電纜額定電壓的選擇Voltage designation  JXN電機引接線
三相系統用電纜的U0 值推薦如下表：The values of U0 re...mened for cables to be used in three-phase systems are listed below.
電纜的額定電壓應適合于電纜系統的運行狀況，用U0/U
（Um）kV 表示。
U0—電纜設計用的導體對地或金屬屏蔽之間的額定工頻電
壓；
U—電纜設計用的導體間的額定工頻電壓；
Um—設備可承受的“最高系統電壓”的最大值。JXN電機引接線
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電機引接線
注：
A 類系統—該類系統任何一相導體與地或接地導體接觸時，能
在1min 內與系統分離；
B 類系統—該類系統僅包括單相接地故障短路時運行的條件，
接地故障時間應不超過1h，但在任何情況下，最長不超過8h，
每年接地故障總持續時間不超過125h；
C 類系統—不屬于A 類、B 類的系統。
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.