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There are some key definitions within the IEC 60950-1 standard that are important to understand when designing a power system. A hazardous voltage is defined as one that exceeds 42.2VAC or exceeds 60VDC without a limited current. An extra low voltage (ELV) is a non-hazardous voltage in a secondary circuit, which is separated using at least basic insulation. A safety extra-low voltage (SELV) is a secondary circuit that cannot reach a hazardous voltage between any two accessible parts or an accessible part and protective earth under normal operation or while experiencing a single fault. In the event of a single fault condition (insulation or component failure) the voltage in accessible parts of SELV circuits shall not exceed 42.4VAC peak or 60VDC for longer than 200 msecs. Also, an absolute limit of 71VAC peak or 120VDC must not be exceeded. SELV circuits must be separated from hazardous voltages (e.g. primary circuits) by two levels of protection, which may be double insulation, or basic insulation combined with an earthed conductive barrier. Limited current circuits may be accessible, even though voltages are in excess of SELV requirements. A limited current circuit is designed to ensure that under a fault condition, the current that can be drawn is not hazardous. Limits for these circuits are detailed here.
