警察必须先表明身份，如果一段时间没人回答，再破门而入。

The police have to knock and loudly announce that they are the police and they have a warrant. If after a reasonable period of time, which isn't much, the police may bust down the door in an attempt to apprehend the person and execute the warrant.

这件事警察没有对这个住址的warrent (by mistake) 就破门而入，屋主正当防卫没问题。