A CT usually has its S1 and S2 terminals connected to an ammeter or transducer for measurement purposes. However, the S2 terminal is also connected to ground. Why is this so? Why does this not affect the current flowing through the ammeter?
Why does this not affect the current flowing through the ammeter?
If you put a voltmeter across a 9 volt battery, the voltmeter would read 9 volts. Now, if you connected either side of the battery to ground (or even a hundred volts) the voltmeter will still read 9 volts.
It's no different with a current transformer - it has induced in the secondary a voltage and that voltage can drive a current into an ammeter and it forms a complete circuit and the ammeter reads so-many amps. If you made a spurious single ground connection, no current will flow into ground because there is no complete circuit to push current into or around and the ammeter will continue to read so-many amps.
Regarding grounding, capacitive coupling can occur between the CT primary and secondary. In some transformers, the capacitance is quite high and the resulting common mode voltage voltage on the secondary side can be higher than the insulation was designed for. In LV and MV installations, the voltage usually is not very high, but grounding is still done. On domestic and most industrial supplies this is less likely to be a problem but is one that is easily eliminated by grounding.
The grounding also prevents any surprises due to capacitive coupling should a technician touch the, supposedly, isolated secondary wiring.