Newsgroups: alt.engineering.electrical,sci.engr.electrical,sci.engr.electrical.sys-protection
Subject: Re: LV Earth Fault
References: <[email protected]>
"Samuel Chan"  writes:

> Anyone know if the earth terminal is talking about 10 Ohm impedance, TN-S earrthing. How can we perform earth fault protection in Main switchboard?


If the earth impedance is high then earth fault protection 
can be made using residual current device
http://en.wikipedia.org/wiki/Residual-current_device

"A residual current device (RCD), or residual current circuit breaker
(RCCB), is an electrical wiring device that disconnects a circuit
whenever it detects that the flow of current is not balanced between
the phase ("hot") conductor and the neutral conductor. Such an
imbalance is sometimes caused by current leakage through the body of a
person who is grounded and accidentally touching the energized part of
the circuit"

"In the United States, the National Electrical Code, requires GFCI
devices intended to protect people to interrupt the circuit if the
leakage current exceeds a range of 4.6 mA of current (the exact trip
setting can be chosen by the manufacturer of the device and is
typically 5 mA) within 25 milliseconds."

"GFCI devices which protect equipment (not people) are allowed to trip
as high as 30 mA of current. In Europe, the commonly used RCDs have
trip currents of 10-300 mA."


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/