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| transformers in that they are of comparatively small capacity and are designed to maintain a much higher degree of accuracy in their ratios of transformation than is necessary in other forms of transformers. To differentiate from other transformers, the load carried by these instrument transformers is referred to as their burden. |

| In using instrument transformers with various meters, relays or controls, it is necessary to know the relative direction of the current flowing in the leads at any particular instant. Hence, it is necessary to know the relative polarity of the primary and secondary windings. This is indicated by marking one terminal of each winding with a mark, usually a colored dot, but the marking may be something else; sometimes the letter H1 is used to mark the high voltage winding and X1 to mark the low voltage winding. The marked leads are of the same polarity when current flows toward the transformer in the marked primary lead, but current will flow away from the transformer in the marked secondary lead, and vice versa. |

| Potential transformers (PT) have a small output and have high accuracy; that is, they have small regulation or voltage drop; the windings are also compensated to give its exact ratio at some specified rated burden. Compensation is an adjustment in the turns ratio to compensate for the voltage drop at the specified burden. They are usually designed for a secondary voltage of 150 or 300 volts. Potential transformers are connected in parallel or across the line as shown in Fig. 6-8. They are used with voltmeters, wattmeters, watt-hour meters, relays, and other controls. Several instruments may be connected to the same transformer if their sum does not exceed the burden for which the transformer is designed. |

| Current transformers (CT) also have a small output and high accuracy. The primary winding may consist of one or few turns and must be insulated to sustain the line voltage; in some instances, the insulated line conductor itself may serve as the primary, passing through the core around which the secondary windings are wound. The usual standard design calls for the secondary current rating to be 5 amperes which is full scale for the ammeter associated with it. Current transformers are connected in series with the line, as shown in Fig. 6-9. They are used in conjunction with the same apparatus as potential |

| Figure 6-8 Potential transformer connection. |

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