Any engineer designing products which uses mains voltages should have a good idea of what is involved in product safety. If one is going to use a test house for assesment of the product then having a good idea of what is involved will help reduce failures and subsequent retest costs. If one is going to carry out self assessment then this will save significant costs and lead to faster design turnaround.
1. Declaration of Conformity (DOC) - When a manufacturer has verified the safety of their product they have to produce a DOC. There is an example of one below for the Mains Switch Widget (MSW).
2. Labelling: It is important to label a product to indicate that it meets all relevant European rgulations (which is indicated by the CE mark). There is an example below for the MSW. The ! mark is an indication that the user has to refer to the instructions which has safety related information.
3. Anchoring of the mains cable: There has been much debate on Element 14 as to what is acceptable or not. In actual fact the test for this is an objective one. For an appliance of less than 1kg, a force of 30N is applied to the mains cable 100 times and for 1 second duration. During this test the power cord should not become damaged and a maximum of 2mm displacement is allowed. The test labes have specialist equipment for this so how does one do this when carrying out self assessment? If you remember that 1kg weight is 9.8N due to gravitational force, then this is easy. You hang enough weights of upto 3kg off the cable a number of times and check for any movement. I find that 1kg bags of sugar tied into a carrier bag works well. When testing the MSW we used 4kg (40N) to be on the safe side and pleased to say that 1 cable tie did the job.
4. Seperating the mains side from the low voltage electronics on the control side: When mains and low voltage circuits are both present on a PCB, it is important to make sure that there is enough seperation between them. This is known as clearance when it's through and creepage when along the surface of insulation. The amount of seperation required depends on the material, working voltage, the application of the product, the pollution degree and other factors. Here is an of the track side of a PCB showing the seperation which provides reinforced insulation for a working voltage of 250V (EN61010-1).
5. Is it necessary to make the Earth wire longer than the Live or the Neutral? This depends on the how the product safety is achieved. On a Class 1 product with an earthed metal enclosure, it is essential that if the cable were to be pulled out, the earth wire is the last to be pulled out. This way if the Live wire were to pull out and touch the metal case, it would still be earthed, thus protecting the user and causing a fuse to blow. On a product in a plastic enclosure this is clearly not the case.
6. Some good books on this topic: 'Electrical Product Safety' (ISBN 0 7506 4604 7) and 'Practical Guide to the Low Voltage Directive' (ISBN 07506 3745 5) are 2 books that I have found useful in my career. Unfortunately both of these have been around for some time and there is a scarcity of good books on this topic. The harmonised standards are a useful source of good information.
Electrical Safety assessment is an important part of designing products which uses mains voltage. Perhaps in the early stages of ones career it's best to use a test house to gain some experience. It's important not to be put off by the many myths that abound on the internet - some of which I have covered above. The great benefit of gaining the experience to carry out self assessment is that small companies can launch products on a low budget.