Ensuring that a chosen power supply is safe and appropriate for a particular product is a complicated process due to the fact that there are an abundance of power supply options and at least 15 separate testing considerations that must be made. Jean-Louis Evans, managing director at TÜV SÜD Product Service, offers some advice
The first stage should be to review a product’s intended environment, including ambient operating temperature, whether it is for indoor or outdoor use, the industry and country for which it is designed.
It is also important to consider if the end user will be able to access the power supply and its output circuits. If it is an open frame design, then a suitable fire, mechanical and electrical enclosure must be provided as part of the final installation. For other designs these enclosures need to be provided as part of the design itself. Provision must also be incorporated to protect installers, users and service personnel from hazardous output circuits if they are anywhere accessible in use.
Design before test
It is vital that you design for compliance and verify by test, and do not test for compliance and then re-design, as this is a costly strategy, both in terms of cost and lengthening the time to market for new products.
At the earliest possible stage of a design all the safety critical components must be identified, such as fuses, mains switches and transformers. Make sure that each component has the appropriate certification and do not rely on manufacturer data sheets alone. Approval certificates and current listings on certification body websites are the only acceptable evidence that components have the correct approvals. The words ‘designed to meet’ or ‘complies with’ are not evidence of compliance.
It is important to pay particular attention to expiry dates on certifications and to ensure that the most up to date standard(s) have been applied. Also, be aware that subtle part number changes can be overlooked, meaning that the certificate applies to another similar component or product, but not the one selected.
Categorise circuits
As part of the design and test process it is important to categorise the output rating and type of circuit being used in the power supply against the following parameters:
- Safety Extra Low Voltage (SELV) – always safe to touch under both normal and single fault conditions.
- Extra Low Voltage (ELV) – safe to touch under normal conditions only.
- Hazardous – not safe to touch (additional precautions required to be taken).
- Limited power – output power limited under normal and single fault conditions.
- Limited current – output is high voltage but current is limited – safe to touch.
It is also vital to identify different circuit types and the appropriate levels of safety isolation required between them. Once the main circuit types have been identified, the next step is to ensure that they are adequately separated. Broadly speaking, your design must adhere to the following:
- Basic insulation – required between hazardous circuits (mains primary) and protective earth. If the basic insulation fails, the fault current would flow to protective earth, thus providing two levels of safety protection.
- Reinforced insulation – required between hazardous circuits and SELV output circuits and provides ‘two degrees’ of safety insulation.
- Reinforced insulation – required between hazardous circuits and functional earth. Required to provide ‘two degrees’ of safety insulation as the functional earth does not provide any degree of safety protection.
Safety insulation between circuits is usually achieved by physical separation, but this is often overlooked in the design process. Creepage is the minimum path between two conductive points along the surface of an insulator e.g. between tracks on the surface of a PCB. Clearance is the minimum path between two conductive points through air e.g. between pins of adjacent connectors. Correct application of distances can only be verified by measurement and the application of hi-potential electric strength tests between circuits.
Fire safety
There are many safety strategies to take regarding fire hazards. The first is the inclusion of a fire enclosure to prevent metallic foreign objects entering a power supply, as well as stopping molten material dripping from it if a fire has started.
The second is the use of components and materials which have the appropriate flammability ratings to minimise fire propagation. The third is the test simulation of single fault conditions to verify that the design is fault tolerant. During such tests the design does not have to continue to work but it must remain safe.
While the considerations outlined here are just a few that must be made to ensure a power supply is fit for purpose, the increasing number of failed goods identified by bodies such as Trading Standards and RAPEX (the EU rapid alert system for non-food products posing a serious risk to health and safety) shows that poor quality power components are becoming endemic. The only way to ensure that your products do not include such items is to undertake stringent tests yourself.
TÜV SÜD Product Service
T: 01489 558 100
