Sometimes the current at a specific site is too much to go through one little meter.
It is possible to let only a scaled amount of current through the meter.
CTs do just that.
They transform the current from something that is very high (more than 100 amps); to something small, usually about 5 amps.
Here are some examples of what CTs look like:


The CT is round and has a hole in the center.
The wire of the main supply goes through the CT.


So, for example, at a site running at maximum 200 amps, we can use a CT to transform that to 5 amps.
That means, 200 amps become 5 amps.
This is called the CT ratio.
The CT ratio is always written in bold letters on the CT.