在中性点直接接地系统中，发生接地短路时零序电流的分布有何特点？
参考答案：
主要特点如下：当系统任一点接地短路时网络中任何处的三倍零序电流（或电压）都等于该处三相电流（或电压）的相量和，即：系统出现非全相运行或断路器三相触头不同合闸时，也会出现零序分量。而在系统正常运行、过负荷振荡和相间短路时，则不会出现序分量。系统零序电流分布与中性点接地的多少及位置有关。断路器合闸线圈的端电压，为什么不能低于额定值的（80～85）%？使用电磁操机构的断路器，其分合速度与操作电源关系很大，合闸线圈的端电压愈低，电磁吸力越小，开关合闸速度也就越慢，此时当开关合闸于故障电流时，故障电流的电动力将严重阻止开关触头闭合，为了确保断路器能顺利合闸，所以要求合闸线圈的端电压不低于额定值的（80～85）%。
People who are outdoors in cold weather should avoid actions like suddenly lifting a heavy basket full of snow. Even walking through heavy, wet snow can strain a person’s heart. Many people aren’t conditioned to the physical stress of outdoor activities and don’t know the dangers of being outdoors in cold weather. Those who like winter sports can suffer accidental hypothermia if they don’t make certain preparations. Hypothermia means the body temperature has fallen below normal. It occurs when your body can’t produce enough energy to keep the internal (内部的) body temperature warm enough. It can kill you. Heart failure causes most deaths in hypothermia. Children, the elderly and those with heart disease are at special risk. As people age, their ability to keep a normal internal body temperature often decreases. Elderly people can suffer hypothermia without knowing they’re in danger because they can’t notice the cold conditions as quickly as the young. Besides cold temperatures, high winds, snow and rain can also steal body heat. Wind is especially dangerous because it removes the layer of heated air from around your body. At 30 degrees Fahrenheit in a 20-mile-per-hour wind, the cooling effect is equal to calm air at four degrees. Similarly dampness (湿气) causes the body to lose heat faster than it would at the same temperature in drier conditions. To keep warm, wear more clothes. This traps air between layers. Also, wear a hat or head scarf. Much of your body’s heat can be lost through your head. Keep your hands and feet warm, too, as they tend to lose heat rapidly. Don’t drink alcohol before going outdoors or when outside. Alcohol makes you feel warm at first because blood vessels in the skin expand. But heat is then drawn away from the body’s important organs. 1. The underlined word “strain” in Paragraph 1 means ________. A. injure B. warm C. control D. burn 2. What is implied in the passage about hypothermia?
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