Current transformers in electrical diagnostics

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All electrical installations, devices and machines rely on the quality of electrical insulation. A failure of insulation usually leads to a failure of the system in which it is used, often with catastrophic consequences.

Apart from the damage to equipment, there could be risks of injury or death, as well as severe environmental consequences if chemical substances are involved.

There are national and international regulations for the design and installation of wires and cables, as well as earthing/grounding. In North America, the National Electrical Code (NEC) is often used. Other countries usually have their own national regulations.

Many of the requirements focus on the safety of people, which can be evaluated in many different ways depending on the application, power and voltage level, point of connection in the installation, and other factors. An important test, however, is the measurement of insulation resistance.

Leakage current and insulation resistance
The underlying principle of insulation testing is a direct application of Ohm’s law. Voltage is applied to a given circuit and the very small resulting current is measured. For good insulation, it can be at the nanoamp level.

The unknown resistance can be then calculated as Rins = V/I. However, there are cases where the test voltages and currents can’t be applied or measured in such a direct way because of conditions such as a leakage current between the source and the object under test, or the configuration of the internal circuitry.

Health and safety regulations often require all mains-powered portable equipment to be kept in a safe condition. One of the ways to ensure this is to carry out regular tests on all portable equipment; so-called Portable Appliance Testing (PAT).

With commonly used mains-powered equipment, the person carrying out the test might have electrical access only to the mains plug, which has connections to live (L), neutral (N) and protective earth (PE) pins.

The internal wiring is unknown and insulation quality can’t be easily evaluated from a direct measurement. For this reason, PAT test instruments are designed to accept the mains plug from the unit under test, and some tests are actually carried out when the unit is powered.

For some well-isolated devices, the quality of the insulation can be assessed by direct measurement of the leakage current that flows through the protective earth (PE) pin. But this may not be possible for non-isolated devices, because there could be an additional leakage path to earth. This might be unintentional (such as a building flooded with water), or intentional (such as a dedicated earthing or bonding connection). In these cases, the direct measurement of current in the PE pin doesn’t give full information about the total leakage current.

The leakage current can be measured indirectly by means of a differential current transformer (CT). The L and N wires are passed through the CT. Any current flowing into the instrument in L and flowing back in N means real energy used by the device. But current flowing in any different path means leakage due to non-ideal insulation.

The difference between L and N currents is therefore directly proportional to all the leakage currents. Such differential measurement is quite challenging. In some cases the measurement can’t be performed by measuring two separate currents.

Such absolute measurement accuracy may be possible to attain in high-precision calibration laboratory, but not in everyday portable equipment!

With a differential current transformer, a high permeability magnetic core automatically performs the task of summing the contributions from L and N conductors threaded through the core as the primary winding.

As a result, the secondary winding produces signal proportional directly to the difference of the two large currents, because they are made to flow in opposite directions. Any additional conductors like PE are routed outside the differential CT, so the currents in these are ignored.

Secondary winding
The differential CT technique is also commonly used in other applications. For instance, most RCDs (residual current devices) operate on the same principle. The L and N conductors are the primary windings, and the secondary winding detects difference between them.

The CT in an RCD can also have the secondary winding wound only around a part of the core. All current transformers are affected by conductor positioning. If the secondary winding is non-uniform, a conductor placed closer to denser part of the winding will induce more signal than a conductor carrying identical current placed farther away.

Performance is improved if good quality magnetic cores are used. However, using very high permeability materials (such as mumetal or nanocrystalline) is costly, and can’t always be commercially justified. Materials with lower permeability force designers to use other methods instead.

For example, the differential CTs in PAT testers are required to have uniform winding, and the two conductors (L and N) are aimed at being positioned at the exact centre using, for example, physical spacers.

It can also be beneficial to “equalize” the contributions from each conductor. Each of the conductors is split in two parts, with each part carrying 50 per cent of the current. This configuration reduces the conductor positioning effect so that lower permeability and lower cost cores can be used.

Dr. Stan Zurek is Manager of magnetic development with Megger.

This article is an edited version of the original, which appears in the November 2015 edition of Megger’s Electrical Tester magazine.

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