Physics of Lightning Strikes
Lightning strike is the final effect of clouds’ polarization process. In typical atmospheric conditions preceding the lightning strike, water droplets moving upward will gradually freeze. Collisions (frictions) between fast moving droplets of water and crystals of ice, strip the latter from electrons. Heavier, negatively charged icy water droplets tend to drift down, while lighter, positively charged crystals of ice will drift to upper parts of the cloud. As a result, bottom and upper fragments of a storm cloud accumulate large electrostatic charges of opposite signs. Such an electrically “polarized” state of the cloud is unstable. At the first opportunity, positive and negative charges driven by their attracting forces will combine to re-create an electrically neutral space (cloud). In the majority of cases it will happen within or between cloud(s). However, at geographical locations with low flying clouds, favorable discharge conditions may also form between clouds and the earth.
Lightning Storm (Source: Wikipedia – Lightnings in Belfort, Author: Thomas Bresson)
Lightning strikes create characteristic flashes of light (extremely hot, ionized atoms of air) marking visible their “paths of least resistance” throughout the air. Additionally, the high air pressure developed along the path of lightning strikes creates familiar acoustic shock waves (thunders).
All that may already sound terrifying, but the worst part of the story is that these otherwise spectacular effects discharge gigantic amounts of energy. As all that happens in a very short time (fractions of a second), the striking power can have catastrophic effect on targeted objects. While we cannot control or even appease caprices of Mother Nature, we may (and frankly for our security we should) protect ourselves from their impact. We like it or not, lightning strikes are part of weather hazards and so we have to find a way to live with them!
The lightning protection system does not protect from strikes. It only protects the structure (house) from the impact of the strike. Its purpose is to provide the low-resistance path able to channel enormous energy of lightning strikes and safely transfer it to the ground (earth). The Earth in this case serves as a “gigantic capacitor” able to “smoothly” absorb the energy of a lightning strike. Given the fact that the soil (earth) is conductive, the locally absorbed electrical energy will be quickly dissipated over large areas of our planet in an attempt to re-create its electrical neutrality.
The lightning protection system consists of:
a) lightning rod(s) – Placed on highest point(s) of the structure they will be primary targets of the strike
b) Path of low resistance to ground – robust copper (or aluminum) wire able to carry very high discharge currents (reaching thousands of amperes).
c) Ground rod – providing robust, efficient and reliable (lasting) electrical contact with the earth in order to dissipate the energy of the channeled lightning strike.
Example of Earthing Rods (Source: Sangdong Industries Co. LTD )
Such systems did not go far from the old idea first proposed by Benjamin Franklin back in 1752. Well, today we obviously use better quality materials that can last longer and may require less maintenance. Especially, the buried ground rod (not available for visual inspections) must guarantee long lasting performance despite all forces of nature (corrosion than can be accelerated in acidic soils).
To make it clear, the Lighting Protection System does not prevent nor repel strikes. What it does is it provides the secure path for discharging the energy of lightning strikes, effectively “bypassing” the structure of the house! Some suspect that such systems may attract lighting strikes, but that scenario has very low probability given that fact that “lightning discharges” span distances of up to few miles (kilometers) from the cloud to the earth. Typical lightning rods will decrease this span by just a few feet, which does not make any significant difference in the length of the discharge path. The bottom line is that events triggering lightning strikes are in the hands of Mother Nature and depend on quickly changing micro-conditions like humidity and pollution in the air, shape of the landscape etc…
Franklin-Type Lightning Protection System (Source: Solarquotes (Australia), Image credit: Erico)
Unfortunately, while the basic Franklin’s idea is still valid, today’s circumstances greatly changed the scope of the protection. Initially, the goal of lightning protection systems was to prevent damage to the house’s structure and potential fire by preventing discharge currents from passing through the structure. These days, a typical house with all its interior electrical installations (cables, appliances and tons of electronics) is much more “fragile” than in Franklin’s times. That’s why each contemporary Lightning Protection System must include one more vital sub-system:
d) Surge Protection (Suppression)
Once the lightning directly strikes a house the protection system (if any) will channel the major part of the energy down to the ground. However, by the nature of physics, all metal (conductive) components of the house’s structure (interior electrical installations, water and gas pipes etc) will “compete” with the exterior protection system to provide extra discharging paths for lightning strikes.
