Current Transformer

Some countries still following the cost reduction concept as they were doing in poor country once even though ecomincally improving like India. we use 3 Cts in each phase.

Hmm, no answers so far, so I'll step out and make an attempt. It's been a long time, but I vaguely remember working on systems that used only two current transformers on a three phase system. To the best of my recollection, these systems were designed without a current carrying neutral (thus, they were typically not at the final distribution level--oops, but maybe sometimes a similar scheme was used on fairly high HP motors as well (well, motors that used CTs instead of just thermal overloads built into the starter).

Anyway, the basic idea was that monitoring two of the three phases also gives you a good idea of what's happening in the third phase.

Maybe somebody can straighten me out and refresh my memory. ;-)

Klocner-Moller designed a motor test panel for me using only two Ct's on phases A and C, feeding a RedLion intelligent ammeter. The selector switch has A,B and C phase positions and it appears to work properly, but I wondered about that to. I haven't looked at the schematic.

As long as there is no current carrying neutral, the current in any one phase is the (negative of) the vector sum of the current in the other two phases. So, to read the current in the B phase, you can have a switching arrangement that connects the two current transformers (on phases A and C) in series (iirc).

But, don't create your own switching circuit without knowing exactly what you're doing--a current transformer must always have a load or it will destroy itself and possibly hurt someone. Switches designed for such switching use things like make before break contacts to make sure that there is always some load (or a short, iirc) on each current transformer.

The destruction of a current transformer under no load conditions is because the voltage will tend toward infinity, eventually reaching a level to arc over the insulation somewhere in the circuit.

Thanks, I knew about open Ct's, I checked the schematic and the switch is indeed M-B-B. You have answered both mine and the OP's question.

Differential protection