Power Supply Basics

Reviewing power supply safety reports

Posted by Dulcie on October 13, 2015
Posted in: Power Supply Basics

The manufacturer’s datasheet provides a variety of information about the features and ratings of a power supply, but more information is needed for a correct and reliable installation. This type of information can be found in the product’s safety reports. If these often over-looked points are not noted, then delays may occur when safety certification is sought for the complete system.

For the purposes of this article, we will just consider a power supply that is certified to the IEC 60950-1 standard.

Typically there are three pieces of documentation, the CB test certificate, an EN 60950-1 test report (often included in the IEC 60950-1 CB report) and a UL 60950-1 report. Often manufacturers will restrict the circulation of full reports, because of confidential information, and may require the signing of a non-disclosure agreement. Fortunately the safety bodies realise this and permit reproduction of a selected number of pages from the report.

The information in the two page CB test certificate is fairly limited. Its primary function is to inform other NCBs (National Certification Bodies) that a sample of the product complies with the requirements of the standard. The name of the applicant, address and manufacturing location is stated, along with the models covered and their ratings. The standard used for evaluation with the appropriate revisions is very important information. This shows that the power supply status is current and is suitable for designing in to a new system or product. It should be noted that as many power supplies now use the CE mark (Low Voltage Directive) to show compliance to EN 60950-1, the CB test certificate may state “Additionally evaluated to EN 60950-1”.

The IEC 60950-1 CB or UL 60950-1 report, even if it is an abridged version of the complete report which often contains 200 or 300 pages, is more useful. Under the section “Engineering Conditions of Acceptability” are the important notes for product installation. Here one will find such information as:

Which model output voltages are SELV (Separated or safety extra-low voltage). Outputs that are not SELV should be insulated to ensure the operator or maintenance person does not come in contact with them, hence avoiding an electric shock.

If any of the outputs have hazardous energy levels. A power supply that has a VA rating of more than 240VA can cause sparking and/or burns if a metallic object shorts the output. Busbars for example, should be protected.

Can the power supply terminals be field wired? For an embedded power supply, the answer is usually “no” and any cabling attached to the product has to be done at the factory by trained personnel. A DIN rail power supply though is designed for field wiring as the task is simpler, often not requiring crimped tags, having a lower risk of stray wire strands and a stronger clamp for the wiring connection.

The maximum investigated branch circuit rating is stated as during power supply abnormal testing, the input will be protected by a circuit breaker. During testing that breaker is not allowed to trip.
The Pollution Degree rating is stated. A PD rating of 2, for example, is common for office, lab and test equipment where only occasional condensation may occur. On the other hand, a PD rating of 4 indicates the power supply is suitable for outdoor applications, and can be subject to rain or even snow. Using a product with the right rating is critical for both reliability and safety.

The requirement for earth bonding is listed. If “required” is stated, then the power supply earth connection to the end-product earth has to be correctly bonded.

As the maximum temperatures of magnetic components are a safety concern, guidance to the inspector on which Class (temperature rating) insulation system is being used is given. Class A for example is 105oC, and measured temperatures when installed in the end-product cannot exceed that.

Generally an industrial power supply is not designed to be a stand-alone part, like say a power supply used on a bench or a phone charger. Clarification is provided as to the type of enclosure the power supply should be housed in.

Other comments may be made by the inspector – what orientation of the product was used for testing, or if the output voltage adjustment range of the power supply was considered.

Gladly, it is becoming more popular among most of the larger power supply manufacturers to post this type of documentation on their websites; even some of the on-line distributors are now doing this too. The challenge is to keep the information current, particularly with the number of amendments and revisions that are required!