[Associated Apparatus]

[Associated Apparatus]

Power Reduction philosophy  for Intrinsically Safe devices operating in  Explosive Atmosphere

References

IEC 60079-Part 0: Equipment – General requirements

IEC 60079-25: Explosive atmospheres – Part 25: Intrinsically safe electrical systems

General Definition

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.

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.

CAUTION

 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.

• Associated Apparatus is suitable for installation in the Hazardous Area with energy limitation facility. The marking will be:-

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)

• Associated Apparatus is suitable for installation in the Hazardous Area with energy limitation is provided from outside the equipment & the Hazardous Area. The marking will be:-

Ex d ia IIC T4 Gb

• Associated Apparatus is NOT suitable for installation in the Hazardous Area.

The marking will be: – [Ex ia] IIC

• The Equipment includes (a) Intrinsically Safe apparatus & (b) Associated Apparatus

The marking will be:-Ex d ia [ia Ga] IIC T4 Gb

(Ref: IEC 60079-0)