5 replies to this topic

[AB2005]

Dear All

It is said that out put of CT must not float. Its winding may burn if float. Is it true? If yes then why the winding of general purpose distribution transformer not burn if there is no load at its secondary side?

[bob]

Hi AB2005,

The secondary side of a current transformer should rather never be left opened on the load side as dangerously high voltage will dvelop across the secondary winding and this could damage the CT.
I hope this answer your question.

Regards.

Bob

[AB2005]

The secondary side of a current transformer should rather never be left opened on the load side as dangerously high voltage will dvelop across the secondary winding and this could damage the CT.
I hope this answer your question.

Hello Bob

I know about this. Read my Question carefully.

[submonkey]

Dear All

It is said that out put of CT must not float. Its winding may burn if float. Is it true? If yes then why the winding of general purpose distribution transformer not burn if there is no load at its secondary side?

Hi AB2005,

The primary winding of a CT often operates at a high voltage with respect
to earth. The secondary winding is usually designed to operate at a fairly
low voltage with respect to earth.

If you allow the secondary winding to float, the capacitance between the
primary and secondary windings may cause a high voltage to be
transferred to the secondary winding. It is possible for a flashover
to occur (depending on the CT and the situation).

CT secondary windings should ALWAYS be earthed to protect humans
and secondary equipment from high voltages.

Note that the same effect can occur in distribution transformers. Every
winding should be earthed somewhere. The secondary windings of
unloaded distribution transformers are usually earthed - often at the
starpoint.

Thanks,
submonkey

[asif]

Dear all,

This is not to answer the ongoing discussion, but to put forward a issue, that there are several meters, like ammeters etc, require that the CT connection is to be done in such a manner that all S2 terminals are to be shorted (star) and grounded and a common earth wire to go to meter.

While I have also come across several load managers and multifunction meters where it is required that all CT terminals (6 wires) must go directly to meter and then only the connection is correct and reading is also correct.

Why these two different connections are required?


Thanks

[chaterpilar]

Dear All

It is said that out put of CT must not float. Its winding may burn if float. Is it true? If yes then why the winding of general purpose distribution transformer not burn if there is no load at its secondary side?

See the difference in the turn ratio of a CT and a normal distribution transformer.....moreover the impedance comes into play ..the secondary of a current transformer when disconnected from its load while current is flowing in the primary will mean transformer secondary to continue driving current across the effectively infinite impedance

This will produce a very high voltage across the open secondary (into the range of several kilovolts in some cases).

Cheers.