Explanation of Countermeasures against lightning surges by equipotential bonding using SPD

Technical information

Countermeasures against lightning surges by equipotential bonding

Basic lightning countermeasures for power lines and communication lines include equipotential bonding by SPDs

What is equipotential bonding?

According to the IEC / JIS standards, equipotential bonding is the means to prevent spark discharge, electric shock, and insulation breakdown of equipment by connecting all metal conductors in common and reducing the potential difference of each part. In practice the ground of an electrical equipment is connected with metal bodies such as building steel frames and piping. Power and communication lines cannot be connected directly to grounding electrodes or metal conductors, so they are connected using SPD. Equipotential bonding is applied in electrical rooms, power distribution boards, MDF rooms, etc.

Specific measures for equipotential bonding ①Integrated grounding

Grounding and bonding are key factors when applying equipotential bonding, which is essentially the core of lightning protection. In order to achieve equipotential bonding with the entire building through grounding, it is desirable to use a structure-based grounding or grounding electrode system (annular grounding electrode, mesh grounding electrode, foundation grounding electrode) and have the grounding of each electrical equipment integrated.

Specific measures for equipotential bonding ②SPD

Since various conductors are drawn into the building. Even if equipotential bonding is achieved between the grounds, a lightning surges can invade from the power line, communication lines, and signal lines. Electrical equipment can be protected by installing the optimum SPD for each intrusion route and equipotentializing it.

Lightning surges also occur when lightning strikes outside the equipment site

Even if lightning strikes outside the equipment site, it must be careful against lightning surge because of the potential difference between the building groundings. Especially, the equipment that is connected over wide area is influenced by lightning surge easily and it is necessary to take countermeasures more sufficiently.

Key point of lightning surge countermeasure

①Reduce lightning surges to a level below the insulation level of the protected equipment ≒equipotential
②Eliminate lightning surge intrusion routes (Insulation)
・Power circuit: Isolation transformer
・Communication / signal line: Optical fiber cable
(It must be careful to handle the tension member if it is made of metal.)

Why the equipment damaged by lightning surges?

Potential difference causes lightning damage

Lightning current flows from a point of high potential to low potential. Therefore equipment gets damage when an abnormally high current (lightning surge) passes through the equipment, from a higher to a lower potential point.

For example, if there are multiple groundings as shown in the above figure, a potential difference will occur between the groundings, and lightning surge will flow in and out via groundings.

Equipotential bonding

By connecting metal conductors (grounding terminals, power lines, etc.) in a building to an equipotential point, the potential difference caused by lightning surges is minimized.

For example, in the above figure, the potential difference can be eliminated by connecting each grounding and making them equipotential.

What is electric potential?

When electricity is considered as physical image, potential energy of electricity (electric charge) means electric potential. Electric potential means the voltage at specific point from zero volt point (typically grounding).
The potential difference is the difference between the voltage at two points. For example, when electricity flows from point A to another point B, point A has a higher potential than point B.