Safety guidelines for building electronics equipments

This document intends to 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.

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.
• 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.
• 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.
• 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

• Generally all equipments need fuse.
• Short-circuit proof transformers do not need a promary fuse.
• 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, S�hk�turvallisuuohjeet, Suomen Radioama��riliitto, 1984, 12 s.

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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.

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Tomi Engdahl <[email protected]>