Grounding of weighing instruments (electronic weighing instruments are just one type, but weighing instruments have their particularities. The load-bearing devices of large weighing instruments mostly work outdoors, and direct lightning strikes have a significant impact on them) is a well-worn issue. There have been 5 to 6 similar articles on "lightning protection and grounding" in the "Weighing Instruments" magazine alone. However, there was almost no discussion on the types of grounding, maintenance, calculation of grounding resistance, and potential reference points. In terms of the power supply grid, there are also some deficiencies. For instance, the national standard has stipulated that the power grid should adopt the "three-phase five-wire system" for power supply. Given that all current power grids are powered by the "three-phase four-wire system", it is impossible to change the current situation at once, and renovation is also unrealistic. Most of the time, the remedial measure of adding "grounding devices" is adopted to achieve this.
Every year during the thunderstorm season, we can hear a lot about the harm and losses caused by lightning to human beings. The most typical example is the lightning strike and fire at the Huangdao Oil Depot in Qingdao, where tens of thousands of tons of crude oil were reduced to ashes, causing huge economic losses. The most important measure to prevent damage caused by lightning is to achieve grounding. Grounding is one of the important measures to ensure the normal operation and safety protection of weighing instruments.
Grounding is achieved by making a good electrical connection between any part of the weighing instrument and the earth (soil). When the power grid is a three-phase four-wire system, an additional grounding device is usually made for the weighing instrument. A grounding device is usually composed of a grounding body and a grounding wire. The part that is in direct contact with the soil (generally a metal body) is called the grounding body, and the wire connecting the weighing instrument to the grounding body is called the grounding wire.
Ii. Grounding types of the system and commonly used types of electronic weighing instruments
According to the provisions of GB14050-2008, there are three types and five grounding types for the system, namely: TN system, TT system and IT system. Among the TN system, it is further divided into "TN-S system, TN-C system and TN-C-S system". The following introduces their grounding types respectively.
The state recommends the use of the "TN-S system" for grounding. However, since the original power grids were all configured according to the three-phase four-wire system, considering the current situation, the TT system is a remedial method for the original three-phase four-wire power supply lines. But in actual grounding arrangements, IT mostly appears in the form of an IT system. Generally, the resistance of the grounding wire (protective conductor) is required to be only 0.5Ω, while the requirement for the neutral conductor is mostly 10Ω. Therefore, after the TN-C system and the TNC-S system are used, their grounding effects are somewhat poor. The resistance of the protective grounding conductor for the IT system should be controlled to the minimum (0.5Ω requirement).