Webinar «Answers to the Designers' Questions (Video lecture)», page 2

The eleventh event of "Earthing and Lightning Protection: Design Issues and Problems" series

Webinar text. Page 2

How is an earthing device for functional earthing of medical equipment made in accordance with SNiP 2.08.02-89?

– The question I am going to answer caused consternation in me. You know, I did not believe the person who asked it. The question was: how is a standalone earthing device for functional earthing of medical equipment is made according to item 36 of the Guidelines for Designing Healthcare Institutions, namely: how to ensure the distance of 15 meters? I will briefly explain to you what it means. I did not believe what the customer wrote. I took this SNiP and saw that high-precision medical equipment, such as X-ray equipment, medical imaging units, equipment for some complex functional diagnostics, really requires a standalone earthing device with the resistance of not more than 20 Ohm, which must be installed at the distance of not less than 15 meters from the functional earthing of the hospital. This is what it states. Then, the fantastic things are provided there, that the bus of this earthing is taken into, for example, an intensive care unit, and a special terminal is made on the wall. And in one case, the terminal is made at the height of 1.6 meters above the floor while in another case, it is made at the height of 1 meter above the floor. And such arrangement is mandatory.

Square earthing circuit with the 10 x 10 m cells

– The estimated graph is shown here. If the soil resistivity in the hospital location is, for example, 100 Ohm m, it means is a very good soil. In this case, you divide 2 Ohm by 100 and obtain 0.02, and if I take these 0.02 then it turns out that the earthing circuit I need will have the dimensions of about 25 x 25 meters. In general, it is not clear for me how, in a densely built town, we may have a free land at the distance of 15 meters from the hospital to do this. It is OK as 25 meters can be somehow found. But if the soil resistivity is 1,000 Ohm m, which is rather typical for the Russian Federation, you need an earthing circuit of 225 x 225 meters to achieve these 2 Ohm. I cannot even imagine how to make it in the urban area. It sounds like a science fiction. And it is very difficult to meet the requirements of such standard. Now, the second question is: what will happen if I ensure this requirement? This is not a trivial thing.

Earthing circuit

– Look at what happens. Now, in this slide, the building’s foundation is shown in bold in the left part of the slide. It will be probably a technological earthing of this building. An earthing circuit is installed at the distance of 15 meters. I have taken a small earthing circuit for the soil of 100 Ohm m, i.e. I have used more than favorable conditions. What do we have as the result? And it turns out that the circuit is not exposed to a zero potential but it is exposed to the potential that is equal to that value. What is written here? It means that each ampere of the lightning current will create 0.3 V of surges. The current is 100 kA, which means that you will have about 28 kV of voltage on the most intensive bus that was taken to the ICU and that is installed on the wall at the distance of 1.6 meters or 1 meter above the floor. And you have a heating device nearby that is attached to the technological earthing circuit in the building. And this device has another voltage. It is defined as 1.1 multiplied by the lightning current. If it is 100 kA, then it will be 110 kV now.

How to design the lightning protection for the structure when the gas pipeline is located nearby, in 10 meters from it?

– This is the question. How to design the lightning protection for the structure when the gas pipeline is located nearby, in 10 meters from it? You know, before answering this question, I would like to ask the customer. How did he manage to get approval for building his facility at the distance of 10 meters from the trunk gas pipeline? I am afraid that you cannot obtain such approval even if you pay the officials for it. You just cannot do that. But if, for some unknown reason, the gas company approved such construction, it must provide the requirements for the arrangement of lightning protection and for the arrangement of the earthing circuit of the building that is being built near their pipeline. They must specify such requirements because they do not exist in any lightning protection standards. What is the point? It is easy. You build a building. And in case of absence of any special requirements your building will use the foundation of this building as a technological earthing. And it means that at the distance of 10 meters from the pipeline, you will have an earthing circuit through which the lightning current will flow. The question is what portion of the current will enter the pipeline? This task can be solved. I have solved it for illustration in the following situation. I have used a building with different lengths of the foundation sides. It is shown here, along x-axis. And the pipeline, a section of the pipeline in interest is 400 meters long. It does not really matter because the result does not depend on its length.

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