Current Transformers are a vital piece in metering and current monitoring applications. Current transformers are commonly used in metering and protective relays in the electrical power industry. And it is very important that the current transformers be installed correctly for the entire process to work properly.

Every current transformer has polarity markings on it (even power transformers). These are commonly called H1 and X1. A current transformer is wound so that when primary current enters H1, then secondary current leaves X1. Therefore, current transformers should be installed with the H1 side pointing towards the power source.

If the current transformer is installed with H1 pointing toward the source (electrically), then the current coming out of X1 = I (or I @ 0°), but if the CT is installed pointing away from the source (electrically) then the current coming out of X1 = -I (or I @ 180°).

The orientation of current transformers in a circuit are dependent on what they will be doing. If the current transformer is feeding single ammeters, then orientation is not important. However, if the currents must be algebraically combined, like in a KWH (Kilowatt Hour) Meter or a ground fault relay, then it is important to to know what direction any one current is flowing versus the other ones.

For example, in a ground fault relay, balanced currents must sum to zero. So Ia+Ib+Ic = 0. But if the C phase current transformer is installed "backward" the ground fault relay is actually measuring Ia+Ib-Ic.

When measuring kilowatts (KW) you must make sure that all of your current transformers are pointed in the same direction and that they correspond to the orientation of the voltage transformers as well as that they are on the correct conductors. Ideally, the voltage transformers would be connected with their H1 lead on the first line. The voltage transformer for Vab should have its H1 lead on A, Vbc should have its H1 lead on B, and Vca should have its H1 lead on C. The current transformers should have their H1 side pointing toward the source of the current.

Current Transformers installed on the three phases on the load side of a power panel. Photo courtesy of the U.S. Coast Guard.

Current transformers are generally located in the main breaker panel or in branch distribution panels where space is always at a premium. Since current transformers do NOT have to be installed 90° to the conductor run (the conductors / wires can go through the current transformer at any angle), they are generally held in place with plastic tie wraps or other similar devices. Current transformers with mounting feet are also available if appearance is important or if there is enough room in the panel to accommodate this type of mounting. If a CT test switch is used, the switch must have a "make-before-break" contact pattern to assure that the current transformers are not open-circuited during transition.

Normally current transformers should NOT be installed on "HOT" services. The power should be disconnected during installation. However, often times this is not possible due to critical loads that cannot be shut down. Split-core current transformers are a great solution for this! But even split-core current transformers should NOT be installed on "HOT" uninsullated bus bars under ANY conditions!

Under NO circumstances may the secondary circuit of a current transformer be opened when current is flowing in the primary circuit. The voltage generated in the primary winding is stepped up by the turns-ratio of the current transformer (typically by a factor of several hundred). The voltage in the primary winding can reach several thousand volts in a fraction of a second if the secondaries are made open-circuit while current is flowing in the circuit being metered. Such high voltages can be dangerous to personnel and can cause serious damage to the transformer or equipment connected to it. Such damage may not be immediately obvious, but will certainly lead to incorrect operation of the equipment.

In LV (low-voltage) installations, it is recommended that the current transformer secondary leads (S2) be grounded as protection against static voltages or insulation failure. Only one connection to ground is necessary and it is general practice to make this at the transformer. In HV (high-voltage) installations, the current transformers must be grounded.

Current transformers used as part of metering equipment for three-phase 400 Amp electricity supply. Photo licensed by 15:52, 13 Dec 2004 (UTC)

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