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.
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."
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 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 sinewave.
- For a sinewave: Vrms = 0.707 * Vpeak, Vavg = 0.637 * Vpeak, so Vrms = 1.11 * Vavg.
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.
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. (For more info see the Introduction to Hipot Testing.)
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. Most of the current flows because of the capacitance from wire to wire. A small amount of current flows because of the resistance of the insulation from wire to wire.
Current
When the high voltage is applied, current will flow for several reasons. Most of the current flows because of net-to-net capacitance. A small amount of current flows because of insulation resistance. These currents add together to create the total current flowing in the test. It's worth looking at each of the currents and how they relate to the voltage.
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."
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.
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."
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.
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.
Figure 5. Enter a hipot voltage (Volts-rms), an insulation resistance (Mohms), and a capacitance (nF).
Results:
- Voltage ___ Volts rms, ___ Volts (peak)
- Resistive Current ___ mA rms, ___ mA (peak)
- Capacitive Current ___ mA rms, ___ mA (peak)
- Total Current ___ mA rms, ___ mA (peak)
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 fairly 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.
Why use AC instead of DC?
Both AC and DC tests are useful in determining how well the insulation is working. Still, there are times when AC would be preferred over DC. A DC test will expose an insulation to a charge in both positive and negative polarities but the test has to change the pattern of voltage connections in the cable to test both polarities. AC exposes the insulation to both polarities without having to change any of the net connections. This difference might make AC better at causing an arc on exposed conductors in certain complex conditions. The IPC/WHMA-A-620A specification recommends using AC when a cable assembly is intended to be used at over 90VAC.
What Total and Real test limits should you use?
If you are creating a specification for a cable here are some important points you should consider. Set your real current as low as your insulation will reasonably allow. Since insulation resistance is usually above 10 Megaohms a fairly low real current limit of 50-200uA is common. Since cable capacitance is usually large and not very consistent from cable to cable set the total current to be a bit above what the cable capacitance will require. Finally, the IPC/WHMA-A-620A specification recommends if the total current is going to be above 2mA you should specify the real current limit for the test.
Hipot Testing on the CH2
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:
- Test voltage (Vrms)
- Frequency (Hz)
- Duration (in number of complete cycles)
- Total Current limit (uA - mA)
- Real Current limit (uA - mA)
Final note - What about safety?
The CH2's internal high voltage power supply 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 more resistive than capacitive to the high voltage supply so setting the Real Current Threshold to a low value helps the tester recognize if you've touched an exposed conductor and shut off even sooner.
The optional xHV Supply for the CH2 can deliver up to 22mA at 1500VAC (2121 V peak). The supply has been designed with safety in mind and it includes many hardware features to maximize safety but it is not intrinsically safe. You are required to use a safety interlock system to prevent contact with the high voltage. The xHV Supply supports single or dual safety switches along with an emergency stop switch to provide flexibility in adapting to your work environment. Just like with the internal high voltage supply you will want to keep the Real Current Threshold and the Total Current Thresholds as low as possible.
For more information on safety see the Hipot FAQ.
