Ex-pert Electrical Technologies

Dynamic elaboration on the Art & Science of Hazardous Area Protection

Intrinsic Safety (IS)

Intrinsic Safety (IS) IS_main02

Technical Guideline : IEC 60079-11 (edition: 6.00 2011-06)

Definition

Type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the Hazardous Atmosphere, to a level below that which can cause ignition by either sparking or heating effects. (Ref: IEC 60076-11)


Intrinsic Safety Dragon Ex


Overview of the philosophy of IS

It has been observed that very small sparks and arcs are incapable of igniting a mixture of Flammable gas/vapor/dust/air mixture. This is due to the fact that the ignition Electrical Energy is very small . Similarly hot surfaces having smaller amount of thermal energy will not ignite a mixture of Flammable gas/vapor/dust/air mixture.

This fact is utilized in designing electrical systems for hazardous areas. The main feature of all such designs is that the associated electrical power (watts = Voltage x current) is of very low value (Approx. 25 Milli-watts) Logically such low-power electrical equipment and systems can only be used for information exchange , measurement and  control, where the operating voltages & currents are of low value. IS Slide 2


Some more definition.

Um= The maximum r.m.s (root-mean-square) value of a.c or d.c voltage. It’s the maximum voltage that can be applied to the non-IS connection facilities (for example: terminals) of associated apparatus without invalidating the integrity of the then existing type of protection. The value of the Um may be different at different sets of connection facilities and may be different for a.c & d.c voltages

Ui= The Maximum input voltage (Peak vale of a.c or d.c) It’s the maximum voltage that can be applied to the connection facilities (for example: terminals) of apparatus without invalidating the integrity of the then existing type of protection.


There are three levels of Intrinsic Safety

1. “ ia “

2. “ ib “

3. “ ic “

The corresponding Equipment Protection Levels are

Ga  or Ma      for Protection Levels            ia

Gb or Mb        for  Protection Level           ib

Gc                   for protection level               ic


What is ‘ia’. With Reference to: IEC 60079-11. LEVEL OF PROTECTION is determined by tests as stated in IEC 60079-11. The tests are for:-

1. Spark Ignition Compliance

2. Thermal Ignition Compliance

3. Maximum Voltage (r.m.s) & Maximum Peak voltage withstand tests .


Interesting details of Test no.3  for “ia” With V(r.m.s) . & V (max.) applied the IS circuits should not be capable of causing ignition in the following scenarios:-   

a.     Normal operation and with application of such NON-COUNTABLE faults which give the most difficult condition  

b.     Normal operation and application of ONE COUNTABLE fault along with those NON-COUNTABLE faults which give the most difficult condition   

c.     Normal operation and application of TWO COUNTABLE fault along with those NON-COUNTABLE faults which give the most difficult condition (The NON-COUNTABLE faults applied may differ in each of the above scenarios)


At this stage let us know a little bit about FAULTS

IS Slide 3 IS Slide 4


What is “ib”

(Tests as specified in ‘ia”)

a. Normal operation and with application of such NON-COUNTABLE faults which give the most difficult condition b. Normal operation and application of ONE COUNTABLE fault along with those NON-COUNTABLE faults which give the most difficult condition


What is “ic” a. Normal operation and with application of such NON-COUNTABLE faults which give the most difficult condition


Any discussion about IS will not be complete without  considering the :-

(ELECTRICAL) Safety Barriers & Isolators

So let us follow the tradition and analyze a SAFETY BARRIER…….

The following example of a break-water (masonry Barrier)  will help us to under stand the working of an Electrical Safety Barrier …

Barrier-1

The electrical equivalent (some-what) of the Water-Barrier is The Zener Safety Barrier . A dedicated ground connection is a must for proper SAFETY operation of the Barrier.

Zener-Barrier


Barrier-ia


Barrier-ib


NOW…let us study another type of barriers . Barriers which connect one side with the other without any physical contact.

The main theme of this type is : GALVANIC ISOLATION

See the figure below .

An example of Galvanic Isolation.

 Isolator-1

The electrical equivalent components ( IS) is The Galvanic Safety  Isolator

 Isolator-Galvanic


Intrinsic Safety ‘s basic concept is based upon the “SOFT & GREEN VALUES” of the Thermal & Electrical parameters .

Basic requirement are LOW VALUES of the THERMAL & ELECTRICAL Energies so that the possibility ofThermo-Electrical Ignition “ is totally eliminated .


The Versatile Safety Concept

Ex i-versatile concept


IS Key 2

 (The IGNITION KEY …never to be used  in an  INTRINSICALLY SAFE domain)

— cont.

About Asif Mirza

electrical engineer artist