The Application of 600 Volt Class Current Transformers in
Medium Voltage Switchgear
August 10, 1999
ANSI Medium Voltage Switchgear commonly uses 600-Volt Class window type current transformers in
medium voltage circuits. For many years manufacturers have installed 600-volt class current
transformers in medium voltage switchgear. The preferred location for the current transformers is the
circuit breaker spouts.
To certify the dielectric capability of the equipment the switchgear type tests are performed with the
maximum number of current transformers mounted on the circuit breakers cell’s feed through bushings.
A typical configuration of the switchgear with the smallest air gaps is then subjected to the series of Basic
Impulse Levels Tests required by ANSI. The success of the BIL testing in the area around the circuit
breaker feed through bushings relies on the following combination of insulation:




The solid dielectric of the circuit breaker’s feed through bushings
The air gap between the conductor and spout
The air gap between the spout and the case of the current transformer
The 600 volt class insulation of the current transformer
This series combination of various insulating mediums provides an insulating system that goes through
the dielectric testing along with the entire switchgear system. During the power frequency test (high
potential) and the impulse test required in ANSI C37.20.2, it is proven that the system dielectric strength
exceeds the nominal BIL rating for the voltage class.
Bus Mounted Current Transformers
Occasionally the client’s current transformer requirements exceed the physical mounting capacity of the
circuit breaker spouts. In these cases it becomes necessary to add additional window type current
transformers. These additional current transformers are mounted in the cable compartment around the
run-back bus.
Since these are non-standard components, they do not get included in the switchgear BIL testing. To
maintain the BIL of the equipment an industry rule of thumb for both factory and field installations
requiring bus mounted window type current transformers is to allow a 1 inch air gap between the
insulated bus and the current transformer housing.
Powell went through the process of testing the 1-inch air gap rule of thumb with various bus insulation
systems that we use in the manufacture of the switchgear. We have established an internal matrix that
applies to the various insulating systems, the associated standard current transformer and a special
"increased dielectric" current transformer. The table below indicates what configurations were acceptable
when tested for 95kV BIL.
Powell Industries, Inc.
PO Box 12818
Houston, TX • 77217
©2005 Powell Industries, Inc. • All rights reserved.
Tel: 713.944.6900 • Fax: 713.947.4453
www.powellind.com
info@powellind.com
The Application of 600 Volt Class Current Transformers in
Medium Voltage Switchgear
page 2
Table of BIL Tests on 6.5 Inch Bus Mounted Window Type Current Transformer
Number of Bus
Insulation
Standard Current Increased Dielectric
Bars per Phase
Material
Transformer
Current Transformer
and Size (inches)
Hipoxy-2000®
GE-Noryl
(1) 1/2 x 3
Passed
Passed
(1) 1/4 x 4
Failed @ 82kV
Passed
(1) 3/4 x 4
Failed @ 78kV
Passed
(2) 1/2 x 3
Failed @ 94kV
Passed
(2) 1/2 x 4
Failed @ 74kV
Passed
(2) 3/4 x 4
Failed @ 72kV
Passed
(1) 1/4 x 4
No Test
Passed
(2) 3/4 x 4
No Test
Failed @ 94kV
(3) 3/4 x 4
No Test
Passed
(sandwiched)
Note: Standard current transformers would be suitable for all switchgear designs requiring 60kV.
Scotch BBI-4A
Jim Bowen
Technical Director
Powell Industries, Inc.
PO Box 12818
Houston, TX • 77217
©2005 Powell Industries, Inc. • All rights reserved.
Tel: 713.944.6900 • Fax: 713.947.4453
www.powellind.com
info@powellind.com