1.3 - Enhanced Protection against Lightning
The NEC/CEC allow for increased protection in high-lightning areas by the optional installation of the following:
- A lightning protection system (LPS);
- Surge protectors on the AC power wiring;
- Additional surge protectors on signal wiring;
- “Supplementary protection” (also called “Point-of-Use” protection) at the equipment to be protected.
Figure 3 shows schematically how the first three above are installed.
Although the lightning protection system is the most visible improvement, it is only useful in the extremely rare direct strike scenario, such as in mode 4 of Figure 1. The basic elements are shown in Figure 3. The lightning strike attaches to the tip of the air terminal, and the lightning current flows via the down conductors into the lightning ground system, which is bonded to the building ground. Properly installed systems should be undamaged by even the largest recorded strikes. They should, however, be inspected periodically to assure that mechanical damage has not occurred.
The design and installation of the lightning protection system is not described by the NEC, but by a related document, NFPA 780-2004. Fortunately there has just been a major recent revision to this code, with strong improvements, especially in requirements to install surge protectors to protect the electrical and electronic equipment inside the house. The new code recognizes only passive strike terminating devices such as metal rods and heavy wires.
The later sections of this Guide provide more detailed information on the selection and installation of surge protectors than is provided in the NEC/CEC and NFPA 780.
AC and signal surge protectors at the building entrance (items 2 and 3 above) serve similar purposes. They collect the major part of the lightning surge currents coming in on external wiring, and direct them harmlessly into the building ground. They also limit the surge voltages that get inside the building, and greatly reduce the burden on the point-of-use protectors, at the equipment.
The effectiveness of this protection system depends on the integrity of the building wiring. A good surge protection system installation should include testing of all the receptacles to be used, for correct connection of the line, neutral, and ground. This should be done using a tester which can detect interchange of the neutral and ground connections, a common problem. Incorrectly wired receptacles can often appear to function normally, but may not allow point-of-use protectors to function properly.
Most new houses are built with power, phone, and CATV entry points close to one another. That is very desirable, and makes it easy to mount the AC protectors and signal protectors close to the main building ground point (Figure 3).
If wiring comes into a building at many different points, it is much more difficult to get proper protection against lightning surges. Even if surge protectors are installed at these alternate entry points, the long ground wires running back to the main building ground greatly reduce the effectiveness of the protectors. In highlightning areas, where lightning protection is a major concern, it is worth routing as many AC and signal cables as possible past the building power entry point, to facilitate good grounding for protectors and cable sheaths.
The coaxial cables carrying CATV signals and small-dish (DBS) satellite signals are often the path for damaging lightning surges to enter the building. For CATV cables, the code-required bonding of the sheath to the building ground is frequently omitted. For the satellite systems, the NEC/CEC require bonding of the antenna mounting hardware, as well as the incoming cable sheath, to the building ground. This is often difficult to do. If the incoming CATV or antenna lines can be routed to a distribution closet near the AC service entry point, the required bonding can be achieved.