问题 问答题 简答题

简述短路故障的危害及主要预防措施。

答案

参考答案:

短路是指电流不流经负荷,而是经过导线直接形成同路。

1.短路故障的危害

短路事故是煤矿常见的恶性事故之一。其特点是电流很大,且产生2500~4000℃高温的电弧。可在极短时间内烧毁线路或电气设备,甚至引起火灾或引发瓦斯煤尘爆炸。同时,短路还可以引起电网电压急剧下降,影响电网中其他电气设备的正常工作。

2.短路故障的主要预防措施

(1)防止线路和电气设备的绝缘破坏,如绝缘老化、绝缘受潮、接线工艺不合格、设备内部的电气缺陷和电缆质量低劣等。

(2)避免遭受机械损伤。如电机车、输送机的撞击,冒落矸石砸伤,爆破崩坏和电缆使用盘放不当等。

(3)杜绝误接线、误操作,如相序不同线路的并联,带电进行封装接地线与带封装接地线送电和局部检修送电等。

(4)井下供电做到无鸡爪子、羊尾巴和明接头。

(5)严禁带电检修和搬迁电气设备。

阅读理解

阅读理解。

     NASA scientists said on June 1 , 2008 they had found ice on Mars-a step towards finding

evidence of life.

    " Sharp  new  images  received  from  their  Phoenix  lander convinced  scientists  that  the 

 spacecraft ' s  thrusters (推进器) had uncovered a large patch of ice just below the Martian

surface, " team members said.

     That bodes (预示) well  for the  mission ' s  main  goal  of digging for ice that can be tested

for evidence of organic compounds that are the chemical building blocks of life.

     Washington University scientist Ray Arvidson said the spacecraft' s thrusters may have blown

away dirt covering the ice when the robot landed one week ago.

      Scientists said a detailed image taken under the lander shows one of the craft's three legs

sitting on rough dirt and a large patch of what appears to be ice-possibly 3 feet(0. 9 metres) in

diameter(直径) -that apparently had been covered by a thin layer of dirt.

      " We were worried that it may be 30-,40-,50-centimeter deep, which would be a lot of

work. Now we are fairly certain that we can easily get down to the ice table, " said Peter Smith,

a University of Arizona scientist who is in charge of the project.

     The spacecraft is equipped with a shovel-like robot arm that will be used to dig into the ground

and collect samples for testing in the lander's small laboratories.

     The lander was sent to a spot on Mars' northern regions in hopes of finding frozen water, but

just how deep underground it would be found was unknown.

     The robot arm is expected to begin its first digging operations after several more days of testing.

Once the arm starts digging, dirt and  ice  it  scoops  up will  be deposited(存放 ) in  several  small

  ovens to be heated. Measuring devices will test the resulting gases.

     The University of Arizona in Tucson is leading and NASA's Jet Propulsion Laboratory is managing

the three-month scientific mission.

1. What was the Phoenix lander meant to do?    

A. To search for frozen water on Mars.

B. To explore life in outer space.

C. To study the minerals in the soil.

D. To discover new resources in outer space.

2. Why did NASA scientists feel worried about the finding?  

A. Because the ice is too large for NASA scientists to dig up.

B. Because it takes some time to deal with the large patch of ice.

C. Because it' s impossible for Phoenix lander to collect the ice.

D. Because it's hard to carry the ice back to laboratories on the earth .

3. "The spacecraft" in Paragraph 7 refers to"_______".

A. The project                      

B. The ice table

C. The robot arm                

D. The Phoenix lander

4. Which of the following is WRONG according to the text?

A. A large patch of ice had uncovered.

B. The goal of sending Phoenix lander to Mars is to look for frozen water.

C. The goal of sending Phoenix Iander to Mars is to discover new resources.

D. It takes some time to deal with the large patch of ice for Phoenix lander.

5. What is the text mainly about?

A. New findings about space exploration.

B. NASA sent another spacecraft to Mars.

C. NASA Phoenix lander finds frozen water on Mars.

D. Washington University scientists study Mars.

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