METERING FOR LINEMEN

 

Electrical Metering for Linemen and Techicians

 

Current Transformers

Purpose

Current transformers are always connected in series with the circuit being measured. A current transformer (CT) has two purposes. First, to reduce the current in the circuit being measured to a lower value. Second, to isolate the meter from high voltages. You might ask, "How could a CT isolate the meter from high voltages"? Suppose you need to meter a 7,200-volt circuit. The current being measured may be less than 200 amps, which could normally be handled with a self-contained meter. However, there is no way to bring the current flow through the meter’s current coil without bringing 7,200 volts with it. Therefore, a CT is used to keep the high voltage out of the meter socket even though the current does not actually need to be reduced. If voltage transformers are used to reduce metering voltage, CT’s will be required to prevent the high voltage from entering the meter socket no matter what the current levels will be. Voltage for potential coils in the meter should always be picked up on the H1 side of the CT. This prevents the CT from registering the energy used by potential coils in the meter as energy used by the customer.

Stated once again, current transformers have two purposes, to reduce the current in the circuit being measured to a lower value and frequently to keep high voltage out of the meter socket.

Caution: The secondary circuit of a current transformer should never be opened when a load is passing through it’s primary!

(See shorting bars, self-shorting devices, and test switches).

Ratio

The ratio of a current transformer refers to the turns ratio of the windings. For example, a

200/5 transformation is equivalent to a 40 to 1 ratio. (200 divided by 5 is 40). The secondary of a CT is always stated to be 5 amps at the rated primary winding current.

Transformer Factor

A current transformer with a 200/5 ratio is said to have a transformer factor (TF) of 40. Knowledge of the TF is required when calculating the dial multiplier.

Rating Factor

Current transformers may be overloaded without a loss of accuracy. This overload rating is known as the rating factor (RF). When sizing CT’s, you should normally use the lowest ratio available while utilizing the RF rating of the CT. Appropriately sized CT’s should always produce from .25 amps (light load) up to the class rating of the instrument meter they are working with which is usually 10 or 20 amps. For example, assume you are going to meter a load that may occasionally reach 1200 amps. However, the load may also be as little as 40 amps. By utilizing a 400/5 CT with an RF of 3.0, the CT can easily handle 1200 amps with no loss of accuracy while sending 15 amps to the meter. When the load drops to 40 amps, the CT will still be sending ½ (.5) amps to the meter. It should be noted that since the CT will produce up to 15 amps, a class 20 meter will be required for this application. If a 1200/5 CT had been used for this application, the CT would only produce .17 amps when the 40 amp load was present. This is below the light load test amp rating for instrument meters and may cause a loss of accuracy.

Many CT’s have a RF rating at 30 degrees Celsius (86° F) and a lesser rating at 55 degrees Celsius (131° F). For example, a 400/5 CT may have a RF rating of 4.0 at 30° C and 3.0 at 55° C. If the CT will be heavily loaded on a hot day or if the CT is in a metal enclosure with little air circulation, the lower rating should be used.

Type

Several variations of current transformers exist. However, there are actually three basic types, window, bar, and wound. Window and bar type CT’s are normally used to meter circuits of 600 volts and less. The wound type is used for high voltage circuits in excess of 600 volts.

When using the window type current transformer the customer’s secondary is passed through the window of the CT. This conductor is considered to be the primary turn of the CT. Often it is necessary to take more than one turn through the window. Generally speaking, each additional turn reduces the ratio. For example, two turns through a 400/5 CT makes it equivalent to a 200/5 ratio. To calculate the ratio when multiple turns are present, divide the ratio as usual and then divide your answer by the total number of turns. For example, if you have two turns through the window of a 400/5 CT, divide 400 by 5, which is 80. Then divide 80 by the 2 turns, and you get 40, which is the transformer factor.

Often, bar type CT’s are just window CT’s with a solid bar installed. Connectors on each end of the bars also allow easy access for picking up voltage for the potential coils in the meter. Some bar type CT’s are designed with a removable bar, which allows the CT to be converted to a window type.

Wound type CT’s are more commonly found in high voltage circuits. Although multiple turns are not possible because of the fixed primary winding, multi-ratio CT’s are available. Although these wound type CT’s are larger because of the additional insulation, the same principles apply as stated for window and bar type CT’s.

Polarity

A meter stator contains a potential coil and one or more current coils. These coils provide both voltage and current signals to the meter. The stator must be able to compare these two signals at any moment in time. Therefore, to establish forward rotation of the meter disk, polarity marks on instrument transformers must be observed. Incorrect polarity connections will result in reverse rotation of the meter disk. All instrument transformers are wound subtractive. This simply means that H1 and X1 polarity marks are physically located directly across from one another. (Additive power transformers have H1 and X1 bushings located diagonally across from one another).

Accuracy

Simply stated, current transformers should be rated for plus or minus .3% (3 tenths of one percent) accuracy when used for metering.

Burden Rating

The burden on a current transformer is the ohm value in the secondary circuit, which passes through the current coil in the meter. The wiring from the CT to the meter is also part of the connected burden. Wire size and clean tight connections are critical. As a general rule, if the meter is within 30 feet of the CT, number 12 copper wire may be used. Distances greater than 30 feet require number 10 copper wire or larger.

The thermal burden rating usually coincides with the primary rating factor (RF). Exceeding this rating will shorten the life of the CT and may cause a loss of accuracy.

Shorting Bars, Self-Shorting Devices, and Test Switches 

Current transformers are designed to have their X1 secondary lug connected to the top of the meter’s current coil. The X2 secondary lug is normally connected to the bottom of the same current coil. When load is passing through the primary of the CT and the secondary is connected properly, very little voltage is present in the secondary circuit. These connections to the current coil in the meter provide a short circuit, which is appropriate for normal operation.

If current is passing through the primary of a CT, and the secondary circuit is not connected to the current coil, a very high and dangerous voltage will be present. The CT becomes a voltage step-up transformer under this condition. Therefore, it is important to always short the X1 and X2 terminals to each other before breaking the circuit. Shorting bars are permanently installed on most CT’s for this purpose. Simply stated, if you need to rewire a metering installation or change the meter while maintaining service to the customer, the shorting bar may be closed from X1 to X2 to prevent dangerous voltage buildup. (Shorting bars that are inadvertently left closed will cause a loss of revenue)!

An alternative (although a poor one) for using the shorting bar when changing a meter, is the meter socket with self-shorting devices. A self-shorting device in a meter socket is supposed to bypass the current coils in the meter as the meter is being removed from the socket. This action is designed to maintain continuity in the CT secondary circuit and therefore prevent dangerous high voltage buildup. Never trust these devices! They are spring loaded and may hang-up due to dirt, cob webs, etc… In addition, these self-shorting mechanisms may get damaged over time and cause partial shorting of the CT which will result in a loss of revenue.

Instrument-rated meter sockets with test switches provide an excellent method of shorting CT circuits as well as disconnecting voltage sources to the potential coils in the meter. By utilizing these test switches, meters may be changed safely and efficiently. Test switches also provide an opportunity to energize individual stators in the meter. This is important when verifying that an instrument-rated metering installation has been wired correctly. Color coded test switch handles may be ordered to match the utilities wiring color code. This enhancement simplifies wiring of the meter socket.

As a final note, CT’s are not like capacitors. They do not hold a charge. However, they are very dangerous when a load is passing through their primary and the secondary circuit is open.

Never open the secondary of a CT while load is passing through the CT’s primary!