Dynamic elaboration on the Art & Science of Hazardous Area Protection
Technical Guideline : IEC 60079-11 (edition: 6.00 2011-06)
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)
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.
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
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 …
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.
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.
The electrical equivalent components ( IS) is The Galvanic Safety Isolator
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 of “Thermo-Electrical Ignition “ is totally eliminated .
The Versatile Safety Concept
(The IGNITION KEY …never to be used in an INTRINSICALLY SAFE domain)