Even though some of your previous posts have portrayed some...

Even though some of your previous posts have portrayed some expert knowledge I'm not totally convinced that you will find the info on your own so I'm happy to help with one of many descriptions available with a cursory search:

Current transformers (CTs) are sensors that measure alternating current. They are particularly useful for measuring whole building electricity consumption (or generation, for that matter).
The split core type, such as the CT in the picture above, is particularly suitable for DIY use, as it can be clipped onto either the live or neutral wire coming into the building, without the need to do any high voltage electrical work.

Hope this puts your concerns to rest.