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SteveA GFCI (Ground Fault Circuit Interruptor) or an RCD (Residual Current Device)
receptacle is designed to help prevent electrical shock to users of appliances plugged into them. The device constantly monitors the currents flowing in the hot and neutral conductors.
The currents flowing in the hot and neutral conductors should match almost exactly, to within a few milliamps. If the current flows do not match, the device disconnects itself within a few milliseconds, to stop the supply of electrical power to any receptacles it protects.
Because the device does not actually need to check for any ground current, it can be installed without a ground wire. (Unlike the earlier type of GFCI device, which only checked for the presence of some fault current flowing to ground and so needed to be connected to a properly installed ground wire.)
If there is no grounded conductor installed, a GFCI wired in this manner, and any receptacles protected by it, are each required to be clearly marked with a label saying:
"NO EQUIPMENT GROUND".
This method of changing two-wire ungrounded receptacles to three-wire grounding receptacles is allowed - even though they are not physically tied to the ground - only in a circuit which is protected by a correctly installed GFCI or RCD.
For more information about MCBs and related topics such as ELCBs, GFCIs and RCDs, see the answers to the Related Questions and the Related Link shown below this answer.
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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
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What is downstream in electrical?
The terminology down stream refers to an electrical circuit and where devices are connected into the circuit. In reference to the breaker that feeds the circuit everything is down stream from it. If a GFCI receptacle is added into the circuit as the first device, the GFCI can be wired to allow all of the regular receptacles to be protected down stream from that GFCI. Every device added to the circuit will be down stream from the one that is ahead of it. The only device that will not have something down stream from it will be the last device in the circuit.
Can you replace an ungrounded circuit breaker with a GFCI circuit breaker?
Yes but it's redundant and may cause unnecessary "tripping" of the circuit. The GFCI circuit breaker is intended to protect an entire receptacle circuit whereas a GFCI receptacle is designed to protect only that receptacle and any which are provided power from its load side. (downstream)
Will a ground fault interrupter work at any outlet with only 2 wires?
Yes. GFCI receptacles do not rely on a ground conductor to work. They sense any difference between current flowing in the hot wire and current returning in the neutral wire. Under normal circumstances, these two currents will be exactly the same. If there is a difference, then some of the current is flowing from the hot to somewhere else, possibly through a person to ground. This causes the GFCI to trip. The National Electric code even permits an old 2-wire receptacle with no ground wire to be replaced with a GFCI 3-prong receptacle. No ground wire is used, and the GFCI must be labeled "No Equipment Ground". See NEC Article 406.3(D)(3)(b) and (c). This is the only legal way to install a 3-prong receptacle in place of a 2-prong without running a new ground wire.
What a short hand way of drawing electrical circuits?
electrical symbol
What symbol is often used for electrical resistance?
I have never come across a symbol for an electrical resistance box. As the resistance box is a test instrument and it would not be drawn on any electrical schematic there may not be a symbol designated for it. The last letter of the Greek alphabet is (Ω omega) and it is used for the electrical symbol for resistance.
