Overvoltage categories in power supply systems

Most users of mains-powered equipment are confident that they can simply plug in their device to a convenient socket, and their device will function correctly, safely, and reliably. Stringent and mandatory international standards for power supplies support this notion. Even a modular product embedded within another equipment is required to meet a minimum level of electrical safety isolation and electromagnetic compatibility including immunity to voltage surges and transients.

The fact is, anywhere in the home, office, or factory where the mains power comes from wall sockets, it is likely to provide an environment where the mains power is relatively ‘clean’ without undue voltage transients that could damage the end-equipment. However, as you move progressively back through the infrastructure wiring to a distribution board and then further to the AC supply connection for a building, the possibility of seeing overvoltage from external events such as lightning strikes and load dumps from other installations on the utility network, which are obvious risks, goes on increasing.

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.

It may have been difficult in the past to locate cost-effective AC/DC power supplies that are rated to installation in OVC III environments, but they are now increasingly available from manufacturers such as RECOM. A discussion with manufacturers’ technical support teams may also show that the products with medical certification may also meet the OVC III requirements with simple modifications as they already comply with the less stringent industrial-grade safety requirements.