UFC 3 -520-01
June 10, 2002
single enclosure. By this design, more protective functions can be implemented in a
9-4.1.1 Instrument transformer design and performance is an important part of relay
design. Protective relays can be no more accurate than the instrument transformers
that provide the input information. Instrument transformers operate on the same
principles as ordinary transformers; however, they are specifically designed to duplicate
the input waveform as closely and predictably as possible.
9-4.1.2 Ensure instrument transformers comply with IEEE C57.13, Instrument
Current Transformers (CT).
9-4.2.1 CTs should deliver a secondary current that is directly proportional to the
primary current with as little distortion as possible. In most cases, the secondary output
current is usually reduced to a level less than 5 amperes. Although CTs are available
with 1 ampere or 10 ampere secondaries, the most common rating of 5 amperes should
9-4.2.2 All CT circuits require a shorting terminal block. Confirm that CT shorting
blocks have been installed.
9-4.2.3 Include an evaluation of CT saturation in the design and selection of CTs.
Design for the highest CT ratio that provides acceptable performance. Multiratio CTs
are acceptable for use.
Evaluate CT accuracy in accordance with IEEE 2 42.
Potential Transformers (PT). For the typical primary voltages used in
facilities, design the PT turns ratio to provide an output voltage of 120 volts.
Fuses can be current limiting or non-current limiting, and can be rated for low
voltage or high voltage applications. Ensure that current-limiting fuses are designed to
operate within their current-limiting range. Refer to IEEE 242 for a detailed discussion
of fuse types and fuse applications.
The interrupting rating denotes the maximum symmetrical fault current
permitted at the fuse location. Generally, both symmetrical and asymmetrical root-
mean-square ( RMS) ratings are given. Select an interrupting rating greater than the
maximum expected short-circuit current at the installed location.