Earthing

需考慮設計問題

如果電器用具有用插囌, 插囌電路裏有一個漏電斷路制, 敏感度為30mA, 當火線鬆脫踫到電器用具殼, 敏感度30mA既漏電斷路制, 已經跳制, 如果中線鬆脫踫到電器用具殼, 敏感度30mA既漏電斷路制, 都好可能跳制, 因爲電源發出部分已經將中線接地, 所以唔會產生強烈電震(觸電)electric shock。

除非你自行違章駁接電流冇裝漏電斷路制又冇裝水線接地則另當別論



圖片參考：http://hk.knowledge.yahoo.com/Figures/5.3b.gif




圖片參考：http://imgcld.yimg.com/8/n/HA00639401/o/700912250066513873386960.jpg


a) 電源發出部分中線冇接地
b) 電源發出部分中線有接地


2009-12-26 07:34:20 補充：
http://www.tlc-direct.co.uk/Book/5.1.3.htm

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Will the same thing happen if the neutral wire becomes loosen?

yep,as Mr.Ho speech,the neutral wire is earthed at the transformer,so if the neutral wire lossen,its as the earth wire,live connect to neutral(earth wire) will action the circuit breaker simultaneously(0.4s)

Even our body has a larger resistance, part of the electric current will still pass through our body, won't it?

if the metal case is earthed,the leakage current will be earthed and the circuit breaker will cut off the power in few time,so when we touch the metal case,we will not electrocuted.
BUT if the metal case is not earthed,when we touch the metal case,the current will pass though or body(although the resistance of body is high),the current is AC0.001A,we will be electrocured.

2009-12-27 21:18:17 補充：
to tonyfan88 :

Situation 1 : no earth
you can light up the bulb without earth wire,because many bulb can work without earth and don't have damage.

Situation 2 : no neutral

the resistance of live to earth is low,it may cause earth fault or earth short circuit,DON'T DO THIS!!

2009-12-27 21:18:24 補充：
to tonyfan88 :

Situation 1 : no earth
you can light up the bulb without earth wire,because many bulb can work without earth and don't have damage.

Situation 2 : no neutral

the resistance of live to earth is low,it may cause earth fault or earth short circuit,DON'T DO THIS!!

2009-12-27 21:20:57 補充：
to hong_kong_money:

two plug socket have the earth wire,but not at the user home,is in the generator and transformer.

Dear Physics 8801,

I have a follow-up question.

Situation 1
If I have a bulb and I just use live and neutral to connect to the bulb, the bulb will light up. In this case I don't use earth at all.

Situation 2
If I use earth and live to connect to the bulb, will the bulb light up too? In this case, I don't use neutral.

Will the electric bills be the same in these two situation?

2009-12-27 13:27:08 補充：
Situation 1
If I have a bulb and I just use live and neutral to connect to the bulb, the bulb will light up. In this case I do not use earth at all.

Situation 2
If I use earth and live to connect to the bulb, will the bulb light up too? In this case, I do not use neutral.

Question 1: Will the same thing happen if the neutral wire becomes loosen?

No. The neutral wire is at earth potential (i.e. zero volt with respect to earth). There would not be any potential difference between the neurtal wire and the person who happens to touch the loosen neutral wire. The person will not get an electric shock.

Queston 2: Even our body has a larger resistance, part of the electric current will still pass through our body, won't it?
No. The earth wire is always kept at earth potential (i.e. zero volt). It is connected to the metal casting of the applicance well before the live wire touches the metal casting. The metal casting is thus at the same potential as earth. When the loosen live wire touches the metal casting, it (the live wire) could not raise the potential of the metal casting because it is connected to earth (as the erath is a very large body at constant potential) .
As a reuslt, there is no potential difference between the person who happens to touch the metal casting. He will not get a shock.