Safety Guidelines for
High Voltage and/or Line Powered Equipment

Version 1.40 (2-Feb-10)

Copyright © 1994-2013
Samuel M. Goldwasser
--- All Rights Reserved ---

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Table of Contents



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    Introduction

    Consumer electronic equipment like TVs, computer monitors, microwave ovens, and electronic flash units, use voltages at power levels that are potentially lethal. Even more so for industrial equipment like lasers and anything else that is either connected to the power line, or uses or generates high voltage.

    Normally, these devices are safely enclosed to prevent accidental contact. However, when troubleshooting, testing, making adjustments, and during repair procedures, the cabinet will likely be open and/or safety interlocks may be defeated. Home-built or modified equipment, despite all warnings and recommendations to the contrary - could exist in this state for extended periods of time - or indefinitely.

    Depending on overall conditions and your general state of health, there is a wide variation of voltage, current, and total energy levels that can kill.

    Microwave ovens in particular are probably THE most dangerous household appliance to service. There is high voltage - up to 5,000 V or more - at high current - more than an amp may be available momentarily. This is an instantly lethal combination.

    TVs and monitors may have up to 35 kV on the CRT but the current is low - a couple of milliamps. However, the CRT capacitance can hold a painful charge for a long time. In addition, portions of the circuitry of TVs and monitors as well as all other devices that plug into the wall socket are line connected. This is actually more dangerous than the high voltage due to the greater current available - and a few hundred volts can make you just as dead as 35 kV!

    Electronic flash units and strobe lights, and pulsed lasers have large energy storage capacitors which alone can deliver a lethal charge - long after the power has been removed. This applies to some extent even to those little disposable pocket cameras with flash which look so innocent being powered from a single 1.5 V AA battery. Don't be fooled - they are designed without any bleeder so the flash can be ready for use without draining the battery!

    Even some portions of apparently harmless devices like VCRs and CD players - or vacuum cleaners and toasters - can be hazardous (though the live parts may be insulated or protected - but don't count on it!

    This information also applies when working on other high voltage or line connected devices like Tesla Coils, Jacobs Ladders, plasma spheres, gigawatt lasers, hot and cold fusion generators, cyclotrons and other particle accelerators, as well as other popular hobby type projects. :-)

    In addition, read the relevant sections of the document for your particular equipment for additional electrical safety considerations as well as non-electrical hazards like microwave radiation or laser light. Only the most common types of equipment are discussed in the safety guidelines, below.



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    Safety Guidelines

    These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage.

    Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage. There are likely to be many sharp edges and points inside from various things like stamped sheet metal shields and and the cut ends of component leads on the solder side of printed wiring boards in this type of equipment. In addition, the reflex may result in contact with other electrically live parts and further unfortunate consequences.

    The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe!



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    Safety Tests for Leakage Current on Repaired Equipment

    It is always essential to test AFTER any repairs to assure that no accessible parts of the equipment have inadvertently been shorted to a Hot wire or live point in the power supply. In addition to incorrect rewiring, this could result from a faulty part, solder splash, or kinked wire insulation.

    There are two sets of tests:

    If the equipment fails either of these tests, the fault MUST be found and corrected before putting it back in service (even if you are doing this for your in-laws!).

    Checking for correct hookup of the Hot, Neutral, and Ground wires to the AC plug should also be standard procedure. There's no telling how it may have been scrambled during a previous attempt at repair by someone who didn't know any better or by accident. Unlike logic circuits, black is NOT the standard color for ground in electric wiring! :)



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