State briefly the consequences of opening the secondary of a current transformer when it is in service. State the precaution to be taken should disconnection be necessary.

A CT has a current ratio of 100/1. If the primary load current is 100 A, determine the excitation current, the maximum ratio error and the maximum phase angle error if the burden is 5 ohms. The CT characteristic gives an excitation current of 0.014 A for a secondary voltage of 5 V.