Safety Guidelines for
High Voltage and/or Line Powered Equipment
Copyright (C) 1994,1995,1996,1997,1998
Samuel M. Goldwasser
--- All Rights Reserved ---
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if both of the following conditions are satisfied:
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Table of Contents
Consumer electronics 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 warings 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.
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 many sharp edges inside this type of equipment as well as
other electrically live parts you may contact accidentally.
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!
- Don't work alone - in the event of an emergency another person's presence
may be essential.
- Always keep one hand in your pocket when anywhere around a powered
line-connected or high voltage system.
- Wear rubber bottom shoes or sneakers.
- Wear eye protection - large plastic lensed eyeglasses or safety goggles.
- Don't wear any jewelry or other articles that could accidentally contact
circuitry and conduct current, or get caught in moving parts.
- Set up your work area away from possible grounds that you may accidentally
- Know your equipment: TVs and monitors may use parts of the metal chassis
as ground return yet the chassis may be electrically live with respect to the
earth ground of the AC line. Microwave ovens use the chassis as ground
return for the high voltage. In addition, do not assume that the chassis
is a suitable ground for your test equipment!
- If circuit boards need to be removed from their mountings, put insulating
material between the boards and anything they may short to. Hold them in
place with string or electrical tape. Prop them up with insulation sticks -
plastic or wood.
- If you need to probe, solder, or otherwise touch circuits with power off,
discharge (across) large power supply filter capacitors with a 2 W or greater
resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor,
use a 20K to 100K ohm resistor). Monitor while discharging and/or verify
that there is no residual charge with a suitable voltmeter. In a TV or
monitor, if you are removing the high voltage connection to the CRT (to
replace the flyback transformer for example) first discharge the CRT contact
(under the insulating cup at the end of the fat red wire). Use a 1M to 10M
ohm 1W or greater wattage resistor on the end of an insulating stick or the
probe of a high voltage meter. Discharge to the metal frame which is
connected to the outside of the CRT.
- For TVs and monitors in particular, there is the additional danger of
CRT implosion - take care not to bang the CRT envelope with your tools.
An implosion will scatter shards of glass at high velocity in every
direction. There is several tons of force attempting to crush the typical
CRT. Always wear eye protection. While the actual chance of a violent
implosion is relatively small, why take chances? (However, breaking the
relatively fragile neck off the CRT WILL be embarrassing at the very least.)
- Connect/disconnect any test leads with the equipment unpowered and
unplugged. Use clip leads or solder temporary wires to reach cramped
locations or difficult to access locations.
- If you must probe live, put electrical tape over all but the last 1/16"
of the test probes to avoid the possibility of an accidental short which
could cause damage to various components. Clip the reference end of the
meter or scope to the appropriate ground return so that you need to only
probe with one hand.
- Perform as many tests as possible with power off and the equipment
unplugged. For example, the semiconductors in the power supply section of a
TV or monitor can be tested for short circuits with an ohmmeter.
- Use an isolation transformer if there is any chance of contacting line
connected circuits. A Variac(tm) (variable autotransformer) is not an
isolation transformer! However, the combination of a Variac and isolation
transformer maintains the safety benefits and is a very versatile device.
See the document "Repair Briefs, An Introduction", available at this site,
for more details.
- The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a
good idea but will not protect you from shock from many points in a line
connected TV or monitor, or the high voltage side of a microwave oven, for
example. (Note however, that, a GFCI may nuisance trip at power-on or at
other random times due to leakage paths (like your scope probe ground) or
the highly capacitive or inductive input characteristics of line powered
equipment.) A fuse or circuit breaker is too slow and insensitive to provide
any protection for you or in many cases, your equipment. However, these
devices may save your scope probe ground wire should you accidentally connect
it to a live chassis.
- When handling static sensitive components, an anti-static wrist strap is
recommended. However, it should be constructed of high resistance materials
with a high resistance path between you and the chassis (greater than 100K
ohms). Never use metallic conductors as you would then become an excellent
path to ground for line current or risk amputating your hand at the wrist
when you accidentally contacted that 1000 A welder supply!
- Don't attempt repair work when you are tired. Not only will you be more
careless, but your primary diagnostic tool - deductive reasoning - will
not be operating at full capacity.
- Finally, never assume anything without checking it out for yourself!
Don't take shortcuts!
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
- DC leakage: Use a multimeter on the highest OHMS range to measure the
resistance between the Hot/Neutral prongs of the wall plug (shorted together
and with the power switch on where one exists) to ALL exposed metal parts of
the equipment including metallic trim, knobs, connector shells and shields,
VHF and UHF antenna connections, etc.
This resistance must not be less than 1 M ohm.
- AC leakage: Connect a 1.5K ohm, 10 Watt resistor in parallel with a 0.15
uF, 150 V capacitor to act as a load. Attach this combination between the
probes of your multimeter. With the equipment powered up, check between a
known earth ground and each exposed metal part of the equipment as above.
WARNING: Take care not to touch anything until you have confirmed that the
leakage is acceptable - you could have a shocking experience!
The potential measured for any exposed metal surface must not exceed 0.75 V.
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