Portable tester and related method for determining the primary winding to secondary winding current ratio of an in-service current transformer

A portable tester for determining the primary winding to secondary winding current ratio of a curent transformer for a watt-hour meter while said transformer is in service with said primary winding coupled to an active power line carrying a first AC signal and with said secondary winding coupled to said watt-hour meter to deliver a second AC signal to said watt-hour meter, said tester comprising:

• a. a current probe having a split core positionable around said active power line to provide a third AC signal which is proportional in magnitude to the magnitude of said first AC signal;
• b. means for deriving first DC signal having a magnitude proportional to the magnitude of said third AC signal;
• c. means for deriving a second DC signal having a magnitude proportional to the magnitude of said second AC signal;
• d. a signal ratio meter having first and second ratio signal inputs and having at least one decimal point signal input with the signal at said decimal point signal input determining the location of a decimal point in said signal ratio meter;
• e. means for coupling said first DC signal to said first ratio signal input of said signal ratio meter;
• f. a reference voltage source;
• g. a ratio switch connected to selectively couple either said reference voltage or said second DC voltage to said second ratio signal input of said signal ratio meter;
• and
• h. means for providing a decimal point signal to said decimal point signal input of said signal ration meter.

A method for field testing the primary to secondary current ratio of a current transformer for a watt-hour meter while the current transformer is operated under actual load conditions, comprising the steps of:

• a. positioning a current probe having split core around a conductor carrying primary current to said current transformer, without disconnection of said conductor from said current transformer, to obtain a first AC signal from said current probe which is proportional to said primary current;
• b. converting said first AC signal to a first DC signal;
• c. inserting a test load in series with the normal watt-hour meter connected to secondary of said current transformer to develop a second AC signal across said test load which is proportional to said secondary current;
• d. converting said second AC signal to a second DC signal;
• and
• e. simultaneously applying said first and second DC signals to a signal ratio meter to display the resultant simultaneous ratio of said first and second DC signals while said current transformer is operated under actual load conditions.