CRAC Class A (2025) — Questions 651–660

VHF and UHF radio waves are electromagnetic radiation at frequencies far below those that can ionize atoms, so they are non-ionizing radiation (非电离辐射, A). Ionizing radiation (B), alpha (C), and gamma (D) are high-energy nuclear/particle radiations, unrelated to radio.

A traditional lightning-protection system consists of the air- termination device (接闪器/避雷针), the down conductor (引下 线), and the grounding electrode (接地体) — the classic three- part path to earth (A). The other options mix in antennas, breakers, or surge components that are not the basic structural triad.

Lightning-protection grounding exists to conduct the captured lightning current safely into the earth (A). Blocking it (C) or blowing a fuse to stop it (D) would be counterproductive — the current must be diverted to ground, not stopped in the structure.

The basic rule is a dedicated grounding electrode with low resistance and a short, thick down conductor (A). Water pipes (B) and the AC neutral (C) must not substitute for a proper lightning ground, and a 0.5 mm wire (D) would vaporize under lightning current.

Modern lightning protection uses the rolling-sphere method; for ordinary buildings the sphere radius is 60 m, and the protected volume is the region not touched by a 60 m-radius sphere rolled over the rod (A). B and C overstate protection ("in all cases"), and D's simple hemisphere is not the correct model.

For bonding ground points (antenna, tower, feed line entry) the best choice is a flat, flexible, low-inductance braided copper strap, preferably silver-plated for corrosion resistance (A). A single steel strand (B) is for tower structurals, while Ethernet pair (C) and coax (D) are signal cables, not grounds.

All coaxial arrester grounds should bond to a single common ground strap taken to the outdoor earth ground — a single- point, low-impedance path (A). Separate ground runs (C) create ground loops/different potentials; switches (B) would interrupt protection; and homemade neon-tube arresters (D) are unsafe and ineffective.

A short, straight ground path minimizes inductance and voltage rise during a strike (A). Right-angle bends (B) add inductance, waterproofing (C) is not the core measure, and placing electrodes far from the tower (D) lengthens the path — all wrong.

Under GB/T 3805-2008, the ELV limit in wet conditions (a person grasping a live part) is 16 V AC (r.m.s.) and 33 V DC (A). The other pairs swap or alter these statutory values.

At equal contact voltage, radio-frequency current tends to flow near the body surface (skin effect), so it is less likely to disrupt the heart/nerves (lower fatal risk) but can cause painful RF burns to skin and subcutaneous tissue (A). RF is still dangerous (C is false), and the threat is frequency-dependent (D false); B reverses the comparison.