问题 问答题 论述题

试述煤矿井下杂散电流的危害?

答案

参考答案:

架线始电机车的牵引电网,是主要的杂散电流源。牵引电网中轨道是其回电体即电机车电流需沿轨道回到牵引变流所电源负端母线。由于轨道与大地不绝缘,在轨道接缝接触不良时,可引起部分电流通过大地(包括敷设金属管道和电缆金属外皮等)等。由于产生电化学效应,金属管道。电缆金属外皮、轨道作为阳极端要被腐蚀。据统计,若常年通过1A杂散电流,在一年中就能腐蚀9kg的钢材。对金属管道和电缆金属外皮来说,腐蚀点集中,情况更为严重。

在矿井杂散电流影响下,轨道与大地间有电位差,大地不同地点有电位差。对于采用电雷管爆破的场合,若两根放炮线两点异地接触,两点间有1~1.5V电位差就会形成电流通路,造成电雷管先期引爆,严重威胁生产和工作人员的安全。

杂散电流还会产生电火花,且有引起瓦斯煤尘爆炸的危险。

阅读理解

 When he thought of the past, my grandfather would sometimes show us photographs of himself at school. They were brown and faded, and it was hard to believe that the blurred(模糊) figure of the little boy in the short trousers and socks could ever have been Grandfather. Besides, he wore a cap --- all the boys in the photographs wore caps pulled so far forward that half of their faces were obscured. When Grandfather asked us to pick him out from the group, we would surely point to the wrong boy.

On one such occasion my younger sister, aged six, burst into tears when Grandfather proudly guided her finger to the right boy. "How could that boy be you?" she cried. "He should have a beard." We were, of course, all convinced that grandfathers should have beards, preferably white and bushy, like our own grandfather's.

"I was a good scholar," Grandfather would say, wagging his beard over the photographs. "I should have been top of the class if I hadn't had to get up at six every morning to milk the cows and chop the wood, and again when I came home from school."

"But Saturdays? What did you do on Saturdays?"

"Saturdays, if it was fine, I'd be out all day in the fields with the men," replied Grandfather. "And if it was wet, I'd be helping my mother with odd jobs round the house. There wasn't much time for studying."

We all tried hard to imagine what it would have been like to have seen Grandfather getting up at crack of dawn and never, obviously, having a moment for himself. It seemed we had learnt something from what Grandfather had said about his childhood.

41. In the first paragraph of this passage, what the author really tells us is that ________ .

A. his grandfather used to wear short trousers, socks and a cap as well

B. it was difficult to tell which of the boys in the photographs was Grandfather

C. he didn't believe Grandfather wore a cap pulled forward when he was at school

D. it was fun to watch boys in the photographs wearing caps pulled forward

42. The author's sister burst into tears because________.

A. she did not get a chance to pick out Grandfather in the photographs

B. she was told which was the right boy before she herself could pick him out

C. other children did not agree with her that Grandfather should have had a beard

D. she found Grandfather in the photographs did not have a beard

43. When Grandfather said, "I should have been top of the class...", he meant ________ .

A. if he had had more time for studying, he would have been the best in his class

B. he should have spent more time studying rather than playing ball games

C. his school days should not have been so hard and miserable

D. he could have never been the best student even if he had studied still harder

44. In the last paragraph the author said, "We all tried hard to imagine..." because ________ .

A. the figures of the boys in the photographs were small and blurred

B. the children had never experienced life like that of Grandfather

C. the photographs Grandfather showed them were brown and faded

D. Grandfather failed to tell them about his childhood in detail

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