CT sensors - An introduction
Current transformers (CTs) are sensors used to 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 straight on to either the live or neutral wire coming into the building without having to do any high voltage electrical work.
Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding.
In the case of whole building monitoring, the primary is the live or the neutral wire (not both!) coming into the building itself, and is passed through the hole in the CT. The secondary winding is made of many turns of fine wire housed within the transformer casing.
The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit .
The current in the secondary winding is proportional to the current flowing in the primary winding:
Isecondary = CTturnsRatio x Iprimary CTturnsRatio = NoTurnsPrimary / NoTurnsSecondary
The number of secondary turns in the CT pictured above is 2000, so the current in the secondary is one 2000th of the current in the primary.
Normally this ratio would be written in terms of currents e.g. 100:5 (for a 5A meter scaled 0 - 100A). The CT above would normally be written as 100:0.05.
A current output CT needs to be used in conjunction with a burden resistor. A burden resistor completes (closes) the CT secondary circuit. The burden value is chosen such that it provides a voltage proportional to the secondary current. The burden value needs to be low enough to prevent core saturation.
The secondary circuit is galvanically isolated  from the primary circuit. (i.e. has no metallic contact)
References and further reading