Even the best lightning protection system cannot prevent side-flashings or “electro-magnetic couplings” from the main discharge path into wires and metal pipes.
Such “infiltrations” of our interior installations will also take place from nearby lightning strikes (which are much more likely to happen than a direct strike). Simply speaking every wire, be it the power grid, telephone lines or coaxial (TV and Internet) cables will pick-up fractions of electromagnetic energy from lightning strikes and spread it along the networks. These short “bursts” of energy will manifest as voltage spikes well above standard 110/220Vac amplitudes and will destroy unprotected equipment. The sad fact is that modern houses are “stuffed” with sensitive electronics so the damage may be very costly.
Note that fiberoptic cables (unless armored with metal sheath for mechanical protection) will not be part of the lightning discharge path.
Surge Protecting Devices (SPD)
The surge protecting devices are connected between power lines and ground. Most typical solutions are based on:
- Nonlinear Components
The resistance of nonlinear components (used as “shunt” configuration to Ground) changes its value in function of the input voltage. Within the nominal voltage range, they have high resistance, so they act as isolators. However, when the voltage increases above the specified threshold level, their resistance sharply decreases so they “create” a by-pass path (short) to ground for all high-voltage transients. In more understandable words, they act like water overflow protection systems in your bathtub and sink diverting water into the drain once it reaches a specified level. Most popular nonlinear protection systems are based on Varistors.
Varistor-based industrial-grade Surge arrester (Source: Zulkurnain Abdul-Malek – “Condition Monitoring of Zinc Oxide Surge Arresters”)
- Air-gap (Spark-Gap)
Mainly used in high-power, high-voltage systems (power grids), air-gap surge protector uses the effect of an “electrical arc”. The electric arc will develop between two wires separated by an air gap when they are exposed to big enough difference of potentials (voltage). The distance (air-gap) and mechanical shape determine the voltage threshold level at which an electric arc develops between the active (line) wire and the one that is connected to the earth (GND). As a result, the excessive energy of lightning strikes will be diverted into the earth.
Lightning Spark Gap Protector – Model KFSU (note that electrodes are encapsulated to eliminate the impact of weather on its function). (Source: Dehn+Söhne, Germany)
- Surge Capacitors
By design, capacitors represent an “Open circuit” (isolator) for DC voltages and conductive elements for AC voltages. Their “conductivity” (ability to transfer signals) increases with the frequency of the signals (correspondingly their impedance decreases). That’s why, they are ideal components to suppress fast voltage transients by diverting them into ground (earth). Surge capacitors are fast, so they often are used in parallel with nonlinear surge suppressors.
ABB’s Industrial-grade Surge Capacitor – Type 2GUS (Source: ABB (ASEA Brown Boveri- Switzerland)
Power companies protect their transmission and distribution systems by using Surge Arrestors. They are designed for high operating voltages (k-Volts) and can suppress large transient energy spikes. While industrial SPDs also offer some level of protection to residential consumers, they do not guarantee full protection. The main reason is that the lightning-induced transients propagate along a power wire in both directions from the place of an incident (strike): towards the transformer (most likely protected by a surge arrestor) and in the opposite direction – towards your unprotected house.
That’s why you will need to install your own, low voltage (110V/220Vac) surge protector that will divert voltage transients to ground, preventing this way their further propagation across your interior electrical installation.
