Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers station grounding and lightning protection per NEC Article 810, NFPA 780, and ARRL station grounding practices. The scope includes: single-point entry panel construction, coaxial gas discharge tube (GDT) arrestors, multi-stage protection (GDT + MOV + TVS), ground rod installation, and bonding strap sizing.

WARNING — LIGHTNING IS LETHAL. This manual describes protection against induced surges and near-miss events. A direct strike cannot be “protected against” — only diverted. All station equipment must be disconnected from antennas and utility power during active thunderstorm activity.

Chapter 2 — Theory of Operation

2-1 Grounding Philosophy

Station grounding serves four purposes in order of importance:

1. Safety: Prevent fatal shock from faulted equipment chassis.
2. RF common-mode elimination: Single-point RF ground prevents loops.
3. Static discharge: Continuous low-impedance path for accumulated charge.
4. Lightning diversion: Guide induced surge energy away from equipment.

A ground system must be: low-impedance at DC and RF (bonding), physically continuous, connected in a star (single-point) topology, and compliant with NEC 250 and Article 810.

2-2 GDT Arrestors

A gas discharge tube (GDT) is a two-electrode spark gap in a sealed tube filled with inert gas (typically argon/neon at low pressure). At normal voltages, the GDT is an open circuit. When the voltage exceeds the DC sparkover voltage (typically 90–230 V), the gas ionizes and the GDT conducts, clamping the voltage. The follow-through current must be broken by a series fuse or the ionization quenches when the surge passes.

2-3 Multi-Stage Protection

A single GDT clamps most surges but may allow brief overvoltages during the turn-on delay (∼1 µs). A two-stage design adds a MOV (metal oxide varistor) or TVS (transient voltage suppressor) in series/parallel after the GDT to clamp faster (<1 ns) at lower voltages. The series impedance between stages slows the surge, giving the TVS time to respond.

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Ground Rod Installation

1. Drive two 8′ ground rods at least 16′ apart (one rod length spacing minimum per NEC 250). Rods must be buried full depth (8′/2.4 m).
2. Bond rods together with #6 AWG bare copper, buried 300 mm below grade. Use irreversible compression connectors (Burndy or equivalent) — never rely on wire wrapped around the rod.
3. Run the ground conductor from the rod cluster to the entry panel via the shortest possible path (<3 m is ideal; avoid right-angle bends).

4-2 Entry Panel

1. Mount the copper panel on the exterior wall at the point where all antenna cables enter the shack. Mount as close to the ground rod as possible.
2. All coaxial feedlines must pass through the panel via GDT-equipped bulkhead connectors. All cable shields must bond directly to the panel at this point.
3. Bond the panel to the ground rod cluster with a minimum 1/2″ copper strap (no wire — strap has lower RF impedance).

Chapter 5 — Operating Procedures

1. During thunderstorm operation: Do not operate within 10 miles of an active thunderstorm. The standard amateur radio recommendation is: if you can hear thunder, disconnect and shelter indoors.
2. When leaving the station unattended: Disconnect all antenna feedlines at the transceiver and connect the coax shields to ground with shorting caps or a dedicated disconnect switch.
3. Annual inspection: Check all bonding connections for corrosion (green copper oxide is normal and protective; white powder indicates aluminum corrosion and loss of continuity). Check GDT arrestors for carbon tracking (indicate previous discharge) — replace if discharged.

Chapter 6 — Calibration

1. Measure ground resistance: use a fall-of-potential test (three-point method) or a clamp-on ground resistance tester. Target: <10Ω per NEC; <5Ω recommended for RF stations.
2. Verify continuity of all bonding straps with a low-resistance ohmmeter: any station ground conductor must read <0.1Ω end-to-end.
3. GDT sparkover voltage: verify with a high-voltage tester if available. The sparkover voltage should match the manufacturer rating (typically 90–230 V depending on type).

Chapter 7 — Verification and Acceptance

1. Ground resistance <5Ω (measure with fall-of-potential method).
2. All bonding straps continuous (<0.1Ω).
3. All coax feedlines pass through the entry panel with no direct path that bypasses the GDT.
4. No RF current on coax shields within the shack (measure with clip-on RF current probe; target <10 mA during transmit on any feedline entering the shack).
5. Log: date, ground rod resistance, number of GDT units installed and tested, bonding strap resistance, RF current check, operator.

Appendix A — NEC Article 810 Summary

• 810-21(f)(1): antenna discharge unit must be connected to ground electrode
• 810-21(f)(2): ground conductor must be #10 AWG copper minimum
• 810-21(h): ground conductor must be run in shortest path, without sharp bends
• 810-20(a): antenna must be kept ≥3 feet from power conductors
• 250-50: all grounding electrodes must be bonded together

Appendix B — Bonding Strap Sizing