Voltage Transformers - are used whenever the line voltage exceeds 480 volts or whatever lower voltage may be established by the user as a safe voltage limit. They are usually rated on a basis of 120 volts secondary voltage and used to reduce primary voltage to usable levels for transformer-rated meters.

Current Transformer - usually rated on a basis of 5 amperes secondary current and used to reduce primary current to usable levels for transformer-rated meters and to insulate and isolate meters from high voltage circuits.

Current Transformer Ratio - ratio of primary to secondary current. For current transformer rated 200:5, ratio is 200:5 or 40: 1.

Voltage Transformer Ratio - ratio of primary to secondary voltage. For voltage transformer rated 480:120, ratio is 4:1, 7200:120 or 60:1.

Transformer Ratio (TR) - total ratio of current and voltage transformers. For 200:5 C.T. And 480:120 P.T., TR = 40 x 4 = 160.

Weatherability - transformers are rated as indoor or outdoor, depending on construction (including hardware).

Accuracy Classification - accuracy of an instrument transformer at specified burdens. The number used to indicate accuracy is the maximum allowable error of the transformer for specified burdens. For example, 0.3 accuracy class means the maximum error will not exceed 0.3% at stated burdens.

Rated Burden - the load which may be imposed on the transformer secondaries by associated meter coils, leads and other connected devices without causing an error greater than the stated accuracy classification.

Current Transformer Burdens - normally expressed in ohms impedance such as B0.1,B-0.2,B-0.5,B 0.9,or B-1.8.Corresponding volt-ampere values are 2.5, 5.0, 12.5, 22.5, and 45.

Voltage Transformer Burdens - normally expressed as volt-amperes at a designated power factor. May be W, X, M, Y, or Z where W is 12.5 V.A. @ 0. 1Opf; X is 25 V.A. @ 0.70pf, M is 35 V.A. @ 0.20 pf, Y is 75 V.A. @ 0.85pf and Z is 200 V.A. @0.85 pf. The complete expression for a current transformer accuracy classification might be 0.3 at BO. 1, B-0.2, and B-0. 5, while the potential transformer might be 0.3 at W, X, M, and Y.

Continuous Thermal Rating Factor (TRF) - normally designated for current transformers and is the factor by which the rated primary current is multiplied to obtain the maximum allowable primary current without exceeding temperature rise standards and accuracy requirements. Example - if a 400:5 CT has a TRF of 4.0, the CT will continuously accept 400 x 4 or 1600 primary amperes with 5 x 4 or 20 amperes from the secondary. The thermal burden rating of a voltage transformer shall be specified in terms of the maximum burden in volt-amperes that the transformer can carry at rated secondary voltage without exceeding a given temperature rise.

Rated Insulation Class - denotes the nominal (line-to-line) voltage of the circuit on which it should be used. Associated Engineering Company has transformers rated for 600 volts through 138 kV.

Polarity - the relative polarity of the primary and secondary windings of a current transformer is indicated by polarity marks (usually white circles), associated with one end of each winding. When current enters at the polarity end of the primary winding, a current in phase with it leaves the polarity end of the secondary winding. Representation of primary marks on wiring diagrams is shown as black squares.

Hazardous Open-Circulating - operation of CTs with the secondary winding open can result in a high voltage across the secondary terminals which may be dangerous to personnel or equipment. Therefore, the secondary terminals should always be short circuited before a meter is removed from service. This may be done automatically with a by-pass in the socket or by a test switch for A-base meters.

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