If I may elaborate...
A CT (current transformer) is a type of instrument transformer that sends a representative current through the meter circuit, rather than the full load current.
A typical CT may have a 400:5 ratio, which translates to an 80:1 ratio. This means that, for every 80 amps that flow through the service, 1 amp flows in the meter circuit.
Most CT's are a simple coil on a core that the load conductor passes through (often called a "doughnut"), or a similar core with a bar that the conductors bolt to.
While considered a current transformer, if the secondary circuit is not closed either by the meter or shorting bars, it will function as a voltage transformer at a 1:80 ratio.
This can step up a typical service voltage to the point that the insulation within the coil can be damaged, or worse, damage nearby equipment. After all, 240 x 80 = 19,200!
