Current transformers for power meters: selection, sizing and wiring

As soon as the load current exceeds what the meter can take directly. The EMU Professional II measures directly up to 100 A per phase. Above that, current transformers measure the current and map it to the 5 A or 1 A the meter can handle.

The primary conductor forms the primary winding, usually with a single turn through the window. The current creates a magnetic field in the core, which induces a proportionally smaller current in the secondary winding. The ratio of the numbers of turns determines the transformation ratio.

At 250 A primary current the transformer delivers 5 A on the secondary side. The ratio is linear: 125 A primary gives 2.5 A secondary. This ratio has to be set in the meter, otherwise the displayed values are wrong.

The solid-core transformer is a closed ring. It is slid over the cable or busbar while the installation is being built, which means the conductor has to be disconnected once. The split-core transformer opens up and can be placed around an existing conductor, so it can be retrofitted without undoing the wiring.

The EMCT solid-core transformers are classified to DIN EN 61869-2, the split-core transformers of the TQ series to IEC 60044-1. Both standards define the accuracy classes and the conditions under which they apply.

EMU recommends a working range of 5 to 120 percent of the rated current. Below 5 percent the relative error becomes large; above 120 percent the core leaves its linear range. What counts is the actual load current, not the fuse rating.

Better not. The fuse rating says what may flow at most, not what usually flows. Installations are often generously fused and run day to day at a fraction of that. A transformer chosen by the fuse then measures permanently in the inaccurate lower range.

For billing and third-party quantity allocation, class 0.5 or better is usual. For pure monitoring and peak load detection, class 1 is enough. The overall accuracy of the measurement is limited by the weaker of the two components, meter and transformer.

The burden is the load the transformer has to drive on the secondary side: meter, terminals and measuring cable together, given in VA. If the actual burden exceeds the rated burden, the transformer no longer keeps to its accuracy class.

At 5 A secondary current, 2.5 mm² is usual, more over longer runs. The conductor resistance goes directly into the burden. At 1 A secondary current this influence is considerably smaller, because the losses rise with the square of the current.

A current transformer behaves like a current source. If the secondary circuit is opened while primary current is flowing, the transformer drives the core into saturation and dangerously high voltage peaks arise at S1 and S2. That can destroy the transformer and endanger people.

A test terminal block with shorting links between transformer and meter makes it possible to close the secondary circuit before the meter is disconnected. Without these terminals the installation has to be switched off for every meter replacement. The block should be planned in at first installation.

First close the shorting links in the test terminal block and check the contact. Then disconnect the leads to the meter, connect the new meter and set the transformer ratio. Finally open the links and check the measured values for plausibility against the known load.

P1 and P2 mark the primary side and thus the direction the conductor passes through: P1 faces the supply, P2 the load. S1 and S2 are the secondary terminals to the meter. If P1 and P2, or S1 and S2, are swapped, the sign reverses and the meter counts consumption as delivery.

In many installations one side of the secondary circuit is earthed, to have a defined potential in the event of a fault. What applies in a specific case depends on the installation regulations and the grid operator. This question belongs before installation, not after.

Under Article 13 paragraph 1 of the FDJP Ordinance on Measuring Instruments for Electrical Energy and Power (EMmV, SR 941.251), the validity of verification of inductive and capacitive instrument transformers is unlimited. For other transformer types the Federal Office may set a re-verification period case by case at type approval.

Yes. The unlimited validity applies only to the transformers, not to the meter: electronic active energy meters must be re-verified every ten years under Article 6 EMmV. The statistical sampling procedure permitted for directly measuring meters under Annex 4 expressly applies only without upstream transformers.

Yes, provided the measuring instruments used meet the requirements of measuring instrument law and the transformer ratio is correctly set and documented. Metering towards the grid remains the responsibility of the distribution grid operator.