3-1
3 System Operation
3.1 General description of the system
An Intrinsically Safe system is composed by an assembly of intrinsically safe apparatus, associated apparatus and
interconnecting cables, where the following definitions apply:
• Simple Electrical Apparatus
Apparatus in which neither of the following electrical parameters are exceeded: 1.2 V 0.1 A
(Generated) 20 mJouIes (Stored) 25 mW (Dissipated), in this case the Apparatus does not require any
certificate or labeling.
(Typical examples can be Thermocouples, Resistance Temperature Detectors, Contacts, Strain
Gauges, Light Emitting Diodes, photocells).
• Intrinsically Safe Apparatus
Apparatus in which all circuits are intrinsically safe. It must have suitable approval for the hazardous
(classified) location where it will be mounted and must have a control drawing that specifies safety
parameters: Ui; Ii; Pi; Ci; Li (i.e. Proximity Switches, Transmitters, Active Sensors, magnetic pick-up)
must be Intrinsically Safe Certified.
• Associated Apparatus
Apparatus in which circuits may affect the energy in the intrinsically safe circuits to which it is
connected. It must have suitable approval for connection to devices in the hazardous (classified)
location. Associated apparatus safety parameters are specified as : Uo; Io; Po; Co; Lo.
Associated apparatus and control room equipment shall not be powered or generate any voltage
greater than 250 Volts.
3.2 System design considerations
The system must be evaluated under the "entity concept" where the following rules apply: To determine
acceptable combination of intrinsically safe apparatus and connected associated apparatus the following
conditions must be valid:
U
i
> U
o
I
i
> l
o
P
i
> P
o
The length of interconnecting cable shall be determinate in accordance with the maximum allowed parameters by
the associated apparatus according to the expression:
C
cable
< C
o
- C
i
L
cable
< L
o
- L
i
When an Hand Held Configurator (HHC) is connected to intrinsically safe terminals, values of C
i
and L
i
for such
equipment must also be considered in calculating the maximum allowed parameters. A HHC terminal must also
have suitable approvals. Otherwise it can not be connected to the intrinsically safe circuit.
3.3 Wiring of intrinsically safe circuits
Installation shall be in accordance with Intrinsic Safety standards and installation rules (e.g. IEC 79-14, ANSI/ISA
RP 12.6), applicable in the Country where the application is made. Intrinsically safe circuits shall be so identified.
Colour coding may be used for identification if the colour used is light blue and no other wiring is colour coded light
blue. Other identification means may be for example: signs, tags, markings which shall be visible after installation.
Wiring of intrinsically safe circuits shall be positively separated from non intrinsically safe wiring (such as power
wiring) by one of the following methods:
• Using separated raceway
• Providing an insulating or grounded metal partition
• Spacing of at least 50 mm and separately tied down
Different intrinsically safe circuits shall not be run in the same multiconductor cable unless at least 0.25 mm thick
insulation is used on each conductor.
Cables, raceways or conduits used to contain intrinsically safe circuits must not transmit flammable atmosphere
from hazardous to non hazardous location. Otherwise they must be sealed or vented.
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