The importance of equipotential bonding in lightning protection technology

The equipotential bonding is also one of the most basic and most important lightning protection measures. It is especially important when the grounding resistance of the grounding system is not easily made small. Its main function is to prevent the electrical discharge from damaging the device due to the high potential difference in the conductive parts of different parts of the device caused by lightning induction.

All kinds of electrical engineering attaches great importance to the role of equipotential bonding, it is necessary for the normal operation and safe use of electrical safety, lightning protection and electronic information equipment. In fact, grounding is also an equipotential connection, it is Equipotential bonding with ground potential referenced to zero potential. The effect of equipotentiality can be proved by the fact that there are few accidents of electric shocks and lightning strikes in aircraft flight. The aircraft does not have access to the earth and the electricity in the aircraft is safe. Earth, but by the equipotential connection to ensure that the body potential as the reference potential in the aircraft for equipotential bonding. Because the aircraft fuselage is a substantially closed metal shell, equivalent to the equipotential cavity metal conductor, shielding effect Very strong, the potential difference caused by the electromagnetic pulse is also very small, so the electrical safety on the aircraft is effectively guaranteed. People live on the earth, so often need to be equipotential with the earth. To connect the electrical system and electrical equipment housing to the earth This is often referred to as "grounding." Terminals on the aircraft can be connected to the fuselage, while on the earth ground poles are used as terminals to connect with other devices.

The equipotential bonding is usually divided into total equipotential bonding and local equipotential bonding. The building equipotential bonding practice is described in detail in the National Building Standard Design Atlas “Equipotential Bonding Installation”. All equipotential bonding installations such as building lightning protection and electronic information equipment against transient overvoltage and interference shall be constructed in accordance with their respective requirements. The role of the total equipotential bonding is to eliminate the potential difference between different metal components and to mitigate the harm caused by overvoltages from the outside of the building via electrical wiring and various metal conduits and other induction facilities. It should pass the total amount of access to the building. The potential connection terminal board (grounding busbar) connects the following conductive parts to each other: PE or PEN busbars of the incoming and outgoing power distribution box; external ties or shields of power, communication cables: metal pipes of utility facilities, above and below, Heat, gas and other pipelines.

Metal structures of buildings; deflectors of lightning protection devices. Grounding wires of natural grounding conductors and artificial grounding bodies. This will make the entire building a good equipotential. When the lightning strikes, it is equipotential inside and near the building, so no internal equipment will be counterattacked by high potential and people will be struck by lightning. In addition, all metal wires associated with the outside world, such as power lines, telephone lines, television signal cables, and computer signal lines, should be connected to a proper overvoltage protection device (SPD), and the ground of the SPD should be connected to the building. The lightning protection grounding device is directly electrically connected to make it equipotential (actually quasi-equipotential because there is lightning residual voltage at both ends of the SPD when a lightning strike occurs).

Local equipotential bonding is to further equipotentially connect the two contactable conductive parts with conductors. The cabinets, racks, equipment enclosures, PE lines, metal trays, and metal pipes of public facilities should all be connected to local equipotential connection terminals.

Equipotential bonding strips (terminal strips EBB) should be made of copper. Due to the softer material, it is easy to crimp and provide extremely low connection resistance when doing equipotential bonding. If it is connected to the steel in the building, it must be brazed.

The section of the equipotential bonding conductor shall comply with the relevant requirements. Using different metal materials or equipotential bonding at different lightning protection partitions, the cross-section of the conductor is different. This is also one of the inspection items for the inspection and acceptance of lightning protection projects.

The equipotential bonding network is used not only for the equipotential bonding of electrical safety, but also for the functional equipotential bonding of the information system from DC to high frequency, but the network form is not the same. The equipotential bonding network for electrical safety is mainly composed of the protective earth (PE line) laying along with the distribution line. The protection earth line must be in multiple places according to the size of the distribution system (for example, each floor or building The equipotential connection is made between the electrical grounding system and the functional equipotential bonding network of the information system. The basic forms of the equipotential bonding network are S-star and M-network, as shown in Figure 1-4.

Figure 1-4 Basic method of equipotential bonding in information systems

In general, S: or M, equipotential bonding networks can be used for relatively small, localized systems. Both systems can be used for systems with low frequency and spurious distributed capacitance secondary effects. They are in the form of single point grounding. Single-point grounding means that only one physical point in a line is defined as a ground reference point. All other points that need to be grounded are directly connected to this point. If the system includes multiple cabinets, the grounding of each cabinet is independent, and inside each cabinet, a single point grounding method is used for each grounding system. Then, connect each cabinet in the entire system to a unique reference point.

This system can be used when the length of the ground connection is much smaller than the operating wavelength of the circuit. In this grounding method, the length of the ground connection is long and many. At high frequencies, the inductance of the ground line is large. This increases the inductance between the ground lines and causes electromagnetic interference, so this system is not used at high frequencies.

If the operating frequency of the system is so high that the operating wavelength can be reduced to the size of the system's ground plane or the length of the ground lead, single point grounding can no longer be used. Because the length of the local line is close to η/4, it is more like a terminal short-circuited transmission line, and cannot play the role of “ground”. At this time, an equipotential connection network with an M-type mesh structure should be adopted. It belongs to multi-point grounding. Multi-point grounding means that each ground point in a system is directly connected to the nearest ground plane, so that the length of the grounding lead is the shortest. The ground plane mentioned here can be either the bottom plate of the equipment or the grounding wire that runs through the entire system, in a relatively large system, and it can also be the structural frame of the equipment.

The perturbation current in each device, device, and circuit of the multi-point grounding can only circulate in itself, and it will not be coupled to other places. Especially in the low-level input stage.

In general, when the frequency of the analog circuit is not greater than 300 kHz, an S-type star structure can be used. When the digital circuit has a rate of MHz, a Mm-type equipotential bonding network should be used.

The ground reference point in the S-star configuration must be the only one between the system equipotential bonding network and the common grounding system. All metal components of a unique information system must be insulated from the components of the common grounding system, except for the equipotential bonding point ERP (ground reference point), such as with insulating brackets or rubber mats. Actual lightning protection works Often do not pay attention to this point, the technical staff responsible for inspection and acceptance should pay attention to this inspection.