CRAC Class A (2025) — Questions 661–669

Power-frequency (50/60 Hz) alternating current is the most dangerous because its frequency lies in the range most likely to trigger ventricular fibrillation of the heart. As the frequency rises into the RF range, the danger from electric shock falls: at HF the current tends to stay near the skin (skin effect) and at UHF it is even more confined to the surface, so it is far less able to stimulate the heart. The threat therefore decreases in the order power-frequency AC > HF RF AC > UHF RF AC, making A correct.

When working live on mains-powered equipment the safe method is to keep both feet insulated, use only one hand, and keep the other hand away from the chassis or any grounded metal, so no current path can be formed across the chest and heart (A). B deliberately creates such a path through the body; C and D are false because the grounding state of the chassis does not remove the shock risk of a live internal circuit.

Correct preventive measures against a live chassis are: bond the metal enclosure of mains-powered gear to a reliable common ground (A); fit a residual-current device (RCD/leakage breaker) so supply is cut on fault (B); and, for three-wire protective- ground plugs, verify the ground is actually effective (C). D is wrong: a water pipe is not a dependable ground (it may be plastic, corroded, or interrupted) and is explicitly not a recommended grounding method.

Even with the plug removed, large filter capacitors in a power supply or linear amplifier can retain a dangerous high-voltage charge for a long time, so the repairer can still receive a shock (A). B (geomagnetic induction), C (fuse blows on opening), and D (ESD damages the ground system) are false.

The minimum safety provision for homebrew 220 V mains equipment is a fuse in series with the live (hot) conductor at the inlet, so an over-current fault opens the circuit (A). A paralleled voltmeter (B), series inductor (C), or paralleled capacitor (D) is not a protective safety measure and a paralleled capacitor across the mains would be hazardous.

Good electrical-safety practice for a homebrew project includes: three-core cord with the metal chassis bonded to mains ground (A); a bleed resistor across HV filter caps so they discharge when off (B); and a case interlock that cuts mains when opened (C). D is wrong and dangerous: spraying water on a live electrical cabinet can cause electrocution and is not a proper fire-protection method.

During charging a lead-acid cell electrolyzes water and releases hydrogen and oxygen; in a poorly ventilated space this mix can reach an explosive concentration (A). 12 V is not a high voltage, so B is wrong; it does not emit ozone (C) nor self- ignite from idleness (D).

Excessive charge/discharge current overheats the cell and accelerates gassing (hydrogen release), raising explosion risk (A). "Reverse polarity" (B) comes from deep over-discharge, not simply fast rate; "memory effect" (C) is a NiCd/NiMH phenomenon, not lead-acid; and 12 V never presents a shock hazard (D).

For an electrical fire the supply must first be cut, then a non- conductive extinguisher used — dry powder, CO2, or 1211 (halon-type) (A). Water (C) and foam (B) conduct electricity and can electrocute the user; simply fleeing (D) is not the correct managed response when the fire is still controllable.