A ground-fault occurs when there is a break in the low-resistance grounding path from a tool or electrical system. The electrical current may then take an alternative path to the ground through the user, resulting in serious injuries or death. The ground-fault circuit interrupter, or GFCI, is a fast-acting circuit breaker designed to shut off electric power in the event of a ground-fault within as little as 1/40 of a second. It works by comparing the amount of current going to and returning from equipment along the circuit conductors. When the amount going differs from the amount returning by approximately 5 milliamperes, the GFCI interrupts the current.
The GFCI is rated to trip quickly enough to prevent an electrical incident. If it is properly installed and maintained, this will happen as soon as the faulty tool is plugged in. If the grounding conductor is not intact or of low-impedance, the GFCI may not trip until a person provides a path. In this case, the person will receive a shock, but the GFCI should trip so quickly that the shock will not be harmful.
The GFCI will not protect you from line contact hazards (i.e. a person holding two "hot" wires, a hot and a neutral wire in each hand, or contacting an overhead power line). However, it protects against the most common form of electrical shock hazard, the ground-fault. It also protects against fires, overheating, and destruction of wire insulation.
For construction applications, there are several types of GFCIs available, with some variations:
The Receptacle Type incorporates a GFCI device within one or more receptacle outlets. Such devices are becoming popular because of their low cost.
Portable Type GFCIs come in several styles, all designed for easy transport. Some are designed to plug into existing non-GFCI outlets, or connect with a cord and plug arrangement. The portable type also incorporates a no-voltage release device that will disconnect power to the outlets if any supply conductor is open. Units approved for outdoor use will be in enclosures suitable for the environment. If exposed to rain, they must be listed as waterproof.
The Cord-Connected Type of GFCI is an attachment plug incorporating the GFCI module. It protects the cord and any equipment attached to the cord. The attachment plug has a non-standard appearance with test and reset buttons. Like the portable type, it incorporates a no-voltage release device that will disconnect power to the load if any supply conductor is open.
Because GFCIs are so complex, they require testing on a regular basis. Test permanently wired devices monthly, and portable-type GFCIs before each use. All GFCIs have a built-in test circuit, with test and reset buttons, that triggers an artificial ground-fault to verify protection. Ground-fault protection, such as GFCIs provide, is required by OSHA in addition to (not as a substitute for) general grounding requirements.
Source: U.S. Dept. of Labor Office of Safety & Health Administration
Ground-kosa hutokea wakati kuna kuvunjika katika chini ya upinzani grounding njia kutoka chombo au mfumo wa umeme. Kisha umeme huo waweza kwenda kwenye ardhi kupitia mtumiaji, na kusababisha majeraha makubwa au kifo. GFCI (Ground Fault Circuit Interruptor) ni kifaa cha kuzima umeme kwa muda mfupi kwa sekunde moja. Inafanya kazi kwa kulinganisha kiasi cha mkondo kwenda na kurudi kutoka kwa vifaa kando ya conductors mzunguko. Wakati kiasi cha kwenda inatofautiana na kiasi cha kurudi kwa takriban 5 milliamperes, GFCI kuvunja sasa. GFCI ni rated kwa safari haraka kutosha kuzuia tukio umeme. Ikiwa imewekwa na kudumishwa vizuri, hii itatokea mara tu chombo kibaya kimeunganishwa. Kama grounding conductor si intact au ya chini-impedance, GFCI inaweza si safari mpaka mtu hutoa njia. Katika hali hii, mtu atapokea mshtuko, lakini GFCI inapaswa kusafiri haraka sana hivi kwamba mshtuko hautakuwa na madhara. GFCI haikukuzuia wewe kutoka kwa hatari za mawasiliano ya mstari (yaani. Mtu anayeshikilia waya mbili "moto", waya moto na moja ya moja kwa moja katika kila mkono, au kuwasiliana na mstari wa umeme wa juu. Hata hivyo, inalinda dhidi ya aina ya kawaida ya hatari ya mshtuko wa umeme, kosa la ardhi. Pia hulinda dhidi ya moto, kupasha joto kupita kiasi, na uharibifu wa insulation ya waya. Kwa matumizi ya ujenzi, kuna aina kadhaa za GFCI zinazopatikana, na tofauti zingine: Aina ya Receptacle inajumuisha kifaa cha GFCI ndani ya moja au zaidi ya viingilio vya receptacle. Vifaa hivyo vinapendwa sana kwa sababu ya bei yake ya chini. GFCI aina portable kuja katika mitindo kadhaa, wote iliyoundwa kwa ajili ya usafiri rahisi. Baadhi ya vifaa vya umeme ni kubuniwa kwa ajili ya kuunganisha katika maduka ya sasa yasiyo ya GFCI, au kuunganisha na kamba na mpangilio wa kuziba. Aina ya kubebeka pia inajumuisha kifaa cha kutolewa kwa voltage ambayo itakatisha nguvu kwa mashirika ikiwa miongozo yoyote ya usambazaji iko wazi. Vitengo vilivyokubaliwa kwa ajili ya matumizi ya nje vitakuwa katika vyumba vinavyofaa kwa mazingira. Ikiwa zitatolewa kwa mvua, lazima ziandikwe kuwa zisizoweza kuharibiwa na maji. Cord-Connected Aina ya GFCI ni plagi ya kiunganishi inayojumuisha moduli ya GFCI. Inalinda kamba na vifaa vyovyote vilivyounganishwa na kamba. Plug kiambatisho ina kuonekana si ya kawaida na mtihani na reset vifungo. Kama aina ya kubebeka, inajumuisha kifaa cha kutolewa kwa voltage ambayo itakatisha nguvu kwa mzigo ikiwa kiyoyozi chochote cha usambazaji kiko wazi. Kwa sababu GFCI ni ngumu sana, zinahitaji kupimwa kwa ukawaida. Jaribu vifaa vya kudumu wired kila mwezi, na aina portable GFCIs kabla ya kila matumizi. GFCI zote zina mzunguko wa majaribio uliojengwa, na vifungo vya majaribio na reset, ambayo huchochea kosa la ardhi ya bandia ili kuthibitisha ulinzi. Ulinzi wa ardhi, kama vile GFCI hutoa, inahitajika na OSHA pamoja na (si kama mbadala wa) mahitaji ya jumla ya ardhi. Chanzo: Idara ya Marekani. Ofisi ya Kazi ya Usalama na Afya ya Utawala