Matching a portable hipot test set to a site requirement means sizing three things against the actual job: the output voltage, which must cover the required test level for the asset class; the output power, which must supply the charging current of the load capacitance at the test frequency; and the site supply, which must support the set’s input demand.

The sizing starts from the asset and the test specification, uses the load capacitance to calculate the current and power, and then checks that a portable set can deliver the requirement from the power available at the location.

Start with the Asset Class and Required Test Voltage

The required test voltage is set by the governing standard or specification for the asset class and the test objective, not chosen from the tester’s catalogue. A cable of a given voltage class has defined acceptance and maintenance test levels, expressed relative to its rated voltage, and the tester must be able to deliver that level with margin. The same logic applies to other assets: the test voltage comes from the document that governs the test, and the tester’s maximum output must exceed it comfortably rather than sitting at the edge of the range.

List the assets the set must cover before comparing models. If the work includes several cable classes and equipment types, the required voltage range is the union of the individual requirements, and the practical choice may be a set whose output covers the largest requirement while remaining light enough for the field. A set chosen for the largest voltage class will often be larger and heavier than necessary for the common jobs, which is why the asset list, not the maximum catalogue rating, drives the decision.

Estimate Load Capacitance and Charging Current

The load capacitance is the engineering quantity that sizes the set. For a cable, the capacitance is the per-unit-length value multiplied by the circuit length, and the manufacturer data or the cable standard provides the figure. For other assets, the capacitance is taken from the equipment data or measured. The charging current at the test voltage is then proportional to the capacitance and the frequency: at power frequency, the current can be large, while at very low frequency it is far smaller, which is why the source family and the frequency are part of the sizing equation.

A worked example makes the scale concrete. A 10 km circuit with a capacitance of 0.25 microfarads per kilometre has a total capacitance of 2.5 microfarads. At a 50 kV test voltage and 50 Hz, the charging current is approximately 2 × pi × 50 × 2.5 × 10⁻⁶ × 50 × 10³ amperes, which is in the order of tens of amperes, requiring a reactive power in the order of megavolt-amperes. At 0.1 Hz, the same circuit needs a current smaller by a factor of five hundred. The example is illustrative, and the calculation should use the actual cable data and the governing test level, but it shows why capacitance and frequency dominate the sizing decision.

Match Output Power and Frequency to the Job

Once the current is known, the output power requirement follows, and the frequency of the source determines which source family can deliver it. A power-frequency requirement on a high-capacitance circuit points toward a resonant system, which supplies the reactive power through resonance while drawing only the active losses from the site. A VLF requirement allows a much smaller portable source because the current at 0.1 Hz is tiny. The choice between source families is therefore not a matter of catalogue preference; it is dictated by the frequency the governing specification requires and the capacitance of the load.

The set’s rated output is only meaningful with its conditions. A hipot set rated at a voltage and a power can deliver the full voltage only into a load whose capacitance is within the set’s current capability, and exceeding the capacitive capability reduces the achievable voltage or lengthens the test beyond the set’s duty. Compare sets on the actual load: the required voltage at the actual capacitance, not the headline ratings alone.

Field Power Supply and Cable Constraints

The site supply is the constraint that defeats otherwise correct selections. A set may be technically capable of the output but draw an input current that the location cannot provide, or require a supply voltage and frequency that the site does not have. Check the input requirement of the set against the available supply, including the effect of long extension cables, which drop voltage under load. Where the supply is marginal, a source family with lower input demand, such as VLF for cables, may be the engineering answer rather than a larger power-frequency set.

Physical constraints belong in the same check: the set must be transported to the test location, set up within the outage window and connected with leads rated for the voltage. A set that fits the electrical requirement but cannot reach the site is not a solution. The practical evaluation includes the weight, the number of cases, the lead lengths and the set-up time, and it should involve the people who will carry and operate the equipment.

Safety and Control Features That Matter

Safety features are part of the rating decision because they determine whether the set can be used safely at the site. The set should provide the controls the procedure needs: a controlled ramp or voltage adjustment, a current limit, an emergency stop, and a discharge function appropriate to the source type. The ground and return monitoring should detect a broken ground path before the high voltage is applied, and the interlocks should prevent energisation while the circuit is incomplete. The safety feature evaluation is developed in the hipot safety features article in this series; at the sizing stage, the question is whether the set includes the features the site procedure requires.

Safety also enters the sizing through the stored energy of the test circuit. A larger set and a larger test object store more energy, and the discharge plan must handle it. The selection record should state the expected stored energy and the discharge method, so that the safety plan is built on the actual equipment rather than on a generic template.

A Sizing Worked Example

Work through the steps with the actual asset data and specification. The example values are illustrative; the decision should be built from the cable or equipment data and the governing document for the test.

The duty rating of the set is as important as its headline output. A portable hipot set that can deliver the required voltage and current for a short demonstration may not sustain the output for the full test duration demanded by the specification, and exceeding the duty rating can trip the set or damage its output stage in the middle of a test. Compare sets on the continuous or intermittent rating that matches your test durations, and ask the vendor how the rating was established. The evaluation should also include the practical serviceability of the set: how easy it is to repair, how available the spare parts and calibration service are in your region, and whether the vendor supports the specific source family you need. A set with an attractive specification but no local support can cost more in downtime than the price difference between models. Finally, keep a copy of the sizing calculation in the equipment file, because the next purchase and the next maintenance decision will both start from the same load data and test requirements.

Frequently Asked Questions

What voltage rating do I need for a hipot test set?

The voltage rating must cover the test level defined by the governing standard for the asset class and objective, with margin. List the assets the set must cover and size to the largest requirement, while considering whether a smaller set is more practical for the common jobs.

How do I estimate the capacitance of a cable circuit?

Multiply the cable’s per-unit-length capacitance, taken from the cable data or standard, by the circuit length. The total capacitance, together with the test voltage and frequency, determines the charging current and therefore the output power the source must provide.

Why does the site power supply matter?

The set must draw its input power from the supply available at the location. A set that is technically capable of the output may still require more input current than the site can provide, so the input requirement, the supply and the extension cable drop must be checked before the set is selected.

For portable hipot and VLF test equipment, see the high voltage insulation and withstand testing page.