When selecting a power supply, it is vital to have the right protection from overvoltages. Depending on the installation site, overvoltage categories II to IV need to be taken into account.
The Problem
With increasing levels of automation, over-voltage protection is become more and more important. Potential troublemakers like variable-speed drives and motors and high-sensitivity electronic control systems are being used more and more in increasingly complex electrical installations. Apart from lightning strikes, other causes of overvoltage conditions in the low-voltage grid include faults in the mains power supply, faulty electrical equipment in an installation, switching off inductive loads (for example motors) and, sometimes, switching on large pieces of equipment. The energy content of the resulting overvoltages in the mains network can vary enormously.
How can the overvoltage category be determined?
A table breaks down the impulse stability for various levels of overvoltages into four categories. Basically, the closer a piece of equipment is to the point where the low-voltage mains is connected, the higher the overvoltage category (OVC) to be taken into account. This can be determined by the kind of application.
• OVC I: devices that are connected to circuits in which measures have already been taken to keep possible transient overvoltages to below the defined limit; e.g. IT equipment, including laptops with an upstream desktop power supply.
• OVC II: devices equipped with a plug and energised via a fixed installation, such as from a power outlet; for example white goods and consumer electronics.
• OVC III: devices in fixed installations; e.g. in the building distribution system, including power supplies in the switching cabinet for building automation.
• OVC IV: for devices connected at the origin of the low-voltage mains network; e.g. smart meters.
All of these points are defined in the IEC/EN60664-1 standard for determining insulation coordination for equipment within low-voltage systems:
Pulse voltage resistance to be taken into consideration with basic insulation according to IEC/EN60664-1
Example
Power supplies from MEAN WELL are often IEC/EN60950-1 certified and are designed as OVC II equipment. However, enquiries about OVC III products are increasing, especially for industrial and building automation. To meet this demand, MEAN WELL has certified a large selection of OVC III power supplies to meet IEC/EN61558-1, or planned their electrical design in accordance with this standard.
• According to the current version of IEC61558-1: 2005 + AMD1: 2009, the following definition applies: a power supply meets the requirements of OVC III if it has double or reinforced insulation between the primary and secondary side.
• In accordance with IEC/EN61558-1, the stated products fulfil the requirements of OVC III up to an operating altitude of max. 2000m. Above that altitude, the datasheet for the devices confirms them as OVC II, although – using the multiplication factor for the clearances – enough space for an operating altitude up to 5,000m has been taken into consideration (factor 1.48).
• This is based on the impulse voltage test of IEC/EN61558-1. The maximum test voltage of 6kV for double or reinforced insulation for OVC III is only specified for an operating altitude of up to 2000m. It is therefore necessary to go down to the lower test voltage for OVC II (4kV for double insulation). At the same time, the required impulse voltage must not exceed 6kV. At a test voltage of 4kV multiplied by a factor of 1.48 for an altitude up to 5,000metres, we arrive at an impulse voltage of 5.92kV. This is guaranteed by an OVC II device up to 5000 metres.
Tip: MEAN WELL offers a broad selection of OVC III power supplies certified in accordance with EN61558-1: the enclosed LRS and RSP series and also the HDR series and KNX-20E for DIN-rail mounting Their UHP series (U-profile) and encapsulated, IP67 OWA series. OVC is also being taken into consideration for new developments, with the aim of increasing the selection of suitable products.
Schukat electronic
As one of the biggest MEAN WELL distributors in Europe, Schukat offers most of their power supplies direct from stock. In addition, the technical sales team at Schukat is at your service to answer questions about overvoltage categories and for any power supply related questions you may have.
