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Cable, Harness and Backplane Testing Made Easy! | |
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.
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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Figure 1. This graph shows how the voltage changes over time for a 120 Vrms/60 Hz source. One cycle of voltage is shown here. 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 "sinewave." 
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What are peak, average, and rms voltages?
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There are several ways of measuring this AC voltage: peak, average, and rms are the most common. Figure 1 shows how all these ways are related.
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. (For more info see the Introduction to Hipot Testing.)
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Figure 2. 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. |
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 the "real current."
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Figure 3. 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. 
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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 (see Figure 4). This current is often called the "out-of-phase current," "capacitive current," or the "imaginary current."
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Figure 4. 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. 
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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.
Figure 5. Enter a hipot voltage (Volts-rms), an insulation resistance (Mohms), and a capacitance (nF).
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Results:
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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.
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. That will increase the capacitive current. Simple manufacturing variations (that have no real affect on quality) can change the capacitive current a lot.
For more information see our page on Causes of Bad Insulation.
During a hipot test the tester is measuring the total current flowing (the sum of the resistive and capacitive current). In typical cables the capacitive current is much bigger than the resistive current. The resistive current is often a good measure of cable quality so separating the resistive current from the total current allows you to monitor the cable quality more directly. It takes a little science and math to separate resistive current from the total current but it can be done.
The CH2 measures the total current flowing during a hipot test, and separates out the resistive current. The easy-wire software gives you control over the maximum allowed resistive current (called the "Real Current") and the maximum allowed total current (in rms mA).
The CH2 makes it easy to do a hipot test with AC voltage. In the test editor you can select:
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 slightly higher than the amount actually needed for the cable to pass, this will help shut off the voltage even quicker if a person comes in contact during testing. Also your body looks resistive to the high voltage supply, not capacitive, so setting the Real Current Threshold to a low value helps the tester recognise you and shut off even sooner. For more information on safety see the Hipot FAQ.