How to Protect Your House and Its Contents from Lightning
IEEE Guide for Surge Protection of Equipment Connected to AC Power and Communication Circuits
IEEE = Institute of Electrical and Electronic Engineers of North America (USA+Canada)
1 - Lightning
Lightning is a natural phenomenon caused by separation of electrical positive and negative charges by atmospheric processes. When the separated charge gets very large, the air between the positive and negative regions breaks down in a giant spark (an intra-cloud stroke), or a charged region breaks down to ground (a cloud-ground stroke). The resulting current flow ionizes and heats the air along the path to ~30,000 K (54,000° F). The ionized air glows brightly (the lightning), and the sudden increase in temperature expands the channel and nearby air, creating a pressure wave that makes the thunder. Most (~80%) lightning strokes are within a cloud; most of the remainder are cloud-ground strokes. Strokes between clouds are relatively rare. Most cloud-ground strokes transfer negative charge from the cloud to ground.
Most lightning properties are beyond normal human experience. The cloud-to ground voltages leading to the discharge are tens of millions volts or more. The peak discharge currents in each stroke vary from several thousand amperes to 200,000 A or more. The current rises to these values in only a few millionths of a second (microsecond), and the major part of each stroke usually lasts much less than a thousandth of a second. Each visible event, referred to as a flash, typically consists of 1–6 (or more) individual strokes, separated by <0.1 second.
Lightning behaves very capriciously. Cloud-ground strokes have been recorded reaching as far as 18.6 miles (30 km) horizontally from the base of the cloud.
The frequency of lightning flashes varies widely with location and season.
1.1 - Damage from Lightning
People generally think of lightning damage as what happens at the point where a cloud-ground stroke terminates on a tree, structure, or elevated wiring. This is generally called a lightning strike. Unless the struck items are protected from lightning, the results of the strike are often visible and lasting. But the lightning current pulse continues into conductive parts of the structure, cables, and even underground wiring and pipes. Because the initial lightning impulse is so strong, equipment connected to cables a mile (1.6 km) or more from the site of the strike can be damaged.
Figure 1 shows four ways in which a lightning strike can damage residential equipment, in order of decreasing frequency of occurrence. The most common damage mode shown in Figure 1 (labeled 1) arises from a lightning strike to the network of power, phone and cable television (CATV) wiring. This network, especially if it is elevated, is an effective collector of the lightning surges. The wiring then conducts the surges directly into the residence, and then to the connected equipment.
While not shown in Figure 1, lightning can also travel through the ground (soil), reaching underground cables or pipes. This is another route for lightning to come into a building, and can also damage the cables.
The second most common mode (2) shown in Figure 1 results from strikes to, or near, the external wiring network common to most suburban and rural houses. Air conditioners, satellite dishes, exterior lights, gate control systems, pool support equipment, patios and cabanas, phone extensions, electronic dog fences, and security systems can all be struck by lightning, and the lightning surges will then be carried inside the house by the wiring.
As shown in Figure 1, lightning may strike nearby objects (trees, flagpoles, signs) that are close to, but not directly connected to the house (mode 3). In this situation, the lightning strike radiates a strong electromagnetic field, which can be picked up by wiring in the house, producing large voltages that can damage equipment.
Finally, Figure 1 shows (mode 4) a direct lightning strike to the structure. This type of strike is very rare, even in high-lightning areas. It can severely damage a structure without a lightning protection system (LPS), and will generally damage most electronic equipment in the house. The structure damage can normally be prevented by a properly installed LPS of Faraday rods and down conductors, but the LPS alone provides little protection for the electronic equipment in the house.
1.2 - Basic Protection Against Lightning
The USA National Electrical Code (NEC) and Canadian Electrical Code (CEC) require certain grounding, bonding and protection features which are intended to protect against lightning. Figure 3 shows certain basic grounding and protection requirements of the NEC and CEC. Figure 2 is not intended to be comprehensive of the NEC and CEC requirements. These safeguards greatly reduce the risk of shock or electrocution to a person in the house, and the risk of fires caused by lightning. However, they are totally inadequate to prevent damage to electrical and electronic equipment.
The main features shown in Figure 2 are as follows:
The main building ground (grounding electrode system, in NEC/CEC terminology) is used as the central ground point to which all lightning currents will be conveyed. Independent, unbonded ground rods are not accepted.
The NEC/CEC requirements are intended to remove most lightning surge currents from all signal wires entering the building from utilities. For coaxial cables, only the sheath must be grounded; for telephone wiring (twisted pair) a special building entrance protector (the “NID” ) limits the impulse voltage between both wires and ground to less than ~1000 V. Sheaths of coaxial cables from satellite antennas must also be bonded to the building ground (NEC Art. 810, and CEC).
