SAFETY TESTING
Testing the safety of power supplies
Ensuring that a power supply is safe and appropriate for a particular product is a complicated process, with typically at least fifteen separate testing considerations to be taken into account, as Jean-Louis Evans explains
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 money and lengthening the time to market for new products. With an abundance of power supply options, what are some of the testing considerations that should be taken to ensure that a product and its power components are safe?
A key step is to review a product’s intended
environment. For example, what will the ambient temperature be, will it be used in a humid environment, and is it for indoor or outdoor use? Also, the industry and country for which the product is designed will affect the power components selected as supply characteristics can vary wildly i.e. military typically operates at 50, 60 or 400 Hz, while domestic supplies operate at 50 Hz. The intended country of use must also be taken into account.
It is also important to consider if the end-user will have access to the power supply and its output circuits, and how it will be used. For example, is the product a mobile phone charger that must be safe for the user to touch, or is it a piece of laboratory equipment where the power output is not accessible? If it is an open-frame design, then consideration must be given to providing a suitable fire, mechanical and electrical enclosure to prevent access to electrical hazards, to provide a rigid mechanical enclosure and to prevent foreign objects from entering and starting a fire and to prevent flaming / molten parts escaping and setting fire to the surroundings. It is also important to categorise the output rating and type of circuit: • Safety Extra Low Voltage (SELV) - safe to touch under normal and single fault conditions; • Extra Low Voltage (ELV) – safe to touch under normal conditions only; • Hazardous – not safe to touch (additional
precautions to be taken when used); • Limited power – output power is limited under normal and single fault conditions, or
Design considerations
At the earliest possible stage of a design the safety critical components must be identified, such as fuses, mains switches and transformers. Also, make sure that each component has the appropriate certification such as NRTL recognition for North America and IEC component approval for Europe and the rest of the world. Of course, it makes sense to ‘source smart’ by using dual certified components. Approval certificates and current listings on certification body websites are the only acceptable evidence that components have the correct approvals. Do not rely on manufacturer data sheets alone. Pay particular attention to expiry dates and ensure that the most up to date standard(s) has been applied. Also, be aware that subtle part number changes can be overlooked, meaning that the certificate applies to another component or product, and not the one being used.
Identification of circuits It is vital to identify different circuit types and the appropriate levels of safety isolation required between them. Typically you will find the following circuits: • Hazardous - operate above the safe to touch limits; • Earth - there are two types of earth: 1. Functional – provided for reasons of functionality or performance (e.g. for EMC), 2. Protective – are proven as performing a protective (safety) earth function;
10 CIE Power Supplement May 2013
• Limited current – output is usually high voltage but current is limited – safe to touch. There are numerous standards relating to
products, everything from specific standards such as IEC 60601 for medical equipment, to IEC 61558, the general power supply standard. It is therefore advisable to consult an expert or certification body to indentify the correct path to follow.
• Output - usually required to be isolated from the hazardous circuit and are typically SELV (safe to touch).
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 the 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 to provide ‘two degrees’ of safety insulation. • Reinforced insulation - required between
hazardous circuit and functional earth. This is required to provide ‘two degrees’ of safety insulation as the functional earth does not provide any degree of safety isolation.
Creepage and clearance Safety insulation between circuits is usually achieved by physical separation, which is often overlooked. 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.
Fault conditions Safety strategy regarding fire is usually tackled in three ways. 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 above 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 |
www.tuv-sud.co.uk
Jean-Louis Evans is Managing Director at TÜV SÜD Product Service
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