The Hazards of Hipot Testing

- Kent Smith, Applications Engineer at XP Power describes some of the hazards of "Hipot" or high potential testing of power supplies and how to avoid them.

Hipot testing is an abbreviation for "high potential" testing - a category of electrical tests used to verify the effectiveness of electrical insulation in transformers, motors, printed circuit boards, electrical sub-assemblies (such as power supplies) and finished equipment. It's also known as "dielectric withstand" testing.

Hipot testing

Electrical equipment that is connected to the AC mains supply must pass a 'type' test requirement by the relevant safety agency. The commonly applicable specifications are IEC60950-1 for ITE & industrial equipment and IEC60601-1 for medical equipment. Before sending equipment for type testing, the manufacturer, or the power supply manufacturer, will usually want to ensure that it meets the requirements by carrying out their own tests. However, test specifications can be misinterpreted, leading to damage to power supplies and delays in gaining the relevant approvals for a product to be brought to market.

This article considers the requirements for testing and provides broad guidelines on what is required of an equipment manufacturer in the type test procedure. It primarily refers to Class I equipment, the differences for Class II equipment are detailed separately.

The safety regulations refer to the following types of insulation:

• Between primary (AC input) and secondary (DC output) reinforced insulation is required
• Between primary and earth basic insulation is required
• Between secondary and earth operational insulation is required

Class I equipment utilizes an insulation system where a protective earth is employed to ensure safe operation. Class II equipment utilizes double or reinforced insulation to ensure safe operation with no provision for protective earth.

Where each type of insulation is required

The diagram below represents a typical class I power supply insulation system.

Typical Power Supply Insulation

R = Location of typical reinforced insulation;
B = Location of typical basic insulation;
O = Location of typical operational insulation.

Safety Agency Testing

Hipot testing requirements are categorized as either type testing (design verification) or production testing.

Hipot Type Testing
This is performed by the safety agency, and tests are intended to prove that the construction of the power supply meets the requirements dictated by the relevant safety standard. For IEC60950-1 (ITE) and IEC60601-1 (Medical) the requirements are shown below:

Insulation Type Testing Levels

Hipot Production Testing
These tests are performed during the manufacturing process and are intended to ensure integrity of safety critical insulation. Production line testing is conducted on basic insulation and on reinforced insulation during the manufacturing process of the subassembly and barrier components.

Reinforced insulation cannot be tested without over-stressing basic insulation on the end product. (Note: see UL60950-1, C5.2.2 or UL60601-1 2nd Edition Section 20.4 or IEC60601-1 3rd Edition Section 8.8.3 for more information.)

Because of this, manufacturers are permitted to test reinforced insulation separately, meaning that they are permitted to test transformers and other primary to secondary isolation barriers separately before other components are incorporated into the product. Only basic insulation or primary to earth insulation is tested on the final assembly prior to shipping the product.

Because only basic insulation exists between primary and chassis ground and only operational insulation exists between secondary and chassis ground, applying 3000 VAC directly from primary to secondary on the finished product will over-stress the primary to chassis ground and secondary to chassis insulation resulting in a potential failure.

To properly test reinforced insulation the power supply needs to be removed from the chassis. In addition, all paths to chassis ground, as far as practical, need to be removed so as not to over-stress basic and operational insulation during the test.

This usually entails removal of all Y-capacitors. The diagram below shows the components that need to be removed.

Power supply disassembly may be required for type testing

On many products not all potential paths can be removed. Printed circuit boards may utilize earth traces between primary and secondary while complying with creepage and clearance requirements. In some instances, when applying the primary to secondary hipot voltage, a breakdown or arcing may be observed which can lead to component failure, rendering the power supply inoperable. This is a breakdown of operational insulation (secondary to chassis ground) only. It does not indicate a failure of primary to secondary insulation that is the focus of the test. Provided this 'fails' in a safe manner, the test is considered successful for safety purposes.

Class II Power Supplies

The previous sections of this document deal with Class I power supplies, which employ a safety earth. In the case of Class II power supplies there is no safety ground and so there is no need, nor ability, to test from primary to earth. Because of the lack of any grounding we also do not have to worry about over stressing any components from the primary to ground, or from the output to ground. The user is able to simply test from the input to the output on the power supply at 3000 VAC (or 4121 VDC) for ITE devices or 4000 VAC (5656 VDC) for medical devices to verify the insulation in the supply.

Kent Smith is an Applications Engineer at XP Power, a company which is committed to being a leading provider of power solutions, including AC-DC power supplies and DC-DC converters. With ISO9001:2000, XP Power offers total quality, from in-house design through to manufacturing facilities around the world. The company offers the widest range of power products available from one source and unrivalled technical and customer support, aiding both vendor consolidation and cost reduction programmes. XP has 27 sales offices throughout Europe, North America and Asia.

XP Power Ltd, a London Stock Exchange listed public company, has design centres in Fyfield (UK), Orange County (California), and Singapore, which also serves as the company's headquarters. Manufacturing is carried out near Shanghai, China. The design teams push back the boundaries of cost and technology, providing market leading power products. Dedicated in-house Engineered Solutions teams provide customer specific solutions, with rapid response times. The applications team provide expert technical support to help customers integrate and use XP power products worldwide.