SAFETY GUIDELINES
A.safety guidelines for High Voltage and/or Line Powered Equipment
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!
A.Safety guidelines for building electronics equipments.
This give you some guidelines how to to make your homebuilt electronics equipments safe. I have tried to make those guidelines to be as caaurate as possible. However, I do not assume, and hereby claim, any liability to any party for any loss or damage, direct or consequential, caused by errors in those guidelines.
The safety requirements refer mainly to the 230V mains voltage and European electrical safety requirements.
Basics
Equipments must be designed and built so that they do not cause danger to the operator or the environment in the normal operation or in case of equipment damage. Especially take ce of the shielding against electrical shocks, high temperatures, explosion and fire.
There are two classes of insulation:
Class I: single insulation which requires three core mains cable with earth
Class II: double insulation which requires no earth
Class I characteristics
Insulation between mains and every touchable part must withstand flashover voltage of 2120V
The distance between mains voltage carrying parts and touchable parts must be at least 3 mm
All touchable conducting parts must be properly earthed
Class II characteristics
Insulation between mains and every touchable part must withstand flashover voltage of 4240V
The distance between mains voltage carrying parts and touchable parts must be at least 6 mm If you are designing electronics product you should aim for making your products class II. They are easier to sell abroad. If you need to provide the equipment as class 1 you should be very clear in the installation instructions of the correct methods for wiring the equipment to a supply.
1.Practical considerations on building safe equipments .
All the parts in the equipment which carry dangerous voltage must be protected so that nobody can touch them. There must not be possible to expose any dangerous voltage carrying parts without using tools to open the equipment.
2.Keep the distance between mains carrying parts and other parts as large as possible.
The distance between mains carrying parts and other parts must not be in any case less than what is required.
3.Try to make the mains carrying parts as compacts as possible.
Use approved parts for mains carrying parts of the circuit (mains etry, fuse holder and switch)
If you do not use an intergrated entry, use strain relief on the mains cable entry point. You must also provide some mechanical protection between the mains cable and the equipment case.
The wires from the mais cable must not be directly soldered to the circuit board.
The gounding wire must be connected so that it will be disconnected last if for some reason the strain relief in the mains cable gets loose.
All wires inside equipment which carry mains voltage must use wire approved for this kind of application.
Use preferably double-pole mains switch in all your circuits. Single pole mains switch is allowed only on equipment that is powered by transformers with isolated promary and secondary windings.
4.Fuses and mains interference supperssors are not needed to be switched off.
Use only the approved color coded wires for carrying the mains voltage. The green/yellow colored wire can be only used as grounding wire.
Fusing
1.Generally all equipments need fuse.
2.Short-circuit proof transformers do not need a promary fuse.
3.Use only approved fuse holders.
It is advisable (though not mandatory) to precede the mains switch with a fuse.
Every fuse must have a label stating the it's rating and type.
The rating of a slow fuse should not be greater than 1.25 times the normal operating current of the equipment.
Every equipment must have a label stating the identity of the equipment, the mains voltage and mains frequency. If the equipments works only on AC power then there must be a symbol stating that. It is a good practice to put also the equipment mains current and/or power rating in that label.
In case of failure the equipment should not be a danger to the user.
Temperature of touchable parts must not be so high that they can cause injury or create a fire risk. The fuse is to prevent overheating or fire hazard in case of a short. The protection for user is not by fuses, instead the ground bonding for the class 1 and double/reinforsed insulation for class II would be to ensure the protection of user against electrical shock.
Mechanical construction
Mechanical construction of the equipment must be sturdy to withstand the equipment operating conditions.
Repeately dropping the equipment onto a hard surface from height of 50 mm must not cause damage.
Greater impacts must not loosen the mains transformer, electrolytic capacitors and other important components.
Materials
Do not ise dubious or flammable materials.
Do not use material which emit poisonous gasses.
The case must be made of such material that does not burn by itself.
Special details on building Class II equipments
Use mains cable with moulded plug.
Use good strain relief on mains cable.
Use an approved mains on/off switch which does not have a metal lever.
Push wires through the eyelets and solder.
Use insulating sleeves to provide extra protection.
The distance between transformer and other parts must be at least 6 mm.
Use wire with insulation of 4 mm or more and core diameter of at least 0.75 mm.
The circuit board must be secured firmly.
Use preferably insulating plastic case.
The equipment label must have the indication that the equipment is double insulated (two squares symbol).
When the power switch is not required ?
Power on/off switch is nit required if the power consumption of the equipments is less than 10W or if the equipment is intended for continuous use.
An on/off switch not in the mains circuit is allowed if the transformer has isolated promary and secondary coils and power consumption in "off" position is less than 10W. There must be a visible indication that the equipment is plugged in.
Information sources
Safety Guidelines for Elektor Electronics magazine. This article is published in very manu Elektro Electronics magazine issues. The article I used is form Elektor Electornics 4/1996 page 53.
Olavi Hokkinen, Sähköturvallisuuohjeet, Suomen Radioamaööriliitto, 1984, 12 s.
Some additional tips when operating with line powered electronic circuits
Three rules when working with line powered electronics equipments
Rule 1: switch the power off
Rule 2: work with one hand
Rule 3: keep the other hand behind your back That way you live longer when you work with mains powered equipments. Failing to follow all 3 rules may shorten your life span.
Additional safety tips when operating with mains powered equipments
Always be very careful when operating with line powered equipments
Use mains isolation transformer always when you must work with equipments when they are powered
If you can't use mains isolation transformer when use Ground Fault Interrupter (GFCI) for your own safety.
To make sure that the power is disconnected do it twice for safety: switch the equipment off and remove the power cord.
When you power down the equipment wait some time to let the dangerous voltage carrying capacitors to discharge. Make sure that the large capacitors are discharged then you start to operate with the equipment (discharge them if necessary).
Do not wear anything which can fall inside equipments and cause short circuits.
reference
http://www.epanorama.net/documents/safety/safety_guidelines.html
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