Overvoltage categories

Electrical distribution systems and loads are becoming increasingly complex. This also results in the likelihood of transient overvoltage increasing. Power electronic modules in particular (e.g. frequency converters, phase angle and trailing-edge control, PWM-controlled power switches) generate temporary voltage peaks in conjunction with inductive loads, which can be significantly higher than the respective nominal voltage. In order to guarantee user safety, four overvoltage categories (CAT I to CAT IV) are defined in DIN VDE 0110 / EN 60664.

The measurement category indicates the permissible application ranges of measuring and test devices for electrical operating equipment and systems (e.g. voltage testers, multimeters, VDE test devices) for application in low voltage network areas.

Defined categories and application purposes in IEC 61010-1:

The following categories and application purposes are defined in IEC 61010-1:

CAT I Measurements on current circuits that have no direct connection to the mains network (battery operation), e.g. devices in protection class 3 (operation
with protective low voltage), battery-operated devices, car electrics
CAT II Measurements on current circuits that have a direct connection by means of a plug with the low voltage network, e.g. household appliances, portable electrical appliances
CAT III Measurements within the building installation (static loads with direct fixed connection, distribution connection, fixed installation appliances in the distribution system), e.g. sub-distribution.
CAT IV Measurements at the source of the low voltage installation (meter, main connection, primary overcurrent protection), e.g. revenue meters, low voltage
overhead lines, utility service entrance box
Fig.: Graphic illustration of the CAT categories
Fig.: Graphic illustration of the CAT categories

The category is particularly significant for safety during measurements, because low-resistance current circuits exhibit higher short circuit currents and / or the measuring device is also required to withstand disturbances in the form of load switching and other transient overvoltages, without the user being endangered by electric shocks, fire, sparks forming or explosions. Due to the low impedance of the public grid, short circuit currents are at their g reatest at the house infeed. Inside the home, the maximum short circuit currents are reduced through the system's series impedances. Technically, compliance with the category is ensured for example through the contact protection of plugs and sockets, insulation, sufficient clearance and creepage distances, the strain relief and kink protection of cables, as well as sufficient cable cross-sections.

In practice:

Our experience and understanding shows that many users are not sufficiently familiar with this subject. In some applications, the subject of overvoltage categories may result in a need to change from a UMG 604 with 300 V CAT-III to a UMG 508 with the overvoltage category 600 V CATIII, i.e. instead of a 4,000-V measurement voltage surge, a 50 % higher measurement voltage surge of 6,000 V is attained! However, it may also result in the shifting of the measurement point.This means additional safety for man and machine!

The combination of the CAT category and the defined voltage level gives the measurement voltage surge.

Rated voltages of power supply systems (networks) with various types of overvoltage limitation