Bjarney,
The Current Transformer is a very common thing here in Commercial and Industrial installations.
A 3 phase meter has a 50A per phase load current rating, above that a CT arrangement is used to step the current down to a level that the meters can use.
Ratio's of 100:5, 200:5, 300:5, 500:10, 700:10, 1000:20 are common here at 400V
11kV bushing type CT's are used for Merz-Price Over-Current Protection, these things really give me the willies!. [Linked Image]