三相系统用电缆的U0 值推荐如下表：The values of U0 re...mened for cables to be used in three-phase systems are listed below.
电缆的额定电压应适合于电缆系统的运行状况，用U0/U
（Um）kV 表示。
U0—电缆设计用的导体对地或金属屏蔽之间的额定工频电
压；
U—电缆设计用的导体间的额定工频电压；
Um—设备可承受的“最高系统电压”的最大值。JXN电机引接线
The rated voltage of the cable for a given application shall be suitable for
the operating conditions in the system in which the cable is used,and is
expressed in the form of U0/U（Um）kV. Where:
U0—The rated power-frequency voltage between conductor and earth
or metallic screen,for which the cable designed；
U—The rated power-frequency voltage between conductors,for which
the cable disigned；
Um—The maximum value of the “highest system voltage”for which
the equipment may be used.JXN电机引接线
注：
A 类系统—该类系统任何一相导体与地或接地导体接触时，能
在1min 内与系统分离；
B 类系统—该类系统仅包括单相接地故障短路时运行的条件，
接地故障时间应不超过1h，但在任何情况下，最长不超过8h，
每年接地故障总持续时间不超过125h；
C 类系统—不属于A 类、B 类的系统。
System category A—This category ...prises those systems in which any phase
conductor that ...es in contact with earth or an earth conductor,is disconnected
from the system
with in 1 min.
System category B—This category ...prises those system which,under fault
conditions,are operated for a short time with one phase earthed.This period should
not exceed 1 h.For a longer period,not exceeding 8 h on any occasion,can be
tolerated.The total duration of earth faults in any year should not exceed 125 h.
System category C—This category ...prises all syatems which do not fall into
categories A and B.