1. Hard-wired surge protection
Typically, they should be installed on power lines feeding your Service Panel (so somewhere between the electric meter and the panel). They should be installed by a professional electrician. Due to their location (at the entrance of the power service), they are often called “Service Entrance Protectors”. They make the first (and most important) line of defense of your house from power transients induced by nearby lightning strikes.
Unfortunately, they may not guarantee full protection of your interior installations because interior wiring may on its own pick-up power transients from direct or nearby lightning strikes. That’s why for safety, you may need the second line of surge protection at the point of use.
Surge Protector for home power lines – Model VAL-MS 230 ST. It’s the Power Entrance level model installed at the breaker panel. (Source: Phoenix Contact, Germany)
2. Power bar (strip) with built-in surge protection.
Most popular and affordable point-of-use surge protectors (in other words portable and deployed where needed) come in the form of power bars (strips). In order to provide surge protection, they must be plugged into a grounded outlet (the ground wire is necessary for discharging line transients). The protected appliance(s) in turn must be plugged into the surge protection power bar. For the highest safety, the low-voltage surge suppressors should be located as close as possible to the protected unit (BTW – some appliances may have their own, built-in surge protection). Note that “point-of-use” surge protectors belong to the Type-3 class of protection.
APC Surge Arrest provides surge protection for power lines, TV coaxial cable, Phone lines and network (All-in-one). Source: APC (American Power Conversion™ Corporation) – brand of Schneider Electric)
3. Surge protectors for signal cables
The mentioned above systems will protect equipment from power transients coming from the grid (commercial 110 /220Vac or your own solar/wind farm). They would not have however any impact on transients coming along fixed phone lines and coaxial cables (TV, Internet…). Typically, these cables carry weak input signals feeding “sensitive” receivers (themselves part of expensive electronics). That’s why they must also be protected from power transients by another version of “point-of-use’ protectors.
Plug-in type cable surge protectors work in a similar way as protectors of electrical lines (in other words they divert transient signals to ground). Commercially available models have 2 plastic prongs and 1 metal GND prong so they are fully compatible with standard grounded electrical outlets. When plugged, they establish the critical contact with the GND wire (part of electrical installation) establishing the discharge path for transients when needed. But in difference to electrical plugs, their input and output signals are TV, Internet or phone ones (in other words, they are isolated from Hot and Neutral part of electrical installation.
Network Lightning Protector – Model DRL RD 24 (Source: Dehn+Söhne, Germany)
- Ultimate Solution
Regardless of how much we “arm” our house with surge protections, when it comes to the impact of thunderstorms, quite often Mother Nature will have the upper hand. The truth is that during direct and even nearby lightning strikes, typical surge protectors won’t help. Most likely they will be destroyed together with appliances they should protect.
This prompts to the following statement: When a powerful thunderstorm advances towards your location, the ONLY viable 100% solution is to unplug all your appliances from electrical outlets and all input signal cables from receivers (TV, Modems, Phones etc…).
Lightning Protection Regulations
- The Lightning Protection System must be compliant with NFPA 780 and UL96(A) Safety Standards. All its components and materials must be UL-listed and manufactured especially for applications in lightning protection systems.
UL – Underwriters Laboratories, Inc.,
- The Lightning Protection System must include the discharge path (lightning rod(s), cable(s) and ground terminal) as well as the Surge Protection sub-system. To be effective, it should be designed as a package!
- The design and installation of the Lightning Protection system is not a DIY job. It should be custom-designed for a given structure and installed by LPI-certified contractors specialized in lightning protection.
Where used above acronyms correspondingly represent: NFPA – National Fire Protection Association, UL – Underwriters Laboratories, Inc. and LPI – Lightning Protection Institute.
Note that improperly designed and installed lightning protection systems may be prone to “side-flashings (discharge path branching-out to other conductive elements of the hose structure). The main goal of the lightning protection system is to contain the discharging current in the protection path (rod, cable and ground terminal). Any branching-out (deviation) from the dedicated discharge path may damage interior installations as well as the structure itself.
