What is voltage rating?

The voltage rating of a cable is the highest voltage that may be continuously applied to a completed cable construction in compliance with the relevant cable standard or specification (refer to this link if you're not sure what the definition of a volt is). It's the voltage a cable can remain stable in operation up to. 

Cable voltage ratings are generally expressed using 2 values U0/U V, such as 300/500V. Voltage rating figures for cables are normally expressed in A.C. RMS. (Alternating Current Root Mean Square). RMS is defined as the square root of the mean square of instantaneous values of the voltage signal.   The RMS value is most important in the case of an AC signal. Because the instantaneous value of an AC signal varies continuously with respect to time. Unlike a DC signal, which is relatively constant.

RMS is defined as the square root of the mean square of instantaneous values of the voltage signal.   The RMS value is most important in the case of an AC signal. Because the instantaneous value of an AC signal varies continuously with respect to time. Unlike a DC signal, which is relatively constant.

An example voltage rating would be what is often just referred to as 0.6/1kV - it would actually be written as 0.6/1 (1.2)kV, or take an example from a higher voltage cable such as the medium voltage 19/33 (36)kV cable. The section in brackets at the end is the maximum voltage the cable can remain stable and safely operate.

Uo = Rated voltage phase to Earth

U = Rated voltage phase to phase

Um = Maximum system

U0 is the maximum allowed RMS voltage between any insulated conductor within the cable and earth (such as a metallic covering of the cable or the surroundings- which are generally considered to be at earth potential).  The U value is the maximum allowed value between any two of the live insulated conductors (phases) within the cable.

The value shown are for AC power supply systems, for example a 300/500V voltage rating describes a cable intended for use in a 230V AC (domestic) distribution system. 

When used in a DC system, it is generally accepted that the cables voltage rating is 1.5 times the AC voltage rating- this can be illustrated in standards such as BS 6724, where in Annex A it states that 600/1000 V cables are suitable for D.C. systems operating at up to 900 V to earth, or at up to 1800 V between conductors.

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