问题 单项选择题

对于相线对地电压为220V的TN系统,手持式电气设备和移动式电气设备末端线路或插座回路的短路保护元件应保证故障持续时间不超过()s;配电线路或固定电气设备的末端线路应保证故障持续时间不超过()s。正确答案是()。

A.0.4;5

B.0.5;5

C.1;5

D.1;10

答案

参考答案:A

解析:

应用保护接零应注意下列安全要求:

(1)在同一接零系统中,一般不允许部分或个别设备只接地、不接零的做法;否则,当接地的设备漏电时,该接地设备及其他接零设备都可能带有危险的对地电压。如确有困难,个别设备无法接零而只能接地时,则该设备必须安装剩余套电流动作保护装置。

(2)重复接地合格。重复接地指PE线或PEN线上除工作接地以外的其他点再次接地。

4)改善架空线路的防雷性能。电缆或架空线路引入车间或大型建筑物处,配电线路的最远端及每1km处,高低压线路同杆架设时共同敷设的两端应作重复接地。每一重复接地的接地电阻不得超过10Ω;在低压工作接地的接地电阻允许不超过10Ω的场合,每一重复接地的接地电阻允许不超过30Ω,但不得少于3处。

(3)发生对PE线的单相短路时能迅速切断电源。对于相线对地电压220V的TN系统,手持式电气设备和移动式电气设备末端线路或插座回路的短路保护元件应保证故障持续时间不超过0.4s;配电线路或固定式电气设备的末端线路应保证故障持续时间不超过5s。

(4)工作接地合格。工作接地的主要作用是减轻各种过电压的危险。工作接地的接她电阻一般不应超过4Ω,在高土壤电阻率地区允许放宽至不超过10Ω。

(5)PE线和PEN线上不得安装单极开关和熔断器;PE线和PEN线应有防机械损伤和化学腐蚀的措施;PE线支线不得串联连接,即不得用设备的外露导电部分作为保护导体的一部分。

(6)保护导体截面面积合格。除应采用电缆心线或金属矿或金属护套作保护线者外,有机械防护的PE线不得小于2.5mm2,没有机械防护的不得小于4mm2。铜质PEN线截面积不得小于10mm2,铝质的不得小于16mm2,如系电缆芯线,则不得小于4mm2。

(7)等电位联结。等电位联结是指保护导体与建筑物的金属结构、生产用的金属装备以及允许用作保护线的金属管道等用于其他目的的不带电导体之间的联结,以提高TN系统的可靠性。等电位联结实现的手段分为主等电位联结和辅助等电位联结。

阅读理解

The history of the Games Olympia

Olympia, the site of the ancient Olympic Games, is in the western part of the Peloponnese which, according to Greek mythology, is the island of "Pelops", the founder of the Olympic Games. Imposing temples, votive buildings, elaborate shrines and ancient sporting facilities(设备) were combined in a site of unique natural and mystical beauty.

Olympia functioned as a meeting place for worship and other religious and political practices as early as the 10th century B.C. The central part of Olympia was dominated by the majestic temple of Zeus, with the temple of Hera parallel to it. The ancient stadium in Olympia could hold more than 40,000 audience, while in the surrounding area there were auxiliary(辅助的) buildings which developed gradually up until the 4th century B.C. and were used as training sites for the athletes or to house the judges of the Games.

The Games and religion

The Olympic Games were closely linked to the religious festivals of the cult of Zeus, but were not a total part of a rite. Indeed, they had a secular character and aimed to show the physical qualities and development of the performances accomplished by young people, as well as encouraging good relations between the cities of Greece. According to specialists, the Olympic Games owed their purity and importance to religion.

Victory ceremonies

The Olympic victor received his first awards immediately after the competition. Following the announcement of the winner's name by the herald, a Hellanodikis (Greek judge) would place a palm branch in his hands, while the audience cheered and threw flowers to him. Red ribbons were tied on his head and hands as a mark of victory.

The official award ceremony would take place on the last day of the Games, at the elevated vestibule of the temple of Zeus. In a loud voice, the herald would announce the name of the Olympic winner, his father's name, and his homeland. Then, the Hellanodikis placed the sacred olive tree wreath(花环), or kotinos, on the winner's head.

小题1:When did Olympia become the site of the Olympic Games?

A. In the 10th century B.C.

B. Before the 4th century B.C.

C. After the 4th century B.C.

D. It was not mentioned here.

小题2:Which one is TRUE according to the passage?

A.Zeus is the founder of the Olympic Games.

B.The ancient stadium in Olympic were used as training sites for the athletes in the 10th century.

C.The Olympic Games have much to do with the religion.

D.The Olympic victor would receive a golden medal.小题3:Can you guess the meaning of this word “herald” in the passage?

A.the author

B.the headmaster

C.the announcer

D.the manager小题4:When the athletes won the game, ______.

A.They were awarded immediately after the competition.

B.They were awarded twice. The first, immediately after the competition; the second, on the last day of the game.

C.They were awarded on the last day of the game.

D.They were awarded on the last day of the game or immediately after the competition.

填空题