Is Your Hipot Tester Safe? Ten Whys of Electric Safety Testing
Author: Stephen Wu
Director of Business Development Department
Foreword
Withstanding voltage, insulation and grounding resistance (Ground Bond) are the test functions of the electric safety tester. Among them, the withstanding voltage test is a required test item for electrical equipment before leaving the factory. Although these tests are widely applied, many engineers still have some questions that they seem to have answers but actually they don't. In this application white paper, we have complied the ten most frequently asked questions and explored the answers to these questions from the fundamentals. The ten questions are:
- What voltage is set for the withstanding voltage test? Why is the voltage of the withstanding voltage test much higher than the working voltage?
- What is the common withstanding test voltage?
- How long should the test time be set?
- How to choose the output capacity of the withstanding voltage tester?
- What is the definition of insulator and insulation breakdown?
- According to the definition of insulation breakdown, how do we avoid misjudgment?
- Does the appearance of ARC and Flashover mean that the insulation has broken down?
- In the test sequence of insulation resistance and withstanding voltage, can the withstanding voltage be excluded after passing the insulation resistance test?
- Will the high voltage withstanding voltage test damage the expensive chips in the circuit?
- What is the difference between grounding and continuity measurements?
In addition to these ten frequently asked questions, due to the safety hazards of high-voltage instruments (instruments used for insulation resistance measurement or withstanding voltage test), IEC has formulated the latest regulations for high-voltage instruments IEC-61010-2-034 to ensure the safety of engineers while using the instrument. This white paper will also discuss these safety hazards and the countermeasures of instrument manufacturers, so that you can safely complete the tests when analyzing insulation materials or conducting production testing. Finally, we will provide effective suggestions for the test plans from R&D to production testing, as well as equipment deployment.
Ten Whys of Electric Safety Testing?
1. What voltage is set for the withstanding voltage test? Why is the voltage of the withstanding voltage test much higher than the working voltage?
Figure 1 - Schematic diagram of the power transmission and distribution system
Red represents power generation facilities (including generator sets and step-up transformers of power plants); blue represents power transmission facilities (the purpose of using 765, 500, 345, 230 and 135kV high-voltage transmission is to reduce losses on transmission lines); green represents power distribution facilities; Black is users (large factories use UHV power 138, 161 or 230kV; medium-sized factories use extra-high voltage power 26, 69kV; small factories use high voltage power 4, 11, 13, 22kV; residential and commercial power consumption is 110, 120, 220, 240V).
Figure 2 - Schematic diagram of residential and commercial power distribution
The withstanding voltage test of insulating materials is based on the maximum transient voltage that may be generated at the location of the transmission and distribution lines where the material is located. The sources of transient voltages include lightning strikes, induced voltage(V=Ldi /dt) caused by inductive load switching in lines, poor grounding, electrostatic discharge (ESD), and troubleshooting of mains power. The maximum transient voltage of each section of the transmission and distribution system is different. Taking residential and commercial 110~240V as an example, the transient voltage will reach 1000 volts (Figure 3), the transient voltage results for 24 hours on the residential power line recorded by GE.
Figure 3 - Transient voltage
The common withstanding voltage test formula is: 1000V + twice the working voltage, which is based on the research in Figure 3. The insulation materials must be able to withstand the possible transient voltage of 1000V at the power distribution location, and twice the working voltage is the working voltage plus the working voltage, which is used as a Margin to ensure that the insulation of electrical facilities is safe.
Another clue of transient voltage can be found from the measurement category of EN61010-2-030 standards. The measurement categories of the instrument are divided into CAT II (socket to primary side of electrical equipment) surge peak voltage of 2500 volts, CAT III (socket to non-fuse breaker of distribution board) surge peak voltage of 4000 volts, CAT IV (distribution board to low voltage distribution equipment) surge peak voltage of 6000 volts.