The test the set produces has to satisfy the standard governing the cable class, and the set has to cover the circuits in the programme. Both constraints are expressed in the specification, and both are commonly under-stated.

Frequency and waveform requirements by standard

The usable frequency band runs from about 0.01 Hz to 1 Hz, with 0.1 Hz and 0.02 Hz doing most of the work in the field. The frequency selected depends on the load, because the charging current a set has to supply is proportional to frequency, capacitance and voltage.

The waveform is a separate matter. A sinusoidal output contains essentially one frequency component; a cosine-rectangular output, produced because it is efficient to generate, contains harmonics as well. At the same nominal root mean square voltage the two stress the insulation differently, which is why the standard defines which waveforms are acceptable and how the test voltage is measured for each.

The practical consequence for selection is that the waveform must be stated, and the voltage measurement method must match the waveform. A set whose waveform is acceptable but whose voltage measurement assumes a different waveform produces a test voltage that is not the one the acceptance criteria assume. The field test requirements that govern this are published as IEC 60502-2 for medium voltage extruded cables, as IEC 60840 for the higher voltage classes and as IEC 62067 for the highest extruded classes, within the general field test framework of IEC 60060-3.

Load range and how to state it

The load range is bounded at both ends. At the upper end, the current capability limits the capacitance the set can drive at the required voltage and frequency. At the lower end, the set has to regulate correctly on a small load, and some sets are less stable there than on a large one.

Stating the range as a single length of cable is unhelpful because the capacitance per unit length varies with the cable construction. The specification should state the capacitance range and the test voltage, and the supplier should confirm the set covers that combination.

Voltage registration and measurement accuracy

The quantity that determines the test verdict is the voltage at the test object. A set that measures the voltage at its own output, or through a divider that has not been calibrated as part of the chain, reports a value that may differ from the voltage at the cable termination.

The measurement arrangement is therefore part of the set’s capability. The divider, its connecting lead and the measuring instrument together form the chain, and the chain is what has to be calibrated. The measurement system requirements that define how a high-voltage measuring system is qualified are set out in IEC 60060-2.

The set should also record the applied voltage against time so that the test can be reviewed. A set that produces only a single final value cannot show whether the voltage was held or whether it drifted during the test, and the current trace is frequently the first indication of a developing defect.

Portability versus capability

Portability and capability trade against each other in a way that is easy to quantify. A set that covers the longest circuits in a programme has more pieces, a heavier control unit and a longer assembly time. A smaller set is quicker to deploy and covers a smaller load range.

Control and safety features that matter in the field

The control features that matter are the ones that prevent an unsafe or invalid test. An automatic discharge sequence with a verified endpoint, an interlock on the high-voltage connection, a current limit that trips rather than sustaining a fault, and a controlled ramp so that the voltage can be raised and lowered deliberately are all features that affect whether the test can be performed safely and repeated consistently.

The recording features matter for the same reason. The set should log the voltage and current against time, store the result against a test identifier, and export it in a form that can be attached to the test record.

The environmental rating decides where the set can be used. A set used in a cable trench or an outdoor compound needs an appropriate ingress protection rating, and one used only in a substation control building does not.

Calibration and verification evidence

The certificates supplied with a set should cover the control unit and the voltage measurement chain, with the traceability chain and the range of use stated. Where the divider is supplied with the set, the calibration should cover the arrangement as it is used rather than the divider alone.

The verification arrangement between calibrations should also be established. A set whose voltage measurement can be checked against an internal reference or against a known source can be verified before a critical test; one that cannot must rely on the calibration interval alone.

The traceability requirements the certificates have to satisfy are described in the NIST handbooks, with the accreditation framework that makes a laboratory’s scope checkable described by the international accreditation bodies.

Comparing two VLF sets on the same job

The comparison should be made on the hardest circuit in the programme rather than on a representative one. The hardest circuit is the one that determines whether the set can do the work at all, and it is the one on which the two sets are most likely to differ.

For each set, establish the highest test voltage at the highest capacitance in the programme, the frequency at which that combination is achievable, the waveform produced, the way the voltage is measured and the assembly time from arrival to first test.

Recording those five values for each candidate converts the comparison from a datasheet exercise into a test of capability. Where a set cannot meet the hardest circuit, the question becomes whether the programme can be reorganised, whether a second set is required, or whether that circuit is tested by a different method.

Specification checklist

A specification that a supplier can quote against states the test voltage range, the capacitance range at each voltage, the required frequencies, the acceptable waveforms with reference to the governing standard, the voltage measurement arrangement with its accuracy, and the recording and export requirements.

It should also state the environment, the supply available on site, the maximum weight per component and the number of pieces, and the calibration evidence required at delivery. Those items determine whether the set can be mobilised and supported, which is as much part of the requirement as the electrical capability.

Finally, the specification should ask for the waveform and the measurement method to be stated explicitly rather than left to be inferred from a general description, because those are the two attributes most often assumed and most often found to differ. The cable classes the set has to cover are defined in IEC 60502-2 and IEC 60840, field-test configuration guidance is published by high-voltage test equipment suppliers such as Haefely, and practice is coordinated through CIGRE study committees. The VLF and resonant test range is grouped on the high-voltage insulation withstand test hub.