This quote on UC Berkley's "Lessons Learned" study:
If an employee needs 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).
I also found the Department Of Energy's DOE Handbook for Electrical Safety (.pdf file).
Lots of stuff about non-flammable clothing, protective gear, GFCI & AFCI breakers, rubberized gloves, etc.
Safety Guidelines for High Voltage and/or Line Powered Equipment
Copyright © 1994-2006
Samuel M. Goldwasser
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 unfortunately 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!
* 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. An insulated floor is better than metal or bare concrete but this may be outside of your control.
A rubber mat should be an acceptable substitute but a carpet, not matter how thick, may not be a particularly good insulator.
* 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 contact.
* Have a fire extinguisher rated for electrical fires readily accessible in a location that won't get blocked should something burst into flames.
* Use a dust mask when cleaning inside electronic equipment and appliances, particularly TVs, monitors, vacuum cleaners, and other dust collectors.
* 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 may 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 GFCI is also a relatively complex active device which may not be designed for repeated tripping -
you are depending on some action to be taken (and bad things happen if it doesn't!) - unlike the passive nature of an isolation transformer.
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!
# Back to Safety Guidelines Table of Contents.
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 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. This corresponds to a maximum leakage current of 0.5 mA.
Note: A true RMS reading multimeter should be used for this test, especially where the equipment uses a switchmode power supply
which may result in very non-sinusoidal leakage current.
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!