Indirect metering and instrument transformers

Indirect low-voltage metering

Metering must be indirect for fuses over 3 x 63 A. When installing instrument transformers, ensure that the order of voltage fuses and current transformers in the feed direction is correct, the voltage connection point is before the current transformers, and that voltage and current wires connected phase by phase correspond to the same phases (L1 voltage / L1 current, etc.). The customer is responsible for the instrument transformers.

Current transformers must be installed for all phases. The accuracy class (CL) of current transformers must be 0.2S, and the secondary rating must be 5 A. Measurement wires must be installed clearly separated from other wires, and the cross-sectional area of voltage and current wires must be at least 2.5 mm². The secondary wiring of the current transformer must be implemented with two wires S1-S2 (do not use a common S1 wire or grounding).

Current transformers must be installed in the switchboard in the correct direction (P1 → P2) so that each nameplate is visible and readable. They must be installed alternately, with the nameplate on the side. All current transformers must be identical in model and rating.

No devices other than those of the network company may be connected to the measuring cores of the current transformer. Compensation equipment must have its own current transformers.

The meter’s operating voltage is taken after the main fuse and before the current transformers, through a 2–10 A short-circuit protection. If the voltage is taken before the main switch, a warning sign must be placed nearby: “The main switch does not disconnect voltage from the meter.”

Reducing the main fuse of the measurement from over 63 A to 63 A

When reducing the main fuse of the measurement, a 63 A usage point can be allowed with indirect metering, provided that the accuracy class and burden of the current transformers are checked and measurement accuracy is maintained. If necessary, the current transformers must be replaced if indirect metering is to be retained. The customer is responsible for the cost of replacing the current transformers.

Secondary nominal burden

Maintaining the accuracy class of current transformers requires selecting devices and wires in the secondary circuit so that they form a burden of 25–100% of the nominal burden of the current transformers. This must be considered when selecting the nominal power (VA) of the current transformers, and the measurement wires must be sized so that calculations show the burden is 25–100% of the nominal burden.

The nominal burden of a current transformer is typically 1–5 VA, depending on its rated current. The secondary-side burden consists of the meter, terminal block, and measurement wires. The burden of modern static energy meters’ current circuit is approximately 0 VA, so it does not need to be considered in burden calculations.

The secondary wiring of the current transformer must be adapted to the burden of the specific instrument transformer (see Table 2).

* Table 2: Secondary wiring of instrument transformers according to transformer burden.

If the burden does not remain within the upper limits of the wiring length in the table, the cross-sectional area of the wiring must be increased beyond 2.5 mm² (maximum for measurement terminal block: 6 mm²).

Note: The secondary side of a loaded current transformer without measurement must always be short-circuited to prevent damage, e.g., when a temporary measured supply is connected to the switchboard during construction!

Transformation ratio 

The transformation ratio of current transformers is determined based on the main fuse of the measured site. The primary current of the current transformers must always be at least equal to or greater than the rated current of the main fuses. For low-voltage metering, current transformers with a nominal value of 250/5 A can be used when the main fuse size is 3x80 A–250 A.

Indirect metering can be installed at a new usage point with 3x63 A main fuses only if it is known that the main fuses will soon be increased to larger than 63 A.

* Table 3: Sizing current transformers according to main fuse size. Maximum and minimum allowed CT sizes.

Current transformers should be sized so that no extra feedthroughs are made through the CT. If feedthroughs are necessary, a maximum of two is allowed. The transformation ratio must always be a whole number divisible by 5. For CTs rated 125/5 A, a second feed-through is not allowed because the ratio would be 12.5, which does not meet the above requirement.

If electricity consumption changes significantly, the customer must ensure that the transformers are replaced with ones suitable for the new load.

Indirect medium-voltage/high-voltage metering (20 kV–110 kV)

The measurement uses three current and voltage transformers installed, one on each phase. For overall measurement accuracy, it is important to select the correct measurement transformers and wires, which must function and withstand installation conditions.

Measurement wires must be installed clearly separated from other wires and have a cross-sectional area of at least 2.5 mm². When using thicker wires, special attention must be paid to the burden on the current transformers.

The secondary sides of the instrument transformers must be grounded with a grounding conductor of at least 4 mm². Grounding points are shown in diagram Appendix 2: “Indirect MV metering, voltage and current transformer connection.”

The accuracy class of current transformers must be 0.2S, secondary rating 5 A or 1 A (recommendation for MV: 5 A), and the safety factor (Fs) preferably 5 or less.

Separate outgoing and return wires must be brought from each current transformer’s secondary terminals to the terminal blocks. No devices other than those pre-approved by the network company may be connected to the current transformer measurement circuit. For example, compensation equipment or protection must have its own measuring cores in the current transformers.

Voltage transformers must have an accuracy class of 0.2 and a secondary voltage of 100 V between phases. Disconnectors or high-voltage fuses must not be used in the primary circuits of voltage transformers. Voltage transformers must be located in the energy flow direction before the current transformers. When selecting measurement transformers, their thermal and dynamic short-circuit withstand must be verified with the network company. Voltage transformers must be equipped with a damping/resonance resistor.

Measurement voltages require their own secondary cabling, protected by circuit breakers or fuses (2–10 A, recommendation 10 A), installed as close as possible to the voltage transformers.

High-voltage and medium-voltage metering devices must have a connection to the mobile network. To ensure coverage, a route reservation for an antenna cable between the metering device room and the outer wall must be made if necessary. If the building has multiple metering rooms, a route for a data transfer cable must be reserved between them or agreed separately with the network company.

The network company must be provided in advance with the technical data of the voltage and current transformers, as well as measurement circuit diagrams and burden calculations for verification.

Secondary nominal burden of instrument transformers

Maintaining the accuracy class of instrument transformers requires selecting devices and wires in the secondary circuit so that they form a burden of 25–100% of the nominal burden of the instrument transformers. If necessary, additional resistors must be used to achieve sufficient burden.

Voltage transformers must have open-delta auxiliary coils, and a suitable damping resistor must be connected to this circuit.

Transformation ratio

The transformation ratio of current transformers is determined based on the apparent power of the transformer at the measured site. The current transformer should be selected so that its primary current rating is equal to or greater than the calculated current value based on power. It is recommended to use a current transformer with two primary current ranges in case the connection’s power demand increases later.

All transformers must be positioned so that their nameplates are readable from the switchgear door. In addition, the connected transformation ratio must be clearly marked on the outside of the measurement cubicle door or at the location of the measurement terminal blocks.

If the electricity user’s power demand changes, necessary actions must be taken to adjust the transformers to match the load. The electricity user is responsible for the costs of replacing transformers.

Auxiliary voltage for the meter

If necessary, auxiliary voltage of 100–240 V AC/DC can be connected to the meter. Connecting auxiliary voltage to the meter is always recommended if available.

Energy measurement pulses 

Energy measurement pulses (P+, P-, Q+, Q-) can be connected from the meter to remote control, building automation, or similar systems. The contact rating is max. 240 V AC/DC and 100 mA.