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Meeting isolator safety requirements

Posted: 12 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Isolator? voltage? Optical? magnetic? capacitive?

Products are ready for use equipment. Examples of safety standards for products (equipment) are IEC 60950-1 for Information Technology Equipment, IEC 61010-1 for Measurement Equipment, and IEC 60601-1 for Medical Equipment. Products undergo a one-minute isolation test voltage, also called dielectric withstand, high potential or hipot test. Isolators cannot operate continuously at the isolation (test) voltage. Product standards also allow partial discharges that can degrade the insulation barrier due to small bubbles or impurities in the insulation. Product standards do not contain the tests that fully stress (test) the insulation of isolator components.

NOTE: Product standards focus on a dielectric withstand test voltage for isolation. This is a one minute transient test, typically at 2,500 C 5,000Vac. Products cannot operate continuously at the test voltage. Do not assume that an isolator rated for 2,500Vac isolation can operate at this voltage.

Components are electrical/electronic parts that are assembled into (built-in) products. The associated isolator component safety standards are IEC 60747-5-5 for Optical Isolators, VDE 0884-10 for Digital Isolators, and UL 1577 for Optical/Digital Isolators. Component standards have different, oftentimes stricter, safety requirements and tests than product standards. Isolator component standards require partial discharge testing, which is performed to check whether the isolator's internal insulation can break down. Partial discharge is an IEC/VDE isolator component test, which can detect small imperfections that could grow over time. Partial discharge testing is a better measure of the internal insulation of the isolator than the isolation test voltage.

NOTE: Working voltage, VIORM, is specified by IEC 60747-5-5 (optos) and VDE 0884-10 (digital) standards. Working voltage is the voltage that can be continuously applied to the isolator during its operation. Working voltage ratings average between 150 and 600Vac. The isolator's working voltage must be as high as the product's operation, supply voltage.

Components and products must fall within a safety standard's scope before the standard can be applied. As shown in Table 1, isolators are components which are not within a product standard's scope. Therefore, product standards are not applicable for isolators.

Table 1: Scope of Component and Product Standards.

1 Reinforced insulation required for safety protection.

Another issue with standards is that component standards are not equivalent between Europe/International (IEC/VDE) and North America (UL), which means that isolators need to meet both the IEC/VDE and UL component standards.

CAUTION: Always look for the correct isolator component standard for safety evaluation and testing. Isolators also need to meet both IEC/VDE and UL component standards for International: IEC, VDE, CE, and for North America: UL. Isolator component safety standards are: IEC 60747-5-5, VDE 0884-10, and UL 1577.

Spacings-clearance, creepage, basic, reinforced
In addition to testing, safety standards require separation of specific circuits for user protection. These are the spacing distances between hazardous voltage and nonhazardous, touchable voltage circuits. Basic, Double, and Reinforced are safety insulation levels. Basic is a single layer and Double is two layers. Reinforced is a single insulation system which is equivalent to Double. Products and components must remain safe even when there is a single fault or failure in the insulation. It is assumed that a single fault can occur in one layer of insulation. Reinforced is required to protect users in case of a single fault of the first (Basic) layer. In other words, if a fault occurs in the Basic insulation, the second insulation will still protect the user. Basic is not adequate for protection of product users. Reinforced solves the single fault issue.

NOTE: Safety standards require Reinforced insulation to protect product users from hazardous voltages (>30Vrms/42.4Vpeak/60Vdc).

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