Power supplies intended for OVC III areas need enhanced specifications for withstand voltages and clearance distances compared to the more common OVC II rated types. These could be met with a preceding isolation transformer to reduce transients to OVC II levels, but this is often an unwieldy and expensive solution. A better solution is to design the power supply such that it can be directly connected to the supply in an OVC III environment.
An implication for
AC/DC power supply design is that clearance distances between line and accessible parts have to be increased substantially. An AC/DC product with basic insulation designed for being used in an OVC II environment requires a 3.0mm minimum clearance for isolation barriers rated at 4kV, but for an OVC III environment, the impulse voltage rating increases to 6kV, requiring a 5.5mm minimum clearance (Tables F1 and F2 in the IEC 60664-1 standard). However, most AC/DC converters have reinforced
isolation barriers. The standard requires that “the value corresponding to the next highest impulse voltage” be used. This means that a reinforced isolation converter needs clearances of 5.5mm/8.0mm for OVC II/OVC III installations respectively.
IEC 60664-1 and IEC 61558-1 also specify dielectric strength test voltages at 50/60Hz that can be as high as the impulse test value. For example, IEC 61558-1 requires a dielectric test voltage of 4200Vrms = 5938Vpk for 300Vrms working voltage and OVC III/reinforced insulation, without specifying the altitude (Table 14 in the IEC 61558-1 standard). This is similar to the impulse test value from IEC 60664-1, which is 6kV at 2000m altitude. However, one test is for the dielectric strength for 60 seconds, while the other is a short transient impulse voltage.
The most appropriate
safety standard that applies to a power converter depends on the actual application. The products for use in an OVC III environment are often best covered by the IEC 61558-1 standard about ‘Safety of transformers, reactors, power supply units and combinations thereof’. In this standard, Annex R references IEC 60664-1’s levels for the impulse test voltages. Fortunately, the safety clearance requirements in IEC 61558-1 are the same as in IEC 60664-1 Both the standards use the same altitude correction factors above 2000m.
The users may also choose to apply the IEC 62477-1 standard about ‘Safety requirements for power electronic converter systems and equipment’ for their products for OVC III areas. This standard also references the IEC 60664-1 impulse values, creepage/clearances, dielectric test voltages, and altitude correction factors.
The standard IEC 61010-1 about ‘Safety requirements for electrical equipment for measurement, control and laboratory use’ is the most appropriate for the test and measurement equipment used in OVC III areas. This standard specifies a clearance distance of 6mm for 300Vac reinforced isolation (Table K.3 in the IEC 61010-1 standard) compared to 8mm in the IEC 60664-1 standard. The altitude correction factors are the same. However, the other test requirements in the IEC standard 61010-1 are a little harsher. For example, the impulse voltage test in a 300Vrms OVC III environment for reinforced solid insulation is 6400V (Table K.6 in the IEC 61010-1 standard) unlike the IEC 60664-1 standard that specifies an impulse voltage test of 6000V.
The general purpose safety standard IEC/EN/UL 62368-1 about ‘Safety requirements for Audio/video, information and communication technology equipment’ references the IEC 60664-1 standard for the requirements for OVC III and OVC IV environments. The obsolete standard IEC 60950-1, however, required 6mm clearance for 300Vrms/OVC III areas (Table 2H of the IEC 60950-1 standard), the same as specified by the IEC standard 61010-1, but now, the replacement IEC 62368-1 standard has come into line with the IEC 60664-1 standard and raised the clearance requirement to 8mm.
