This paper describes the mechanism of discharge development in non-uniform field gap in SF 6 under positive impulses with different rising steepness. Based on theoretical consideration, an expression of the transition delay time from streamer to leader is presented, which can help to better understand the process of streamer to leader transition under impulses with different rising steepness in SF 6 . Also, the calculated results of the proposed expression for the transition delay time are quite in agreement with the experimental ones.
clinical diagnosis (degree I-III). 141 healthy subjects from Xi'an city were chosen and matched by age and sex. The study was performed in accordance with the Declaration of Helsinki and approved by the Human Ethics Committee of Xi'an Jiaotong University. Informed written consent was obtained from each subject. The staple food ( flour, rice and corn hazel) and hair from children and healthy subjects were collected. The selenium level in hair and staple food was tested with Atomic Fluorescence Spectrophotometer. Results: About 65 children which the total number of children was 1743 were KBD X-ray positive. The selenium level in hair of children from xunyi county was significantly lower than healthy subjects from Xi'an city (0.24 and 0.62 μ g/g respectively, P< 0.05). No difference was found in selenium level of staple food between children from xunyi county and the control group (P> 0.05). ConClusions:The abnormal ration of X-ray in children from xunyi county was lower than 5%. Although prevention and control measures have been implemented, the selenium level in hair of children from xunyi county was still lower. Children lived in xunyi county should intake more Se to prevent KBD.
clinical diagnosis (degree I-III). 141 healthy subjects from Xi'an city were chosen and matched by age and sex. The study was performed in accordance with the Declaration of Helsinki and approved by the Human Ethics Committee of Xi'an Jiaotong University. Informed written consent was obtained from each subject. The staple food (flour, rice and corn hazel) and hair from children and healthy subjects were collected. The selenium level in hair and staple food was tested with Atomic Fluorescence Spectrophotometer. Results: About 65 children which the total number of children was 1743 were KBD X-ray positive. The selenium level in hair of children from xunyi county was significantly lower than healthy subjects from Xi'an city (0.24 and 0.62 μ g/g respectively, P< 0.05). No difference was found in selenium level of staple food between children from xunyi county and the control group (P> 0.05). ConClusions: The abnormal ration of X-ray in children from xunyi county was lower than 5%. Although prevention and control measures have been implemented, the selenium level in hair of children from xunyi county was still lower. Children lived in xunyi county should intake more Se to prevent KBD.
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