Power Supply Safety

[Bakmeel]

Many threads in the Power Supplies Forum deal with mains connected equipment and/or equipment capable of producing dangerous voltages and powers. This thread is here to tell you about the safety rules you need to know about before you embark on your “power supply adventures”. Please also note that these rules also apply to all other forums on diyAudio, as many electronics discussed on this website may expose you to the very same threats. (So, this is to make sure a "forum thread" does not become a "safety threat" )


What is electrical safety?
The European Standard “IEC 60950-1” defines that electrical safety relates to protection from:
• A hazardous voltage, that is, a voltage greater than 42.4 V peak or 60 V DC;
• A hazardous energy level, which is defined as a stored energy level of 20 Joules or more or an available continuous power level of 240 VA or more at a potential of 2 V or more;
• A single insulation fault which would cause a conductive part to become hazardous;
• The source of a hazardous voltage or energy level from primary power ;
• Secondary power (derived from internal circuitry which is supplied and isolated from any power source, including DC)

On top of that, there are many other normative reference that stipulate rules and regulations dedicated to electronic equipment safety, such as fire hazards, risk of injury, etc.


Use a wall-mart supply!
Whenever possible, it is recommended to use commercially available and approved (UL, VDE, KEMA) power supplies to power your equipment. These power supplies are tested for safety by independent institutions, they are readily available and usually quite cheap as well.


Safety Classes
If it is necessary to build a mains-connected application, protection against electric shock can be provided in two classes:

Class I equipment uses basic insulation; its conductive parts, which may become hazardous if this insulation fails, must be connected to the supply protective earth. Examples are computer power supplies and kitchen appliances.
In summary, Class I isolation requires adequate insulation between mains and any user accessible part, capable of withstanding a test voltage of at least 2120V peak. An isolation distance of at least 3 mm must be maintained between any user accessible part and mains carrying parts. Furthermore, any conductive, user accessible part must be adequately connected to the safety ground.

Class II equipment uses double or reinforced insulation for use where there is no provision for supply protective earth. Examples are power tools, hair dryers and cell phone chargers.
The double insulation requirement means that the insulation between mains and any user accessible part is capable of withstanding a test voltage of at least 4240V peak. An isolation distance of at least 6 mm must be maintained between any user accessible part and mains carrying parts.

In practice, it is recommended to separate mains carrying parts and user accessible part as much as possible, but never less than required (see above).

When building a power supply, the use of a Class II insulated transformer is preferred, but note that when this is fitted in a Class I equipment, this does not, by itself, confer Class II status on the equipment.


When the mains enter your project
It is always recommended to use an approved mains entry with an integrated fuse holder and switch. If this is not possible, you should do the following.

- In case you intend to route the mains cable directly into your equipment, use a proper strain relief at the point of entry.
- The first thing after the mains enter your equipment must be a properly sized fuse in an approved fuse holder. If you use a mains plug, keep the wiring to the fuse as short as possible. Use properly insulated wiring, and be sure to make firm and mechanically strong connections.
- A separate mains switch should be of the double pole type. In case of three-phase appliances, all phases and neutral must be disconnected simultaneously.
- It is not required, but strongly recommended to place filtering components after the mains switch. Placing them before the mains switch will expose them to continuous stress and reduces their lifetime.
- A secondary on/off switch is allowed if the equipment consumes less than 10W in the “off/standby” state. In such case a visible means (e.g. a stand-by LED) must be provided to indicate that mains are connected.


Safety Ground
Anyone constructing a Class I equipment, must pay special attention to proper safety grounding:

- Use yellow/green insulated wire (if necessary, green with a narrow yellow stripe. Do not use yellow wire with a green stripe)
- If you route the mains cable directly into your equipment, the ground wire must be the longest, such that it is the last one to come free if someone accidentally pulls the wire.
- Any user accessible metallic part must be adequately grounded.
- Adequate grounding may require, for example, an additional ground wire to be connected between the front plate and the mains ground wire.
- If the entire enclosure is conductive, and all parts are firmly connected to each other using metal screws, one grounding point is considered adequate.
- Take care when using metallic switches or potentiometer shafts. They must not provide any hazard!



Source: Elektor Construction and Electrical Safety Guidelines (Dutch, English)


!!! DO NOT MESS WITH MAINS !!!

[Bakmeel]

This info now also eternified in This Wiki Page

[Electrone]

Good of you!

Use a wall-mart supply!

It looks like the spelling checker might have gotten you. Could you have written wallwart (wall wart) originally?

[Bakmeel]

I think that should've been wallwart yes... I think that is a typical US or UK English term that is not tutored in our English classes... thanks for the tip I've corrected it in the Wiki Page.

[cbdb]

Electricity kills! There should be a test on this for all new DIYers.

[Electrone]

That is more good of you. "Wallwart" is supposed to be a little humorous for people who live in a country largely colonized by the British and therefore has a long history with the English language. It is a relatively new term for describing a power adapter that is supported on the wall by its power socket prongs.

[DigitalJunkie]

Because it looks like a wart,on the wall.
http://www.horrorseek.com/home/hallo...l_WallWart.jpg

[Geek]

There was a saying when I took my training in the TV shop... "There are old technicians and there are bold technicians, but there are no old, bold technicians!"

Cheers!

[Bakmeel]

Originally Posted by Geek

"There are old technicians and there are bold technicians, but there are no old, bold technicians!"

I like that one...

[Corben]

I could be wrong, but I think class II requires using two different insulators, not just twice the thickness.

I've seen designs that place an NTC with a smallish 'cold' resistance (5 ohm) between the PE pin on the power inlet and the device ground to prevent ground loops. This should be quite safe, but does anyone know what IEC 60950-1 actually says about this?