Damage cause of s7300 analog input module

S7300 analog input module (331-7KB02-0AB0 6ES7) 3 years of bad 2, the input signal 4-20mA, consult the module is easy to damage the reason?

Best answer
Reference: analog quantity module damage
http://www.ad.siemens.com.cn/service/answer/solution.aspx? Q_ID=63278&cid=1029?