725Ex
Users Manual
4
such as transmitters and positioners (I/P devices). These
barriers are identified with a maximum open circuit voltage
parameter (Voc) and a maximum short circuit current
parameter (Isc). The matching criterion requires that Voc
of the barrier not exceed 30 V and Isc not exceed 100 mA.
The Model 725Ex calibrator will itself be a source of
voltage and current. Each set of terminals has a Voc and
an Isc rating as shown on Fluke 725Ex CCD. When
connecting terminals to other equipment, the Vmax and
Imax ratings on the other equipment must exceed the Voc
and Isc ratings for the terminals connected to on the
725Ex calibrator.
In addition to matching voltage and current entity
parameters, it is also necessary to verify that capacitance
and inductance has not been exceeded. Again, Fluke
725Ex CCD identifies the maximum capacitance (Ca) and
maximum inductance (La) that is permitted based either
on the intrinsic safety barrier ratings or on the 725Ex
calibrator ratings for the specific terminals used. As an
example, Fluke 725Ex CCD
explains that the capacitance
of each unit connected in the circuit (Ci) plus the
capacitance of the cable in the circuit must not exceed the
maximum allowed capacitance (Ca). Similarly for
inductance in the intrinsically safe circuit.
When connecting the 725Ex calibrator into a powered
circuit, i.e. when the circuit is powered by an intrinsic
safety barrier, then the maximum circuit voltage used for
the entity parameter evaluation will be the higher of either
the 725Ex calibrator Voc or of the barrier Voc. The
maximum current will be the sum of the 725Ex calibrator
Isc and the barrier Isc. In this case, the maximum allowed
inductance (La) will be reduced. This value will have to be
determined using the ignition curves found in standards
such as CSA C22.2 No. 157 or UL 913.
For additional information about Ex Hazardous Areas,
refer to ANSI/ISA-12.01.01-1999 Definitions and
Information Pertaining to Electrical Instruments in
Hazardous (Classified) Locations and to ANSI/ISA-
RP12.06.01-2003 Recommended Practice for Wiring
Methods for Hazardous (Classified) Locations
Instrumentation Part 1: Intrinsic Safety.
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