Dielectric Withstanding Voltage Leakage Current Limit
Are you setting your DWV leakage current pass/fail threshold too low?
The Dielectric Withstand Voltage (DWV) test, also sometimes referred to as a “Hipot” test, evaluates the ability of electrical insulation to prevent sparks. The test accomplishes this by applying high voltage across the insulation (a dielectric), to ensure it has sufficient strength to withstand momentary over-voltages that might occur from switching surges or other transient events during normal operation.
The DWV test assesses several key factors:
- Insulation Integrity: It checks if the insulation is free from defects or degradation that could lead to electrical breakdown.
- Safety Compliance: It ensures that the insulation can prevent hazardous currents from reaching users, thereby meeting safety standards.
- Quality Assurance: It helps identify any material or workmanship defects in the insulation during the manufacturing process.
If a spark occurs during the application of high voltage, an electric current enabling the spark will briefly flow. This current is known as leakage current and its intensity, measured in amperes, is the defining factor of the dielectric material’s strength. Therefore, the measurement of leakage current is the key element in the design of hardware and software required for reliable DWV testing.
As a background point, the term “spark” is being used here instead of “arc.” As the two are commonly confused, it’s worth noting that a spark is the discharge of a limited supply of energy through a plasma while an arc is the discharge of a constant supply of energy through a plasma. Further distinction between the two is unnecessary as by definition, “spark” is the appropriate term here as elaborated below.
An electric spark is characterized by a sudden, brief electrical discharge that occurs when a high electric field creates an ionized, electrically conductive channel through a normally insulating medium, such as air or other gases. This discharge produces a bright flash of light and a sharp sound, often described as a “snap” or “crack.” As the gas in the gap is ionized when the spark forms, its resistance is drastically reduced. As a result, the electrical resistance through an electric spark is very low, almost negligible, allowing current to flow easily for the duration of the spark.
For example, assume this resistance is as high as 1 ohm while the applied test voltage is 1000 Volts. Using Ohms law, it can be calculated that the resulting leakage current will attempt to reach an extremely high, and unsafe, 1000 Amperes. However, in typical DWV test systems, the voltage supply limits the available current. The point here is that a dielectric event producing a spark can be easily detected by monitoring and evaluating the leakage current as the voltage is applied. In fact, a reasonable leakage current limit specification in the range of milliamps is enough to detect the dielectric failure event.
There is a tendency to specify a leakage current limit for DWV testing by using Ohm’s law to convert the insulation resistance specification, which is usually several hundred million ohms, to current. The resulting leakage current limit specification is hundreds or thousands of times lower than necessary or practical and often produces erroneous failures that mistake normal charging current for a spark that did not, in fact, occur.
To summarize, the leakage current limit arrived at by dividing the applied voltage by the insulation resistance too often results in false DWV failures and unnecessarily complex and expensive test instruments.
The IPC/WHMA specification recommends 1 mA at 1000 VDC for Class 2* assemblies and 1mA at 1500 VDC for Class 3** assemblies.
*Class 2 Dedicated Service Electronic Products includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically, the end-use environment would not cause failures.
**Class 3 High Performance Electronic Products includes products where continued performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support systems and other critical systems.
