问题 单项选择题

TT系统配电线路接地故障保护的动作特性应符合下面()公式的要求。式中RA--外露可导电部分的接地电阻和PE线电阻,Ω;Ia2--保证保护电器切断故障回路的动作电流,A;U0--相线对地标称电压,V。

A.RAIa2≤50V

B.RAIa2≤2U0

C.RAIa2≤U0

D.RAIa2≤100V

答案

参考答案:A

单项选择题
阅读理解

阅读理解

     When a storm is coming, most people leave the area as quickly as possible and head for safety…But

there are a few people who will get into their cars and go straight for the center of the storm.These

people are willing to risk being killed by floods or 100-Kilometer- an-hour winds for the excitement of

watching the storm close up.

     "Storm chasing (追逐)"is becoming an increasingly popular hobby,especially in the Midwest of the

United States, where there are frequent storms between March and July.A storm chaser begins the day

by checking the Internet for the latest weather reports, and then drives up to l,000 kilometers to where the

storm will be and waits for it to develop.

     Although anyone can do it,storm chasing is extremely dangerous.The power of a big storm  can throw

a cow into the air or destroy a whole house in seconds.Storm chasers are also often hurt in accidents

caused by driving in a heavy rain.If you are a beginner, it is much safer to join a group for

storm - chasing vacations during the storm season.

     Even then,storm chasing is not all adventure and excitement."Storm chasing is 95%driving,"says

Daniel Lynch,who spends most of his summer storm-chasing."Sometimes you

Can sit around for hours waiting for something to happen,and all you get is blue sky and a few light

showers."

     However,for storm chasers,it is all worth it. "When you get close to a storm,it is the most exciting

sight you will ever see in your life,"says Jasper Morley."Every storm is an example of the power of

nature.It is the greatest show on Earth."

1. For storm chasing,the first thing storm chasers do is to _____.

A. head straight for the center of the storm    

B. get into the car for safety

C. wait patiently for the storm to develop

D. collect information about a coming storm       

2. Beginners of storm chasing are advised _____.

A. not to drive in a heavy rain    

B. to do it in an organized way

C. not to get too close to a storm  

D. to spend more time on it in summer

3. By saying "it is all worth it" in the last paragraph,the author means that  _____.

A. storm chasing costs a lot of money

B. storm chasing is worth hours of waiting

C. efforts in storm chasing are well paid

D. a storm presents the greatest show on Earth

4. What can we learn from the text?

A. Sometimes storm chasers get nothing but disappointment.  

B. Many storm chasers get killed in the storms.

C. Storm chasing is becoming popular around the world.

D. Storm chasing is only fit for young people