10. What is the difference between grounding and continuity measurements?
Good insulation is the first line of defense for equipment users (no leakage, no electrical shock to people). When there is a problem with the first line of defense, good grounding is the second line of defense to protect users (After electricity leakage, lead the electricity to the ground), When the equipment leaks current, people touch the outer casing of the product, and if the human body is a relatively high resistance to the ground, it is known from the equivalent circuit that the two resistances are connected in parallel, and the shunt of the large resistance is small, which is the meaning of the second line of defense. Therefore, the ground wire of the electrical equipment must be installed correctly, and the current can be quickly conducted to the ground when there is leakage.
Ground bond (GB) test is through a large current of 25 to 40 amperes to have the voltage drop on the grounding wire by 4~12 volts. Via the conversion of Ohm's law, this grounding resistance is mΩ level, GB test time is two minutes required by some standards, or at least 5 to 10 seconds.
Safety tests include single-function models with AC withstanding voltage (ACW) or two-in-one models with AC withstanding voltage (ACW)/DC withstanding voltage (DCW); AC withstanding voltage (ACW)/DC withstanding voltage (DCW)/insulation resistance (IR) three-in-one models; and AC withstanding voltage (ACW)/DC withstanding voltage (DCW)/insulation resistance (IR)/grounding resistance (GB) four-in-one models. If you want to have the grounding resistance GB test function, you must choose a four-in-one model, so the price will be more expensive.
However, there are some withstanding voltage analyzers such as GW Instek's GPT-10000 series electrical safety analyzers, its ground Continuity test (GC) is the standard configuration. The ground continuity test (GC) uses a small current to measure the resistance, which is the same as the continuity test principle of the multimeter. That is, to measure whether the grounding path is connected, and the test time is fast (about 1 second). But the measurement accuracy (in Ω level) is not as good as GB. When your device does not have the GB function, the test function with GC can at least evaluate the continuity of the ground.
Instruments used for insulation resistance measurement or withstanding voltage test are to output high voltage, measure small current, and then convert resistance through Ohm's law. The past standards did not consider many safety hazards to users. These hazards include:
- The original basic insulation requirements are not enough to ensure the safety of users
- Accidental output: the button for operating the high-voltage output of the instrument may be pressed by mistake
- The instrument may be in a dangerous state of automatic output after the power system is cut off and the power is restored
- The device fails to effectively discharge the residual voltage tested to a safe potential
- No warning is provided on the rear panel when using rear panel for outputs
IEC/EN 61010-1:2010 safety requirements for electrical equipment for measurement, control and laboratory use - Part 1: General requirements. The instrument itself must comply with the IEC/EN61010 standards. Due to the above safety hazards, the IEC 61010-2-034: 2017 Safety Requirements for Measurement, Control and Laboratory Electrical Equipment was added to insulation resistance and withstanding voltage test equipment in 2017. Special requirements for insulation resistance measurement equipment and withstanding voltage testing equipment, provided that the instruments for insulation resistance measurement equipment and withstanding voltage testing equipment with an output voltage exceeding 50 volts (AC) or 120 volts (DC) shall apply this new standard.
After the standard was published, it took effect in 2021. On July 12, 2019, UL released the UL61010-2-034 standards with reference to the IEC standards. On December 24, 2020, China issued the fourth batch of recommended national standards for 2020. (File No. National Standards Committee Fa [2020] No. 53) serial number 115 complied IEC61010-2-034 as plan number 20204746-T-604, which shows that this safety issue has been taken seriously by major countries.
On April 16, 2020, GW Instek released the GPT-10000 series electrical safety analyzer that is the world's first safety analyzer to comply with the IEC-61010-2-034 standards, and provided a complete countermeasure against the above-mentioned safety hazards.
- High-voltage relays and high-voltage transformers adopted double-insulated parts, and the trace distance of high-voltage boards also meets the requirements of double-insulation. The design of the measurement circuit is changed, and protective impedance is added to isolate the dangerous voltage on the high-voltage to meet the double-insulation requirements.
- One of the following countermeasures can be taken to deal with the provisions of the new IEC standard for unexpected output.
2-1: The high-voltage output switch must be pressed for 1 second before it can output (Considering the convenience and efficiency of safety and production testing, GW Instek GPT-10000 series adopt this countermeasure).
2-2: Output switch requires a key to activate
2-3: The output switch needs to be under a spring cover. When starting, the spring cover needs to be opened before pressing the output switch.