The NEC/CEC requirements for connecting all metal piping and large metal parts of the structure to the building ground serve two purposes: If there is metallic buried water piping, bonding it to the building ground improves the quality of that ground. Also, in the rare event of a direct strike to the piping, or to a metallic part of the structure, the ground bond conducts the lightning currents safely into the building ground. This greatly reduces the voltage differences between the parts of the structure, and therefore decreases possible injury to the residents, and reduces the possibility of a fire within the structure due to surge currents and voltage flashovers.
These requirements greatly reduce the likelihood of injury to the residents, and damage to the structure itself, from lightning. However, there are many loopholes in the basic NEC/CEC requirements. Most obviously, there is little mitigation if there is a direct strike to the building, especially if the upper sections of the building have no wiring or conductive material to terminate the strike. (Because of the extremely high lightning voltages and surge currents, building distribution wiring built to NEC/CEC standards is inadequate to terminate direct strikes safely.)
More importantly, most buildings now have many additional outside connections—exterior lighting, remote gate controls and security monitors, electronic dog fences, auxiliary buildings, etc., which are often not dealt with in the codes. Any of these connections can bring damaging lightning currents into the building.
Finally, and most significant for many people, modern houses have from $5,000 to, in rare cases, $500,000 of electrical/electronic equipment, such as in utility systems, home entertainment systems, computers, security systems, and building automation systems. All of these are extremely vulnerable to lightning surges brought in on power or signal cables, and the basic NEC/CEC requirements do little to protect them.
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.
2 - AC Power Fluctuations
In addition to lightning, there are a number of other disturbances that can come in on the AC power lines and damage equipment. Some surge protectors can reduce or eliminate damage from some of these perturbations. There is considerable confusion about the overlap between damage from AC power disturbances and from lightning.
Five different anomalies in AC power can damage equipment commonly found in homes. They are as follows:
1. Open neutral events - It is not widely appreciated that “open-neutral events” are a very common cause of damage to customer equipment, at least in some areas. Open-neutral problems arise when the neutral wire (see Figure 2) between the center tap of the distribution transformer and the neutral at the service equipment becomes loose, broken, or disconnected, or where the neutral-ground bond inside the house is defective. At the transformer, the 240 V full-phase output is evenly divided into two 120 V phase voltages, with the neutral wire being common for both phases. If the neutral connection is disconnected or broken, the 240 V full-phase voltage at the house will no longer be divided into two equal 120 V phases. The division will be determined mainly by the relative load on the two phases,4 and the phase-neutral voltages can easily be as different as 200 V on one phase, and 40 V on the other. The excess voltage on one phase can easily damage 120 V equipment.
The AC service entrance protectors alone do not provide useful equipment protection against these events. However, a combination of entrance protectors and some point-of-use (plug-in) surge protectors can protect or reduce damage to equipment plugged into them.
2. Catastrophic overvoltages - Rare “catastrophic” overvoltages can result from accidental contact between high-voltage lines and low-voltage AC distribution lines, due to icing, traffic accidents, falling trees, etc. In such situations, voltages up to thousands of volts can be brought into houses.
AC building entrance protectors may provide some protection against these events. Again, the combination of entrance protectors with the point-of-use (plug-in) surge protectors can offer better protection to the equipment plugged in than either one alone.
3. Sustained AC overvoltages - Sustained overvoltages (typically, over 135 V on 120 V service) can result from malfunction of utility regulators or damaged distribution transformers.
AC building entrance protectors do not provide useful protection against these events. Electronically controlled point-of-use (plug-in) surge protectors described in Section 5.1 disconnect for AC voltages outside a specified range, and offer useful protection to equipment plugged into them.
4. AC undervoltages/brownouts - AC undervoltages (typically, below ~100 V) may result from overloaded transformers or utility or building wiring, or malfunctioning regulators. Undervoltages can lead to equipment damage because motor-driven appliances and some electronic power supplies draw higher current at low voltage and will overheat.
A few of the point-of-use (plug-in) surge protectors are electronically controlled, and will disconnect at low voltage and should protect equipment plugged into them.
5. Utility switching transients - Utility switching transients that come into homes are generally of relatively low voltage and energy.
Switching transients large enough to damage customer equipment will normally be adequately controlled by either building entrance protectors or plug-in protectors.
In areas where the environment is very rugged and utility lines are long and subject to frequent damage, the protectors can greatly decrease damage to residential equipment, for a modest expense. So even if there is little lightning risk, it can be worthwhile to install these protectors.