Dynamic elaboration on the Art & Science of Hazardous Area Protection
Power Reduction philosophy for Intrinsically Safe devices operating in Explosive Atmosphere
IEC 60079-Part 0: Equipment – General requirements
IEC 60079-25: Explosive atmospheres – Part 25: Intrinsically safe electrical systems
An associated apparatus is a device typically placed in the non-hazardous portion of an industrial facility, between the automation /control systems’ input/output side (modules ) and the FIELD Devices in the Hazardous Area .
The main purpose is to limit the POWER (which is being delivered to the field devices) to Intrinsically Safe energy level.
Electrically it is the link between the control room & the corresponding device located in the field.The basic purpose is to provide communication through VOLTAGE / CURRENT (POWER) at such a low WATTAGE so as not to provide any ignition-capable energy .
Definition FROM IEC 60079-0: Electrical apparatus which contains both energy-limited and non-energy-limited circuits and is constructed so that the non-energy-limited circuits cannot adversely affect the energy-limited circuits
Following , two types of Associated Apparatus are most commonly used in this context .
1. Safety Barriers
The barrier is a simple , passive device consisting of a ZENER diode , resistors and a network of fuses.
2. Galvanic Isolator
Along with providing intrinsically safe connections, these devices also isolate, transform, electro-mechanically or optically, the hazardous and nonhazardous location circuits. These devices sometimes provide signal conditioning functions as well.
A small but important note about the FIELD DEVICES.
( Common Examples: Switches , transmitters, positioners, solenoid valves, LED , thermo-couples etc.)
There can be TWO types of field devices :
Simple devices are the one which meet the following requirements:
a. Device does not store more than 1.2 Volts or 20m Joules.
b. Device does not draw more than 100 mil-Amps.
c. Device does not dissipate more than 25 mil-Watts.
Examples : Switches , thermocouples , LEDs
Devices which does not full-fill the requirements of a , b , c.
These devices require evaluation and Intrinsically Safe (I.S) Certification from a certifying body.
More about the location of Associated Apparatus
There can be following scenarios as to the suitability for installation of the associated apparatus in the Hazardous Area.
Ex d [ia] IIC T4 Gb (Group of the equipment & associated apparatus is same)
Ex d [ia IIC Ga] IIB T4 Gb (Group of the equipment is different from that of the associated apparatus)
A good example is an Ex d enclosure containing a “Safety Barrier”. (Safety Barrier is the associated apparatus)
Ex d ia IIC T4 Gb
The marking will be: – [Ex ia] IIC
The marking will be:-Ex d ia [ia Ga] IIC T4 Gb
(Ref: IEC 60079-0)