AC Hipot Testing

Hipot testing involves using a high potential (voltage) to test insulation. A hipot test can be very useful in helping you develop a process that builds high quality cables and wire harnesses. There are two popular hipot voltage sources: DC and AC. This page introduces some of the concepts you'll find in AC hipot testing on the easy-wire CH2.

What is "AC Hipot?"

Hipot testing uses a high voltage source to make sure insulation is doing its job. An AC hipot test uses a voltage source that continually changes its voltage. The voltage alternates from positive to negative and back again. Generally there are 50 or 60 complete cycles of the alternating voltage each second.

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The graph above shows how the voltage changes over time for a 120 Vrms/60 Hz source.

One cycle of voltage is shown above (right). The voltage starts at 0V. Note how the voltage climbs to +170V, then drops to -170V, and finally finishes at 0V again. This shape of waveform is called a "sine wave."

What are peak, average, and rms voltages?

Peak voltage (Vpeak) is the highest voltage magnitude reached during a cycle (170V in this case).
Average voltage (Vavg) isn't very useful for AC waveforms since the average of one complete cycle is 0V (that's why RMS is used). Still, many meters rectify the AC waveform and take the average. Rectify-then-average produces a voltage reading similar to the RMS voltage.
RMS voltage (Vrms) is the "root mean square" of the voltage. It's like the average of the rectified voltage except it takes the average of the square of the voltage, then takes the square root. RMS is useful for calculating the power and work that can be done by the sine wave.
For a sine wave: Vrms = 0.707 * Vpeak, Vavg = 0.637 * Vpeak, so Vrms = 1.11 * Vavg.
Converting a hipot test from AC (Vrms) to DC

This page is about using AC hipot testing, but you may find you have a specification in AC (Vrms) even though your tester only has DC hipot testing. The usual thing to do is to convert the AC specification to Vpeak and test at that voltage with your DC tester.

DC Test Voltage = 1.414 * AC (Vrms) Test voltage
AC (Vrms) Test Voltage = 0.707 * DC Test Voltage

Hipot Testing Review

A hipot test starts with all of the points in a cable held at 0V. Then a single wire or network of connected points (called a net) is disconnected from 0V and connected to the AC source (see Figure 2). The total current that flows is measured. If too much current is flowing the cable fails the hipot test.

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The high voltage is connected to the red wire while both the green and blue wires are held at zero volts.

The high voltage is applied and the Total Current is measured. Some of the current flows because of the resistance of the insulation from wire to wire. Some of the current flows because of the capacitance from wire to wire.

Current

When the high voltage is applied, current will flow for several reasons. The two biggest reasons are insulation resistance and net-to-net capacitance.

The current flowing through the insulation resistance is proportional to the voltage. When the voltage is zero the resistive current is zero. When the voltage is at its peak, the resistive current is at its peak (see Figure 3). This current is often called the "in-phase current", "resistive current", or "real current."

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A graph of the Voltage and the Resistive Current for 120 Vrms/60 Hz driving 10 Mohms.

The peak voltage is 170V, the peak current is 17uA. Note that the resistive current tracks the voltage. When the voltage is zero, the current is zero. When the voltage is at peak, the current is at peak.

The current flow through the net-to-net capacitance is proportional to the change in the voltage. When the voltage is going up, current is flowing into the capacitance. When the voltage is steady, no current is flowing. If the voltage is going up fast, a lot of current must be flowing into the capacitance. This current is often called the "out-of-phase current," "capacitive current," or the "imaginary current."

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A graph of the Voltage and Capacitive Current for 120 Vrms/60 Hz driving 15.6nF of capacitance.

The peak voltage is 170 V, the peak current is 1.0 mA. Note that if the voltage is increasing rapidly, the current is large and positive. If the voltage is steady, the current is zero. If the voltage is decreasing rapidly, the current is large and negative.

Experiment with the Resistance and Capacitance

Figure 5 is a simple calculator that allows you to enter the hipot voltage, resistance, and capacitance and it will show the voltage, total current, resistive current, and capacitive current. You can use the calculator to experiment with the relationship between resistive, capacitive, and total current.

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What about an arc?

Up to now we have focused on the two major causes of current flow, insulation resistance, and net-to-net capacitance. An arc is also something that causes current to flow. The current flow from an arc can be confused with resistive or capacitive current, raising both current readings. Arcs generally start suddenly, creating an abrupt increase in current. So arcs can be detected by either the resistive or capacitive current exceeding an allowed limit or a sudden large increase in current.

Which current is more important for assuring quality?

The resistive current is generally considered a good measure of cable quality. The resistive current should be very low on every cable. The resistive current should not vary much from cable to cable.

The capacitive current will vary from cable to cable. The position and spacing of the wires in a bundle will change the capacitance. Moving the wires closer together will increase the capacitance. Simple manufacturing variations (that have no real effect on quality) can change the capacitive current a lot.

Final Note: What About Safety?

The CH2 is designed to comply with safety regulations and be "intrinsically safe." If you come in contact with a wire while the high voltage is applied, the tester will shut off the voltage very quickly. It is a good practice to set the Total Current Limit as low as you can and still pass a good cable. This will help shut off the voltage even quicker if a person comes in contact during testing. Also, since your body looks resistive (rather than capacitive) to the high voltage supply setting, the Real Current Limit which is set at a low value, helps the tester recognise you and shut off even sooner.