# Why Current Transformer (CT) Seconday Should not be Open ?

- Link: https://studyelectrical.com/2018/06/why-secondary-of-current-transformer-should-not-be-open.html
- Published: 2018-06-10T18:43:00+00:00
- Author: 

⌊Why%2BCT%2Bsecondary%2Bshould%2Bnot%2Bopen⌉⌊Why%2BCT%2Bsecondary%2Bshould%2Bnot%
2Bopen⌉

Amazed why secondary of current transformer should not be open? If it is left open
it may destroy the current transformer.

So it is very important that the secondary of Current Transformer should not be 
kept open.  The reason is quite interesting.

 Either it should be shorted or must be connected in series with a low resistance
coil such as current coils of watt meter, coil of ammeter etc.

> [Related: Why Transformers are Rated in kVA, Not in kW?](https://studyelectrical.com/2014/08/22/why-transformers-are-rated-in-kva-not-in-kw/)

If it is left open, then current through secondary becomes zero hence the ampere
turns produced by secondary which generally oppose primary ampere turns becomes 
zero. 
  As there is no counter m.m.f., unopposed primary m.m.f. (ampere turns) 
produce high flux in the core. 

 * This produce excessive core losses, heating the core beyond limits. 
 * Similarly heavy e.m.f.s will be induced on the primary and secondary side. This
   may damage the insulation of the winding. 

This is danger from the operator point of view as well. It is usual to ground the
C.T. on the secondary side to avoid a danger of shock to the operator. 
  Hence 
never open the secondary winding circuit of a current transformer while its primary
winding is energized.   Thus most of the current transformers have a short circuit
link or a switch at secondary terminals. When the primary is to be energized, the
short circuit link must be closed so that there is no danger of open circuit secondary.

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