How do I perform separate grounding?
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I need to build a multi-purpose building, which will include laboratories. The laboratory equipment requires separate grounding, and I have never done this before. May I ask whether \"separate grounding\" means reserving a dedicated grounding plate in the laboratory and connecting it to the grounding electrode shared with the building, or does it mean that the grounding electrode cannot be shared with the building either? Are there any standard catalogs or something like that? Thank you. Another question is: how can we determine during construction whether the grounding resistance meets the 1 ohm requirement? Does one have to bury all the grounding elements first in order to conduct a test, and only add more grounding electrodes around when it is found that there aren’t enough? Is that so?
Some devices have specific requirements regarding grounding; generally, there are two scenarios: one is when it is a requirement set by the device manufacturer as part of early-stage technical specifications, and in such cases the required grounding resistance is not high； Another issue is that the grounding requirements for this device are quite high; typical electrical grounding systems cannot meet the required grounding resistance levels. For the first case, it is recommended to adopt combined grounding (equipotential bonding); this way, when the ground potential rises (due to lightning strikes or lightning induction), the reference potentials rise as well, preventing current flow between different grounding elements. In the second case, a separate grounding electrode is required, but care must be taken in selecting its location; it should be kept at a sufficient distance from the combined grounding (building grounding) to avoid current leakage. When designing a grounding system, it is necessary to know the soil conditions at the location of the grounding electrodes (is there a sand map? Clay? Factors such as the presence of stones and debris, together with soil resistivity, as well as changes in soil moisture caused by seasonal variations, must also be taken into account. Based on these parameters, the type and quantity of grounding electrodes are selected (such as steel pipes, angle irons, copper-clad steel, graphite, etc.), as well as the type of grounding grid (when multiple electrodes are used), along with the choice and quantity of resistivity reducers. After the grounding electrode is buried, it is necessary to measure the grounding resistance. If the measured grounding resistance does not meet the requirements after the grounding electrode is buried, simply adding more grounding electrodes will not suffice to resolve the issue. When the grounding electrodes are too close to each other, stray current interference occurs.
Thank you. When I designed the grounding electrodes, I specified a distance of 5 meters between them. The same requirement is applied when additional grounding electrodes need to be added after the resistance measurements prove to be unsatisfactory. Is that acceptable?
Adding additional grounding electrodes can still be done by increasing the distance to 5 meters as per the design; if the soil conditions are poor, the resistance-reducing agents can be increased or replaced. There are also cases where medium-pressure water pipes are buried next to the grounding electrode, to supply water when the soil is dry.
There are quite a few questions, so I will reply to them one by one: 1. An equipotential bonding box can be reserved in the laboratory for equipotential bonding of the equipment; the protective grounding and functional grounding of the equipment can share the same grounding system, with the rebar in the building columns being used as down conductors and the foundation rebar forming the grounding grid. 2. When designing, a grounding test card should be included and connected to the down conductors, to facilitate the testing of the grounding resistance in the future. 3. Grounding resistance testing should be carried out after the foundation grounding is completed as well as after the entire building’s lightning protection grounding system has been installed; regular tests should also be conducted during operation. 4. If the grounding resistance does not meet the required standards, it is possible to add more grounding electrodes. For this purpose, steel pipes or angle irons can be used, with 3 electrodes in each set, each measuring 2.5 meters in length. The electrodes should be placed at a distance of 5 meters apart from one another, with their tops buried at a depth of at least -0.8 meters, below the frost line.