The strange story of the Ex nA and Ex nL their demise. Part II: nL

This note works best as a complement of the previous one, which is available here.

The Spanish version of this note is available here

Introduction

The IEC is working on a process of rationalization to recategorize the EN/IEC 60079 set of standards. The purpose is to regroup the protection methods around their common physical principle, instead grouping them by their level of compatibility with the existing Zones.?

The first type of protection that was moved from the “n” family of standards to the standard better suited in accordance with the physical principle employed was the “nL” method. The process of how “nL” became “ic” works very well as an example of the restructuring process that the EN/IEC 60079-15 standard is going through.

Non-incendive, energy limited, “nL”, many names for the same thing

According to the NEC 500 Code (NFPA 70 standard) a non-incendive circuit is defined as “a circuit in which any arc or thermal effect produced, under intended operating conditions of the equipment, is not capable, under specified test conditions, of igniting the flammable gas, vapor or dust-air mixture."

The standard describes a non-incendive circuit as a system composed by:

·????????An associated non-incendive field wiring apparatus

·????????A non-incendive field wiring apparatus

·????????Non-incendive field wiring

To check whether such a circuit is non-incendive the following evaluation must be performed:

The maximum Input voltage of the non-incendive field wiring apparatus (Ui) must be equal or greater than the maximum voltage output (Uo) supplied by the associated non-incendive field wiring apparatus.

The maximum Input current of the non-incendive field wiring apparatus (Ii) must be equal or greater than the maximum current output (Io) supplied by the associated non-incendive field wiring apparatus.

After informing that these criteria shall not applied to non-incendive loop powered devices, the standard goes on the topic of the interconnection of a non-incendive field wiring apparatus with an associated non-incendive field wiring apparatus using non-incendive field wiring.

It was intrinsic safety, but under covered

The verification of the non-incendive behavior of the system described requires checking the following equations:

Uo ≤ Ui

Io ≤ Ii

Co ≥ Ci + Ccable

Lo ≥Li + Lcable

The non-incendive equivalent components to an intrinsically safe circuit would be the following:

• Associated non-incendive field wiring apparatus => associated apparatus (intrinsically safe barrier)
• A non-incendive field wiring apparatus => intrinsically safe field device
• Non-incendive field wiring => intrinsically safe wiring

Additionally, the standard describes a method for spark ignition testing of non-incendive circuits. Users are advised to follow the CSA/UL 60079-11 standard to perform this test, which is performed in a spark generating apparatus.

In other words, the non-incendive circuit explosion protection method described in the UL 12.12.01 standard is a simplified implementation of the intrinsically safe explosion protection method.

It does away with the power verification and ensures that a non-incendive circuit will not become into an ignition source under normal operational conditions. It also changes the value of the safety factor of 1,5 employed to determine the safety parameters for intrinsically safe systems. The factor employed in non-incendive circuit verification is 1,0.

That means the concept does not allow for any faults, like surges, equipment faults or static electricity, therefore its use is restricted to Div 2 applications.

The associated non-incendive apparatus (the non-incendive equivalent of an intrinsically safe barrier) consists of a current limitation device, which requires the use of a SELV (Safety Extra Low Voltage) power supply to limit the available voltage in the circuit. Between the SELV power supply and the current limitation device, power could be limited to non-incendive values.

There are very few Ex “ic” intrinsically safe certified barriers in the market, therefore the common approach employed in Ex “ic” installations is to use a Ex “ia/ib” barrier with a Ex “ic” field device. The advantages of this practice are the following:

• Since Zone 2 compliant values of equivalent inductance and capacitance of the system are higher in “ic” applications than in “ia/ib” ones and the safety factor employed for the entity parameters is also lower, the possible length of the field device cable run can be increased in the order of 40%.
• New devices specifically designed as “ic” devices will be able to handle a similar increase in available power.

An excess of responsibility

The main problem that the traditional non-incendive approach presents is that the verification of whether a circuit is non-incendive or not, is something that must be determined by the end user. Since most end users will not accept the responsibility of this task, the majority of non-incendive circuit applications use either equipment that features additional protection methods, being the most typical the hermetically sealed method, or limit the use of this technique to applications based on simple electric apparatus.

Intrinsically safe field devices come with certifications that detail their entity parameters precisely measured at the factory by the manufacturer, evaluated and certified by an approved certifying test laboratory.

But that was not the case with “nL” or energy limited non-incendive devices. In fact, the UL 12.12.01 standard requires the end user to verify if a particular combination of elements is non-incendive or not.

