CT Nominal System Voltage 1

[zimmerDN]

Is it common and acceptable practice to install fully insulated medium voltage wires through a LV CT?

There is a 4160V starter that requires a 50:5 CT for ground fault detection to a protection relay. The majority of the instrument current transformer I've come across are LV, only 600V rated.

How is the CT protected if the wire insulation breaks down at the 600V rated CT?

Please advise.

 

[stevenal]

Yes.

 

[DRWeig]

The CT is protected by the insulation on the MV cable.

Best to you,

Goober Dave

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[zimmerDN]

What was wrong with his post?

 

[DRWeig]

Whose post, zimmerDN?



Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[living2learn]

Check this article out - good explination.

 

[living2learn]

noticed the article wouldn't work....
PTB #85 - The Application of 600 Volt Class Current Transformers in Medium
Voltage Switchgear
©1997 by Powell Electrical Manufacturing Company. All rights reserved.

ANSI Medium Voltage Switchgear com monly uses 600-Volt Class window type current transformers in
medium voltage circuits. For many years m anufacturers have installed 600-volt class current
transform ers in m edium voltage switchgear. The preferred location for the current transformers is the
circuit breaker spouts.
To certify the dielectric capability of the equipm ent the switchgear type tests are perform ed with the
maxim um num ber of current transform ers m ounted on the circuit breakers cell’s feed through bushings.
A typical configuration of the switchgear with the sm allest air gaps is then subjected to the series of Basic
Im pulse 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 com bination 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 transform er
• The 600 volt class insulation of the current transform er
This series com bination of various insulating m edium s 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 im pulse test required in ANSI C37.20.2, it is proven that the system dielectric strength
exceeds the nom inal BIL rating for the voltage class.
Bus Mounted Current Transformers
Occasionally the client’s current transform er requirem ents exceed the physical m ounting capacity of the
circuit breaker spouts. In these cases it becom es necessary to add additional window type current
transform ers. These additional current transform ers are m ounted in the cable com partm ent around the
run-back bus.
Since these are non-standard com ponents, they do not get included in the switchgear BIL testing. To
maintain the BIL of the equipm ent an industry rule of thum b for both factory and field installations
requiring bus m ounted window type current transform ers is to allow a 1 inch air gap between the
insulated bus and the current transform er housing.
Powell went through the process of testing the 1-inch air gap rule of thum b with various bus insulation
system s that we use in the m anufacture of the switchgear. W e have established an internal m atrix that
applies to the various insulating system s, the associated standard current transform er and a special
"increased dielectric" current transform er. The table below indicates what configurations were acceptable
when tested for 95kV BIL.