Safety Guidelines for High Voltage and/or Line Powered Equipment


[Document Version: 1.25] [Last Updated: 05/25/1998]

1. About the Author & Copyright

Safety Guidelines for High Voltage and/or Line Powered Equipment

Author: Samuel M. Goldwasser
Corrections/suggestions: | Email

Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.

2. Introduction

Consumer electronics equipment like TVs, computer monitors, microwave ovens,
and electronic flash units, use voltages at power levels that are potentially
lethal.  Normally, these are safely enclosed to prevent accidental contact.
However, during servicing, the cabinet will likely be open and safety
interlocks may be defeated.  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 strobelights 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!

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, fusion generators, and other popular hobby type projects.

In addition read the relevant sections of the document for your particular
equipment.  Specific safety considerations have been included where

3. 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 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 100-500 ohms/V approximate value (e.g., for a 200 V capacitor,
  use a 20K-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-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.

* 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!

4. 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:

* 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.  With your multimeter set on ACV across this combination
  and the equipment powered up, touch 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.

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!).

Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server [Copyright] [Disclaimer]