Medium Voltage Application Guide

SWITCHGEAR
NEMA/IEEE Ratings
These ratings are typically used for current transformers manufactured or used in North American installations. As
well as a stated primary to secondary nominal current ratio, the device also carries an overall accuracy rating in the
format
Accuracy class
AC-CR-BU
Where:
AC = accuracy class
CR = class rating
BU = maximum burden (ohms)
Designates the accuracy of the secondary current with respect to the primary rated current. This accuracy is only
guaranteed provided the maximum burden is not exceeded.
Class rating
Accuracy class Tolerance at 100% primary current
1.2 ±1.2%
0.6 ±0.6%
0.5 ±0.5%
0.3 ±0.3%
Designates the intended application of the device.
B =
for metering applications
H = for protection applications. The CT secondary accuracy is guaranteed at 5 to 20 times the nominal
primary rated current
Burden
The maximum load allowed to be connected to the current transformer secondary, to guarantee the accuracy class.
The maximum burden includes secondary cable/wire, connectors and the load. The following table converts burden
in ohms to VA, for a 5 A secondary.
0.04 0.06 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 0.40 0.48 0.56 0.64 0.72 0.80
VA 1 1.5 2 3 4 5 6 7 8 9 10 12 14 16 18 20
Examples
0.5-B-0.1 indicates a current transformer with an accuracy of ±0.5%, and a maximum allowable secondary burden of
0.1 (or 2.5 VA on a 5 A secondary CT). This is a metering class rated current transformer.
1.2-H-0.2 indicates a current transformer with an accuracy of ±1.2%, and a maximum allowable secondary burden
of 0.2 (or 5 VA on a 5 A secondary CT). This is a protection class rated current transformer.
Page 118 Medium Voltage Application Guide 710-12280-00A