The original ATEX Directive did not seem to encourage the use of the “n” protection methods. Since the ATEX Directive does not require a third-party certificate for equipment designed for Zone 2 applications, and only asks the manufacturers to issue a declaration of compliance, there were no significant advantages for suppliers on investing in Zone 2 certifications. The consequence of these factors was that few “nL” certified field devices were ever available in the market.

Here come the fieldbus wars

In the 2000s, fieldbus technology according to IEC 61158-2 (i.e., FOUNDATION Fieldbus and Profibus PA) was at its zenith, competition among fieldbus infrastructure suppliers was at its highest point, and the ample scope of quite different distribution topologies was creating a specifications mess, especially with Zone 2 Div 2 applications.

The British company MTL tried to impose a de facto standard by promoting the FNICO (Fieldbus Non-Incendive COncept) concept, which was a simplification of the FISCO "Fieldbus Intrinsically Safe COncept) concept, but using “nL” field devices instead of “ia/ib” devices.

This attempt ended up in the modification of the standards on intrinsic safety. To avoid further confusion, in 2006 the EN/IEC introduced the "intrinsic safety for Zone 2" protection method, which became identified by the “ic” marking.

This was done with the purpose of integrating the energy limitation protection method “nL” with the existing and well-known methods and practices of the intrinsic safety protection method “ia/ib.”

An integrated solution finally arrives

The intrinsic safety standard EN/IEC 60079-11 featured two levels of protection:

• “ia”: this is the highest protection level, which ensures that the system will continue to be safe to operate even under two unrelated faults. This feature enables its use in applications up to Zone 0.
• “ib”: this is the next protection level, which ensures that the system will continue to be safe to operate even under one fault. This feature enables its use in applications up to Zone 1.

The logical extension of the concept is:

• “ic”: this is the lowest protection level, which ensures that the system will be safe to operate under normal operation conditions and does not consider faults due to the low probability of such an event. This feature enables its use in applications up to Zone 2.

By replacing the “nL” protection method with the “ic” method, all the accumulated experience and well-known working procedures and practices of the intrinsic safety protection method could eliminate the ambiguity that tainted the “nL” protection method previously.

In 2010, the “nL” method was removed from the EN/IEC 60079-15 standard and declared obsolete. New installations must use the Ex “ic” approach instead of the Ex “nL” method and new equipment could not be assessed and certified as a “nL” device. Manufacturers could still produce Ex “nL” devices, but they were to be used only as spares for existing installations. New “ic” certified devices were made available, because in most cases, existing “ia/ib” devices could be employed after the entity parameters verification was done.

Also, existing “nL” devices could be recertified a “ic” with minor modifications, being the most notable the 50 mm minimum separation distance between intrinsically safe circuits and non-intrinsically safe ones.

Aftermath

The direct consequence of this changes in the standards was that the already scarce group of non-incendive field wiring apparatus or Ex “nL” certified devices became suddenly obsolete in all the world, except for the USA.

At the same time, control system and interface technology suppliers were presenting their new generation of smart I/O systems, which could be easily adapted for intrinsically safe applications.

Some vendors designed their new I/O systems to be “ic” compliant (Ex ec [ic]) in their standard versions, but also featuring “ia/ib” compliant versions for Zone 0/1 (ia/ib) or Div?1 (ia) applications. This means that a standard I/O card could also work as an “ic” associated device, thus eliminating the need for separate intrinsically safe barriers.

Since the most common use of “ic” devices was in IEC 61158-2 fieldbus-based applications. Most of the available fieldbus power supplies were already SELV compliant devices, and the use of current limiting device couplers allowed users to have segment topologies with “ic” spurs.

The current ATEX/IECEx standards for intrinsic safety (60079-11) covers all the range of applications of this protection method for any Zone. It integrates the “intrinsic safety for Zone 2” approach that was the basis of the non-incendive circuit method into a coherent set of standards based on the single common physical principle of energy limitation.

From my personal point of view, these changes look very logical.

Mirko Torrez Contreras?is a Process Automation consultant and trainer who still remembers the instant when his brain finally managed to understand the inner workings of intrinsic safety. That instant was his Eureka moment.

According to Wikipedia, "Eureka" comes from the?Ancient Greek?word?ε?ρηκα?heúrēka, meaning "I have found (it)", which is the?first person?singular?perfect?indicative?active?of the verb?ε?ρ?σκω?heurískō?"I find". It is closely related to?heuristic, which refers to experience-based techniques for problem-solving, learning, and discovery.

This last linguistic related digression is completely unrelated to the article's main topic, but it is never late to learn one more thing.

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