2-4: Need to use both hands to press two switches at the same time to output - After the power supply at the input end of the device fails and then recovers, even if the output switch is pressed, the device should still be in a safe condition (no harm). After GW Instek GPT-10000 series detect the disappearance of the input power supply for 10 cycles, output will stop.
- Add a 56k ohm discharge resistor to the discharge circuit to ensure that the discharge mechanism after the large capacitance test in DC mode must be discharged to a safe voltage lower than 30% within 10 seconds.
- The IEC standard requires the use of flashing (50-300 times/min, at least 40% of the flashing time) or a permanent red warning light, or adopting either a visible/variable indicator in a high contrast color, or a high volume audible warning (above 70 dBA/less than 5kHz). GW Instek adopts the method of installing output warning lights on the front and rear panels.
Self-protection in the use of high-voltage instruments
Even if an instrument that complies with the latest safety standards of IEC61010-2-034 is used, if the safe installation and operation methods are not followed at the use site, the instrument operator will still have safety concerns. The instrument must be properly grounded. The importance of grounding has been discussed in question 10 of this white paper. But, how do we determine the grounding condition of the building? Is it grounded when you see three holes in the socket?
Note : This figure is referenced from Amprobe’s website
Figure 10 - Power Socket Checker
A multimeter can be used to check or use the simple power checker in Figure 10. This checker can confirm whether the grounding is properly grounded, and whether the live wire and neutral wire are reversed and other power distribution issues. GW Instek GPT-10000 series have an additional ground connection terminal on the rear panel. When the three plugs of the power supply are not properly grounded, you can use this terminal to make another grounding. Furthermore, the instrument itself has a grounding failure detection function. When the grounding failure is detected, the output will be stopped to meet the protection requirements (this function can also be turned off).
Figure 11 - GW Instek GPT-10000 series rear panel
Safety measures such as insulation table mats, insulation floors, insulation gloves, and goggles are indispensable. Since insulation gloves are required during operation, it is not recommended to use the touch screen operation interface design for the withstanding voltage tester.
Test planning and equipment deployment from R&D to production testing
Research and development stage: Through destructive experiments to understand the margin of insulating materials. The safety analyzer must have the V-I curve scanning function to confirm insulation breakdown. Next, through the statistical function, the average value is calculated, and the production test acceptance standard is set. The insulation breakdown V-I curve in Figure 7 can be reviewed, and the statistical analysis is shown in Figure 12.
The GPT-10000 series provide screen capture function, which can store test images to simplify your standard operating procedure (SOP) work for making test reports or production testing so as to save time. Deployment of production testing: When the process is stable and the yield is high, the efficiency of production testing can be improved through an external multiplex scanner boxes or a built-in multi-channel safety analyzers (see Figure 13). If the safety analyzer for production test supports the setting copy function of USB flash drive, it can save the test deployment time of the production line.
Figure 12 - Statistical analysis
Figure 13: Multi-channel testing solution
Conclusion
Selecting the withstanding voltage/insulation test equipment that complies with the latest safety standard IEC 61010-2-034:2017 and correct grounding is the first priority to ensure safe use. The only way at the application level is to understand the test standards and the working principle of the instrument. Through the explanation of these ten questions, we hope that the work of safety engineers will be benefited. According to the statistics of Taipei City from 2017 to 2019, fires caused by electric factors accounted for 20.1% (783 cases/2560 cases), and the casualties caused by electric fires were 29 injured, accounted for 46.8%, and 15 deaths, accounted for 34.1%. These data told us that safety engineers should strictly inspect the electrical safety of products, so that these unfortunate incidents can be minimized.
References
1. February 2013 Fire Protection Monthly: Discussion on Electrical Accident Handling (Part 1) Author: Xu Zhongxian
2. An introduction to the safety regulations of Cerpass EN 61010
3. The Hipot test by Richard Nute : October 15, 2014
4. Schneider Electric - Data Center Science Center White Paper 18 Seven Types of Power Problems By Joseph Seymour and Terry Horsley
5. Wikipedia
6. Electric Strength Test to Determine Insulation Characteristics Products Safety Test Column (4) Author: UL
7. Electric Power Issue 16 November 2014 Power Technology Column: High or Low Insulation Resistance Authors: Huang Shenglu, Qiu Minyan, Li Changxing, Yan Shixiong
8. NTS Corporation Website https://www.nts.com/services/testing/electrical/electrochemical-migration/
9. Amprobe Corporation Website
10. Central News Agency reported: 44 deaths in Taipei City fires in 3 years, more than 30% are electrical causes, reporter Huang Liyun April 26, 2020