1.?????? Rated Voltage of Power Cable

Uo is the AC rated voltage(effective value)of metal screen,metal armor and metal sheath of conductor used in power cable design.The insulation structure design of cable and electric test all adopt Uo as the standard.It reflects the insulation level of power cable.

U is the AC rated voltage(effective value)of phase conductor in power cable design.It is not only related to insulation level of power cable,but also to system earthing style and power cable category in using power cable.

Um is the maximum value of the highest system voltageto be borne by equipment.

2.?????? Category of Power Cable

According to the sustained time of earthing breakdown,power cable is divided into two categories:

Category 1:The sustained time of earthing breakdown does not exceed 1 minute,that is,it requires a system when any phase conductor contacts the earth or earthing conductor,can separate the phase conductor from the system in 1 minute.

Category 11:The sustained time of each earthing breakdown usually should not exceed 1 hour,the longest not exceeding 8 hours and the accumulated time in a year should not exceed 125 hours.

For the insulation level of two categories of cable,see the following table.

Table 1 Category and Rated Voltage of Power Cable

U	Um	Uo	KV
kV	kV	Category 1	Category 11
6	7.2	3.6	6
10	12	6	8.7
15	18	8.7	12
20	26	12	18
35	42	21	26
110	?	64	?

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Table 2 Thickness and Requirements of Insulation Layer of 6-110kV XLPE Insulation Power Cable

?? ??????Rated Voltage Conducting????? kV Wire Core			6/6, 6/10		8.7/10, 8.7/15		12/20		18/20 18/30		21/35		26/35		64/110
Cross Section Area mm2	No./Wire Dia. mm	Extruded Overall Dia. Mm	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Av. Thickness ??/p>	Thinnest Thickness ??/p>	Core Shift Degree ??/p>
25	7/2.25	6.0	? ? ? ? ? ? ? 3.4??????? 2.96?????? 4.5???? 3.95??? 5.5???? 4.85?? 8.0????? 7.0??? 9.3???? 8.27??? 10.5??? 9.35												?
35	7/2.62	7.0
50	7/3.10	8.3
70	19/2.25	10.0
95	19/2.62	11.6
120	19/3.0	13.0
150	36/2.36	14.6
185	36/2.62	16.2
240	36/3.00	18.4													19.0	17.10	? ? ? ? 0.10
300	36/3.36	20.6													18.5	16.55
400	60/3.00	23.8													17.5	15.75
500	60/3.36	26.6													17.0	15.30
630	60/3.75	30.0													16.5	14.85
800	60/4.26	34.0													16.0	14.40
1000	60/4.75	38.2													16.0	14.40
1200	60/5.2	42.0													16.0	14.40
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