Factory Earthing & Lightning Protection — Designing to IEC 62305 / EIT to Stop Equipment Damage and Pass TOR & Utility Requirements

Is your factory/building at risk? — Check these 6 signs

Thailand sits in one of the most lightning-dense regions on Earth (many areas exceed 80–100 thunderstorm days/year), while modern factories are packed with VFDs, PLCs, servers and solar inverters that are sensitive to voltage surges. Check whether you see these signs:

• Electronics / PLCs / control cards failing in batches after thunderstorms — the classic surge symptom
• Old air terminals on the roof but ground resistance never measured — or no test records at all
• Separate earth pits for "lightning" and "electrical" (separated pits = dangerous, violates equipotential principle)
• Electric shock / tingling when touching machine frames, or RCDs/earth-leakage breakers tripping often
• A new TOR, inspector or insurer asks for earthing/LPS test reports — and you have none
• You extended the building / added a production line / installed solar without revisiting the existing earthing

If two or more apply, it's time to assess and manage your earthing/lightning system systematically — not just "drive in another ground rod" and hope.

Earthing ≠ Lightning protection: the two systems people confuse most

The number-one misconception is "we have a ground rod in the soil, so we're protected from lightning." These are two different systems, in two different standards, with different jobs:

The key point: both systems and the SPDs must bond to one common point (Main Earthing Terminal + equipotential bonding). Separating the "lightning" earth pit from the "electrical" earth is a dangerous mistake — during a real strike the two potentials diverge enormously and side-flash into equipment and people.

flowchart TD
  A["⚡ Direct strike"] --> B["Air Termination
rod / mesh"]
  B --> C["Down Conductor"]
  C --> D["Earth Termination
LPS earth"]
  L["Utility mains"] --> S1["SPD Type 1
(main board)"]
  S1 --> S2["SPD Type 2
(sub-board)"]
  S2 --> S3["SPD Type 3
(near equipment)"]
  S3 --> E["Electronics
PLC / Server / Inverter"]
  D --- MET["Main Earthing Terminal
+ Equipotential Bonding"]
  S1 --- MET
  G["Electrical earth
(IEC 60364)"] --- MET
  MET --> EARTH["((Earth))"]

Why this matters in Thailand (and why it pays back)

• High climatic risk — dense lightning; electronic equipment damage is frequent, expensive to repair, and causes production downtime
• Legal/standard mandate — building codes require lightning protection for certain occupancies (tall buildings, flammable stores, fuel tanks)
• TOR and utility conditions — public projects usually require earthing/LPS test reports as deliverables, and grid connection (especially solar via MEA/PEA) requires correct earthing
• Insurance conditions — many property/fire policies use the annual LPS inspection record as a condition for paying claims

Earthing/lightning work is a "prevention" spend that is cheap relative to the equipment it protects — a single PLC or production line lost to a surge usually costs more than an SPD system for the whole plant.

Step 1: Do you even need an LPS? — Risk assessment per IEC 62305-2

Not every building needs a full LPS — IEC 62305-2 requires a risk assessment first, so you neither over- nor under-invest:

• Compute risk R1 (loss of human life) from: local lightning density (Ng, strikes/km²/year), building size/height (collection area), occupancy, number of people, and existing measures
• Compare against the tolerable risk RT (for loss of life the standard uses RT = 10⁻⁵ per year)
• If R1 > RT → an LPS is required, and you pick the Class that drives risk back below RT
• The assessment also tells you what level of SPD / bonding / other measures to add

flowchart LR
  A["Risk assessment
IEC 62305-2"] --> B{"R1 > RT ?"}
  B -- "No" --> C["No LPS required
(still need electrical earth)"]
  B -- "Yes" --> D["Choose LPS Class
I / II / III / IV"]
  D --> E["Design air term +
down conductor + earth"]
  E --> F{"R1 < RT now ?"}
  F -- "No" --> D
  F -- "Yes" --> G["Add SPD + bonding
per 62305-4"]

Have an electrical engineer produce the real assessment document — especially if you must submit a TOR or permit. Not "looks like Class III to me."