Protecting Container Houses
Container houses, especially high multilevel units by nature of their metal-based structures represent an easy target for lightning strikes. Sitting on the ground or on low-profile foundations they will usually have some physical contact with the earth and so may be perfect magnets for atmospheric discharges. Well, to put it in the right perspective – under normal circumstances the potential danger of direct strikes is quite small. In general, container houses surrounded by tall trees, or located in areas where thunderstorms are rare may never experience such terrifying events. On the other side – houses located in an open space visibly exposed to the forces of nature (like for example on the top of the hill) may act as “magnets” for lightning strikes.
Now, let’s face the truth, regardless how brutal it may be – given the gigantic amount of electrical energy discharged to the earth, most direct strikes will damage the targeted object. In the case of trees or traditional houses, lightning strikes may set fires, endanger human lives and most likely seriously affect its internal electrical installation (and connected to it equipment). Even the best lightning protection system will not be able to smoothly “channel” the energy from the direct strike. In other words – it can’t guarantee the full safety of the protected object.
Eaton CHSPT2ULTRA Whole House Surge Protector (Type 2) is one of the best Power-Entrance level protections on the market (Source: Eaton)
However, a good lightning protection system will do a great job in cases of nearby strikes (trees or any tall object in proximity of the container house). In such a case, energy induced in the metal structure of the house will be much smaller compared to direct strike, so it will simply flow to the earth via the protection system with most likely no effect on your house nor its electrical installation. That’s why it is of utmost importance to protect your home from such natural disasters.
Some may think that containers’ metal structure by itself makes a sort of “lightning protection system” at least protecting its interior. They may also point to the fact that the interior electrical installation is grounded and earthed (GND wire) so the whole structure should act as one big, distributed lightning protection system guaranteeing the safety.
Well, nothing can be more wrong than such assumptions for the following reasons:
a) For safety, the discharge current must be confined in designed for this task grounding path. Anything else leads to an uncontrolled, random process, where the discharge paths will be distributed all over the container’s metal structure with high probability of arcing, side-flashing, as well as branching to interior electrical installations and plumbing.
b) The grounding/earthing system designed as a protection from 110Vac (220Vac) electrical shocks can carry just a few tens of amps (depending on fuses in the interior electrical distribution system). It cannot handle discharging currents of thousands of amps characteristic for lightning strikes (such currents will simply melt the whole system).
Protection of Off-Grid Houses
Protection of off-grid container houses does not differ much from those connected to the grid. The main difference is that now you can’t count on surge protections built-in into commercial power grids. While your private grid is most likely “contained” within the perimeters of your property, so it has very limited exposure to the forces of nature, you cannot exclude the possibility of direct or nearby lightning strikes.
Your wind turbine (if any) installed at some distance from the home and well above the ground level, any midair wires and also roof-mounted solar panels and their wirings – all of them may be targets for direct strikes. While we should hope that it will never happen (because there is no 100% protection from such violent forces), most likely your system will be rather exposed to nearby lightning strikes. They will “insert” significantly lower amounts of transient energy into your private grid, well within the limits of commercial surge protection systems.
However, it is worth to mention that container-based houses have much lower susceptibility to nearby lightning strikes compared to traditional houses. Due to the metal structure, they represent sort of “Faraday’s Cage” protecting interior installations from electromagnetic couplings (infiltrations) of exterior signals. Obviously, windows (and for sure large glass-walls) will weaken the electromagnetic shield.
1. you will have to protect the Inverter and Controller as they will be the first victims of lightning strikes if left without any surge protection system.
2. Solar panels must have their metal frames grounded (once again, we are not talking here about a typical bare copper wire used in fuse-protected electrical networks, but about a robust and reliable connection to the earth able to handle large transient energy).
3. Wind-turbine (far end of your private grid) may also need a good surge suppressor. While in commercial grid it is the responsibility of a Power & Distribution Company, now it’s up to you to protect your investment.