為維護用電安全，一些工作場所的用電設備之金屬外殼必須進行接地，即稱設備接地。其 目的為當該設備因絕緣破壞或其他因素使金屬外殼帶電時，將該外殼電壓降至安全電壓之下， 保護人員，避免感電。此外如該用電迴路有安裝漏電斷路器時，因設備之金屬外殼有接地時， 漏電電流較大，能使漏電斷路器較易動作，將電源切離，達到保護人員安全的目的。但是如果 設備接地設計不良，或是考量接地線脫落或接線不良，接地線接地電阻值經長時間後變大等， 以及可能的誤接線方式，都可能會造成接地線出現危害電壓。本文將以電路工作條件分析的方 式探討單相三線式系統(單相二線式系統亦可使用相同之方法），找出接地線出現危害電壓之 工作條件，並與安全且正確接線的工作條件比較電壓值之差異。藉此找出可以判斷接地線出現危害電壓之電壓值，再利用此電壓值來設計出偵測模組，希望藉由此偵測模組降低感電事故的發生。

To ensure the electricity safety, the metal casings of some electrical devices on working place should be grounded. It is called as device grounding. When the insulation layer is not as good as desired, a certain voltage would appear in the metal casing. The device grounding approach could reduce the casing voltage to safe voltage as possible. It can protect people from electricity hazard.
If a leakage current circuit breaker has been installed in the circuit branch, the conditions of device grounding can increase the leakage current value to trip the circuit breaker easily to protect people. However, if the design of the device grounding is not suitable, disconnection appears in the grounding wire, or the grounding resistance becomes higher after a long time, the hazard voltage would appear in the grounded wire. This paper will discuss the possible working conditions of single-phase three-wire systems (single-phase two-wire systems can also use the same way). In addition, we have found some working conditions where the hazard voltage will appear in the grounded lines. It is also to investigate the difference of voltages in the grounded lines among correct working conditions and error working conditions. In order to reduce the accident of electric shock, we design the detecting module based on the results from analysis of these working conditions to determine which voltages will be possibly dangerous.