Electrical System Design
Design and check the earthing and lightning protection of a comms tower, rooftop, shelter, or cabinet — earth-electrode resistance from soil resistivity, lightning risk to a protection level, rolling-sphere protection of mast-mounted antennas, and bonding and surge protection.
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Earthing and lightning protection on a communications site is its own discipline. The earth electrode has to reach a low resistance in whatever soil the site sits on, the tower or rooftop is a tall, exposed, lightning-attractive structure full of sensitive electronics, the antennas are mounted up where a direct strike is most likely, and every metallic service entering the shelter is a path for surge current. A site that earths and bonds poorly does not fail gracefully — it loses equipment to a nearby strike, raises dangerous touch voltages at the tower base on a power fault, or couples lightning energy straight into the radios.
The noIM₃ Earthing & Lightning Protection Calculator is built for exactly that work, for the systems integrator rather than the substation or building-LPS specialist. It starts from the earth electrode: resolve the resistance of driven rods (single, or an array with the mutual coupling between rods properly accounted for), a horizontal strip, or an area grid or ring using the IEEE 80 equation, from a soil resistivity you enter directly, pick from a soil type, or reduce from a Wenner four-pin survey. It tells you how many rods reach your target resistance and screens the ground potential rise against the IEEE 80 tolerable touch and step voltages.
From there it covers the three things that decide a comms site’s lightning design. A risk mode estimates the collection area and the expected number of direct strikes from the ground flash density and screens them against a tolerable frequency to recommend a Lightning Protection Level. An air-termination mode applies the rolling-sphere method to check whether mast-mounted antennas sit inside the protected zone, sizes the down-conductors, and computes the separation distance for parallel feeders. A bonding mode picks an ITU-T K.27 topology, sizes the bonding conductors, works out the lightning current each service carries, and coordinates the surge protective devices against the equipment withstand. Everything runs in the browser, copies to the clipboard for the design file, and is labelled as planning-grade guidance to confirm by measurement and a full risk assessment.
Resolve the earth resistance of a single driven rod (Dwight / IEEE 142), a rod array, a horizontal strip (BS 7430), or an area grid / ring (IEEE 80 Sverak). Soil resistivity is entered directly, picked from a soil type, or reduced from a Wenner four-pin survey, so the result is anchored to the soil you actually have rather than a blanket assumption.
Adding rods does not simply divide the resistance by the number of rods, because each rod sits in the potential field of the others. The tool combines the rod self-resistance with the average mutual resistance, so the array efficiency falls as rods are packed closer, shows the resistance-versus-rod-count curve, and finds the smallest rod count on a given spacing that reaches the target.
Enter the prospective earth-fault current and clearing time and the tool computes the ground potential rise and the IEEE 80 tolerable touch and step voltages for a 50 or 70 kg body, with a surface-layer derating for a crushed-rock or asphalt finish — a personnel-safety screen at the tower base.
Estimate the structure collection area and the expected number of direct strikes from the ground flash density (entered, or from thunderstorm-days) and a location factor, screen against a tolerable frequency, and recommend a Lightning Protection Level from I to IV by the simplified AS/NZS 1768 / IEC 62305-2 method.
Apply the rolling-sphere radius for the protection level to a vertical air terminal at the mast top, get the protected radius at the antenna height, and check whether an antenna at a given outrigger offset sits inside the protected zone, with a protected-zone chart that makes the geometry obvious.
Size the number of down-conductors for the structure perimeter at the level spacing, never fewer than two, and compute the separation (isolation) distance from the induced-voltage, current-distribution and material coefficients, so you know whether a parallel coax or feeder run must be bonded to the down-conductor.
Pick a common-bonding-network, mesh, isolated-bonding-network or single-point topology per ITU-T K.27, and size the lightning-current-carrying and functional equipotential bonding conductors against the IEC 62305 / AS/NZS 1768 minimums for copper, aluminium or galvanised steel.
Work out the lightning current each AC, DC, coax and data line carries from the current-sharing rule, recommend an SPD type for each, and coordinate the SPD protection level against the equipment impulse withstand with a recommended margin so the let-through voltage stays safely below what the equipment can take.
Reach for the Earthing & Lightning Protection Calculator in any of the following situations.
It depends on the site and the equipment. A common general target for an LV installation earth is 10 Ω or less, many communications sites aim for 5 Ω or less, and sensitive or co-located sites sometimes specify 1 Ω or less. The right figure comes from the equipment requirements, the protective-device disconnection, and the lightning design, not from a single rule, so the tool lets you set the target and shows what electrode it takes to reach it. Whatever the design figure, the installed resistance must be confirmed by measurement.
Because the rods are not independent. Each rod sits in the potential field of the others, so the effective resistance of an array is higher than a naive divide-by-n suggests. The tool combines the rod self-resistance with the average mutual resistance between rods, which is why the array efficiency falls as rods are packed closer together, and why spreading rods further apart (ideally by at least their length) recovers more of the benefit.
The apparent soil resistivity is ρ = 2π·a·R, where a is the equal probe spacing and R is the measured resistance, and the result represents the soil to a depth of roughly the probe spacing. Surveying at several spacings reveals how the resistivity changes with depth, which matters when a deep rod passes through a low-resistivity layer. The tool reduces a single spacing for you and you apply it as the design resistivity.
No. The risk mode is the simplified screening method: it estimates the collection area and the expected number of direct strikes and compares them against a tolerable event frequency to recommend a Lightning Protection Level. It is intended to establish quickly whether protection is needed and at roughly what level. The final determination of protection measures should come from a full R1 / R2 risk assessment to AS/NZS 1768 or IEC 62305-2 with the site-specific loss and probability factors.
An imaginary sphere of a radius set by the protection level (20 m for Level I up to 60 m for Level IV) is rolled over the structure; anything it cannot touch is protected. For a vertical air terminal that gives a protected radius at each height, and the tool checks whether the antenna, at its height and outrigger offset from the mast, falls inside that radius. If it does not, you either raise the air-terminal finial, add a terminal, or move the antenna.
When lightning current flows down a down-conductor it induces a high voltage on nearby metalwork. The separation (isolation) distance is the minimum air or material gap that keeps that induced voltage from flashing over to a parallel conductor such as a coax or feeder. If the run is closer than the separation distance, it must be bonded to the down-conductor so the two rise together rather than flashing over. The tool computes it from the protection level, the number of down-conductors, the material between, and the run length.
Two ways. First, the impulse current rating must be adequate for the lightning current that line carries, which the tool estimates from the current-sharing rule (about half the stroke goes to the earth electrode and the rest divides among the services). Second, the SPD voltage-protection level Up, plus the volt-drop on its connecting leads, must stay below the equipment’s rated impulse withstand Uw. Good practice keeps Up at or below 80 percent of Uw, which is the margin the tool checks.
No. The calculator runs entirely in your browser. No site data is submitted to a server, which suits commercially confidential work and security-restricted communications sites.
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