withstand or create.  

These parameters need to be taken into consideration when selection of equipment for Intrinsically Safe loops.  

Voltage 

Maximum input voltage Ui is the maximum voltage that can be applied to the terminals of the field device. 

Maximum output voltage Uo is the maximum voltage that can appear at the terminals of the barrier. 

Current 

Maximum input current Ii is the maximum current that can be applied to the terminals of the field device. 

Maximum output current Io is the maximum current that can be taken from the terminals of the barrier. 

Power 

Maximum input power Pi is the maximum power that can be applied to the terminals of the field device. 

Maximum output power Po is the maximum power that can be taken from the barrier. 

Capacitance 

Maximum internal capacitance Ci is the maximum equivalent internal capacitance created by the field device or by the cables within the intrinsically safe loop. 

Maximum external capacitance Co is the maximum capacitance that the barrier can withstand.   

Inductance 

Maximum internal inductance Li is the maximum equivalent internal inductance created by the field device or by the cables within the intrinsically safe loop. 

Maximum external inductance Lo is the maximum inductance that the barrier can withstand. 

Inductance/Resistance Ratio 

Maximum internal inductance to resistance ratio Li/Ri maximum value of ratio of inductance to resistance created by the field device. 

maximum external inductance to resistance ratio Lo/Ro maximum value of ratio of inductance to resistance that can be connected to the barrier. 

The Calculation 

The Calculation

This is where most people stop, and most automated spread sheets do not cover the extra requirements – what is shown in this next part of the calculation, as described in AS/NZS 60079.14:2017 S16.2.4.3 and AS/NZS60079.11:2011(+A1) S10.1.5.2. 

Extra requirements (1% rule)  

The 1% rule covers capacitance and inductance (shown below) these are explained in the standards mentioned. 

Extra Requirements

An alternative to calculating inductance values the L/R ratio may be used unless the inductance ratio is above 1%. 

Calculations must still be performed for circuits that only contain simple apparatus. 

Read Brad’s earlier article on Hazardous Areas Audits here. 

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