Step 2: Design the LPS — 4 levels + 3 core parts

Once an LPS is needed, the Lightning Protection Level (LPL) splits into four classes, with Class I the most stringent. The main design parameters per IEC 62305-3:

The three core parts of an LPS:

1. Air Termination — three methods (often combined):
   • Rolling Sphere — roll a sphere of the class radius over the structure; where it touches is exposed and needs a terminal — most accurate for complex shapes
   • Mesh — run conductors as a grid over a flat roof, sized to the class mesh
   • Franklin Rod — protective cone angle, suited to peaks/stacks
2. Down Conductor — carries current from roof to ground, spread in multiple paths around the building per the class spacing (more paths = current divides, lower voltage); include a test joint for isolated measurement
3. Earth Termination — two arrangements:
   • Type A — individual rods/plates per down conductor
   • Type B — a ring electrode around the building or a foundation earth electrode — recommended for new builds / high-resistivity soil because it gives a low value and good equipotentialization

Step 3: Safety earthing (IEC 60364)

This is the system "every building must have" even without an LPS — to prevent shock and let protective devices (breakers/RCDs) operate on a fault:

• Earthing system per IEC 60364: in Thailand the utility mostly uses TN-C-S at the service entrance; isolated installations (rural/remote) may be TT, which requires an RCD because the earth loop impedance is high
• Ground resistance — general/utility work usually requires ≤5 ohm; electronics/telecom/server rooms often need ≤1 ohm
• Soil resistivity — measure with the Wenner 4-pin method before designing, to set electrode count/depth/spacing. Wet clay reads low (good); sand/rock reads high (needs more electrodes or soil enhancement)
• Main Earthing Terminal (MET) + Equipotential Bonding — bond every metallic part (frames, pipes, racks, tanks) to one point so potentials equalize, cutting touch/step voltage

Step 4: SPD — the final layer that actually protects electronics

Even with full air terminals and earthing, most equipment damage comes from induced surges travelling along lines — handle it with coordinated SPDs across three stages per IEC 61643 / IEC 62305-4:

SPD golden rules:

• Install coordinated — skipping Type 1 and fitting only a Type 3 at the end = both fail
• Connecting leads must be as short as possible (≤0.5 m total) — long leads add voltage (L·di/dt) until the SPD barely helps
• The Up (voltage protection level) must sit below the equipment's impulse withstand (e.g. Category II gear withstands ~2.5 kV → you want a comfortably lower Up)
• Don't forget SPDs for data/comms lines and the solar DC side, not just AC power

High-surge-risk sites (e.g. plants with many VFDs or rooftop solar) should design SPDs alongside power-quality/harmonic work from the start.

Materials and connections — the overlooked part that sets system life

Almost every weakness in an earthing/lightning system is at the joints, not the conductors:

• Conductors — bare copper, copper-clad, or galvanized steel, with cross-section per standard (copper down conductors often start at 50 mm²)
• Ground rods — copper-bonded steel, typically 16 mm diameter, 2.4–3 m long; high-resistivity soil uses parallel rods or chemical earth electrodes
• Connections — exothermic weld (permanent fusion, high current capacity, no corrosion) for buried/main-current points vs clamps (inspectable) for test joints
• Every component (clamp, conductor, electrode) should be a grade tested to IEC 62561 — cheap, non-compliant parts corrode/loosen within 2–3 years and silently push resistance up

Testing and annual inspection

Earthing/lightning systems "degrade silently" — they look fine while the real values worsen, so measure regularly:

• Measure ground resistance by the fall-of-potential (3-pin) method, or clamp-on for parallel electrodes — log year over year and test in the dry season (worst case)
• Re-measure soil resistivity after earthworks/extensions
• Inspect the LPS per IEC 62305-3: visual (corrosion, loose joints, broken conductors) + measurement — generally visual yearly + full check every 1–2 years for Class I–II
• Keep the test records — they are your TOR deliverable, utility condition, and insurance-claim condition

Cost ladder + common mistakes

Invest from basics upward:

1. Measure and document first — soil resistivity + ground resistance + risk assessment (cheapest, highest return)
2. Fix electrical earthing to spec + complete MET/bonding
3. At least a Type 2 SPD at the main board (best surge protection per baht)
4. Full LPS + coordinated SPD Type 1/2/3 per the assessment

Frequent, money-wasting mistakes:

• Separating the lightning earth pit from the electrical earth → side-flash destroys equipment
• Driving extra ground rods without measuring soil resistivity → value won't drop in high-resistivity soil, wasted effort
• SPD leads longer than 0.5 m → barely effective
• Forgetting SPDs on the solar DC side / comms lines → equipment dies despite AC-side protection
• Installing once and never re-testing → resistance climbs within 2–3 years unnoticed

Summary

Earthing and lightning protection are two different systems that must work together on one common earth — earthing per IEC 60364 is mandatory for every building, an LPS per IEC 62305 is required when the risk assessment says so, and SPDs are the layer that actually protects electronics. The correct order is measure → assess → design to Class → install IEC 62561-rated materials → test and document — not "drive in another ground rod" and guess.

Sahawatthanakit (1988) supplies complete earthing and lightning protection equipment — copper-bonded ground rods, ground bars, exothermic weld kits, air terminals/down conductors, and SPD Type 1/2/3 — with selection guidance to meet standards and pass TOR. Talk to our engineering team to match the right solution to your site and budget.