AC Hipot Testing
Hipot testing, also referred to as Dielectric Withstand (DW) testing, uses DC or AC high voltage to stress test insulation. This article focuses on DW testing using AC voltage, particularly on the CH2 test system.
What is AC Hipot?
Alternating Current (AC) changes direction in regular cycles over time. AC current is generated by a voltage source that continuously alternates from positive to negative and back again. The number of times the voltage cycles each second is referred to as frequency, which is measured in hertz (Hz). Depending on the country, the electricity delivered on the power grid typically has frequency of 50 or 60 cycles per second (50 Hz or 60 Hz).
Definition of Terms
Plotting the AC voltage over time produces a graph like the example here for one cycle of 120 Vrms at 60 Hz. Notice that the voltage starts at 0V, then climbs to +170V, drops to -170V, and finally finishes at 0V again. Graphing the voltage over time produces a waveform, which for true AC is a sine wave. Using the graph, we can identify different terms used to describe AC voltage.
Instantaneous voltage is the voltage at any instant during the cycle.
Average voltage (Vavg) for a full cycle is always zero, which is not very helpful in describing AC waveforms. Therefore, when the term is used it refers to the average of all instantaneous voltages for the positive or negative half cycle.
Peak voltage (Vpeak or Vpk) is the highest voltage magnitude reached during a cycle (170V in the example).
RMS voltage (Vrms) is also referred to as the Effective Voltage, is the “root mean square” of the voltage. When working with sinusoidal waveforms like AC voltage, RMS is useful for calculating the power and work that can be done by the sine wave. Therefore, the RMS value is typically used to describe AC voltage.
Converting AC voltage values
Vrms = 0.707 * Vpeak
Vavg = 0.637 * Vpeak
Vrms = 1.11 * Vavg
Converting a hipot test from Vrms to DC
Although the topic of this article is AC hipot testing, if a specification cites voltage in AC Vrms, and the capability to perform AC Hipot testing is not accessible, it may be acceptable to perform the test using an equivalent DC voltage. The conversion of AC Vrms to a DC equivalent and vice versa can be calculated using the formulas:
DC Test Voltage = 1.414 * AC Vrms
Vrms Test Voltage = 0.707 * DC Voltage
The Hipot Testing Process
On a CH2 AC hipot (DW) test, the test voltage is specified in Vrms, the frequency in Hz, the dwell time in cycles, and the pass/fail threshold for Total and Real current as mciroamps (µA) or milliamps (mA). The test starts with all terminations in the Device-Under-Test (DUT) held at ground (0V). Then each net, one at a time, is brought to high potential by connecting all the points in the net to the high voltage AC source while all other nets are held at ground (see Figure 2). During each step, the current that flows from the source to ground is measured. If the measured current exceeds the specified maximum, the test fails.
Total vs. Real Current
The example below shows the red wire connected to the high voltage source while the green and blue wires are held at ground. The current flowing from the red wire to the blue and green wire (ground) consists of two main components – resistance and 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 below). This current is often called the “in-phase current”, “resistive current”, or “real current.”
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 flowing 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.”
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.
The graph below includes the total current which adds the resistance and capacitive current to produce the total current curve.
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.
What About Safety?
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.