On Sunday 20 March 2005 11:10, Danny Sauer wrote:
On Sunday 20 March 2005 06:49 am, Kevanf1 wrote:
Here in the UK most places - I have them fitted at home - are now geared up with micro residual current circuit breakers too which trip out at the slightest hint of a short. It probably wouldn't save equipment but it would save your life from an electric shock.
Those are called GFCI (ground fault circuit interrupter) outlets in the USA, and are typically only installed near water (in the kitchen and bathroom), as they cost about $7 instead of $0.50 for regular outlets. Many (most?) new houses are built as cheaply as possible over here. :(
--Danny, who generally uses GFCI outlets when feasible
Note: A ground fault interupter is designed to sense a small current flow from the hot side (not neutral) to ground. There should never be a current flow through this path . . . unless, of course, you are standing barefoot on the wet bathroom tile floor and the "hair dryer" in you hand has an internal short to one of the screws that holds it together, which you are touching. In that case, the GFI circuit will shut off faster that the current can screw up the beating rythym of your heart. Pretty neat, huh? And all this for $7.50. Ain't technology neat? John Sowden American Sentry Systems. Inc. 1221 Andersen Drive San Rafael, CA 94901 U.L. Listed Central Station Alarm Service Serving the San Francisco Bay Area Since 1967 mail@americansentry.net http://www.americansentry.net
John Sowden wrote:
Note: A ground fault interupter is designed to sense a small current flow from the hot side (not neutral) to ground. There should never be a current flow through this path . . .
I thought they detected a current imbalance in the hot & neutral conductors. It is possible to have leakage to other than ground.
James, On Monday 21 March 2005 03:55, James Knott wrote:
John Sowden wrote:
Note: A ground fault interupter is designed to sense a small current flow from the hot side (not neutral) to ground. There should never be a current flow through this path . . .
I thought they detected a current imbalance in the hot & neutral conductors. It is possible to have leakage to other than ground.
Correct (well, half correct). They're often used when there is no ground reference lead in the building's wiring system. However, there is no other leakage path than to ground (however indirect or--no pun intended--circuitous that path is). GFCIs essentially _infer_ that current is traveling to ground because there's unequal current flow through the two power wires. (Kirchoff's laws to the rescue). The _assumption_ (a conservative one) is that the current is flowing through a human. Humans are notoriously ill-suited for use as electrical conductors. Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time average the current measurement over several cycles? Randall Schulz
The Monday 2005-03-21 at 06:56 -0800, Randall R Schulz wrote:
GFCIs essentially _infer_ that current is traveling to ground because there's unequal current flow through the two power wires. (Kirchoff's laws to the rescue). The _assumption_ (a conservative one) is that the current is flowing through a human. Humans are notoriously ill-suited for use as electrical conductors.
English is a nice language for some kind of humor :-p
Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time average the current measurement over several cycles?
The kind that are in use in Spain (they are mandatory here, as I mentioned) are simple electromagnetic devices. Both wires, live and neutral, are coiled with the same number of turns around the same nucleous of a relay - manual on, automatic off. If both currents are equal, the magnetic fields that both coils produce are equal, but oposite, with a combined nil efect. Of course, as soon as there is a current leak there is a current diference, and one of the fields is stronger than the other. As soon as the difference is enough (20mA for home use, perhaps 50 for industry, I'm unsure) it trips over. Thus, they are current sensitive devices. They don't sense voltage, only instantaneous current (really average), and thus know nothing about reactive load (ie, they can not measure the I·A phase difference), they are all the same to it. They would trigger. More or less :-) -- Cheers, Carlos Robinson
Hi, Carlos, On Monday 21 March 2005 12:40, Carlos E. R. wrote:
The Monday 2005-03-21 at 06:56 -0800, Randall R Schulz wrote:
GFCIs essentially _infer_ that current is traveling to ground because there's unequal current flow through the two power wires. (Kirchoff's laws to the rescue). The _assumption_ (a conservative one) is that the current is flowing through a human. Humans are notoriously ill-suited for use as electrical conductors.
English is a nice language for some kind of humor :-p
Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time average the current measurement over several cycles?
...
Thus, they are current sensitive devices. They don't sense voltage, only instantaneous current (really average), and thus know nothing about reactive load (ie, they can not measure the I·A phase difference), they are all the same to it. They would trigger.
Right, right. That's why I switched from EE to CS. Del dot B = 0 my ass. Electrostatics was tolerable, but electrodynamics was just not worth it. And what's the deal with exponentials in the current / voltage relationship in semiconductor junctions? That's nuts!
Carlos Robinson
Randall Schulz
The Monday 2005-03-21 at 17:10 -0800, Randall R Schulz wrote:
Thus, they are current sensitive devices. They don't sense voltage, only instantaneous current (really average), and thus know nothing about reactive load (ie, they can not measure the I·A phase difference), they are all the same to it. They would trigger.
Right, right. That's why I switched from EE to CS.
Del dot B = 0 my ass.
I got lost there O:-)
Electrostatics was tolerable, but electrodynamics was just not worth it. And what's the deal with exponentials in the current / voltage relationship in semiconductor junctions? That's nuts!
Ough. I have forgotten those, never used them. Come to think of it, I don't think I have worked much with electronics. -- Cheers, Carlos Robinson
On Monday 21 March 2005 07:10 pm, Randall R Schulz wrote:
Right, right. That's why I switched from EE to CS.
Heh. I made that switch after completing "analog signal processing". :) --Danny, who wouldn't go back and change a thing, though
The Thursday 2005-03-24 at 09:09 -0600, Danny Sauer wrote:
On Monday 21 March 2005 07:10 pm, Randall R Schulz wrote:
Right, right. That's why I switched from EE to CS.
Heh. I made that switch after completing "analog signal processing". :)
Ouch, don't remind me of Mr Fourier... -- Cheers, Carlos Robinson
Carlos E. R. wrote:
The Thursday 2005-03-24 at 09:09 -0600, Danny Sauer wrote:
On Monday 21 March 2005 07:10 pm, Randall R Schulz wrote:
Right, right. That's why I switched from EE to CS.
Heh. I made that switch after completing "analog signal processing". :)
Ouch, don't remind me of Mr Fourier...
Or LaPlace transforms, in servo systems! ;-)
On 24/03/05 03:09 PM, Danny Sauer
On Monday 21 March 2005 07:10 pm, Randall R Schulz wrote:
Right, right. That's why I switched from EE to CS.
Heh. I made that switch after completing "analog signal processing". :)
Wimp, I stuck it out and got my BEng. . . . It was only after I realised the mistake I had made :-) Best, Ben
Randall R Schulz wrote:
James,
On Monday 21 March 2005 03:55, James Knott wrote:
John Sowden wrote:
Note: A ground fault interupter is designed to sense a small current flow from the hot side (not neutral) to ground. There should never be a current flow through this path . . .
I thought they detected a current imbalance in the hot & neutral conductors. It is possible to have leakage to other than ground.
Correct (well, half correct). They're often used when there is no ground reference lead in the building's wiring system. However, there is no other leakage path than to ground (however indirect or--no pun intended--circuitous that path is).
I don't know about where you are, but in North America, a split phase 240/120V system is used, where the center tap of the line transformer is grounded and becomes the neutral. The two "hot" lines are 180 degrees apart, and have 240V between them. It is possible to have leakage to the opposite phase.
GFCIs essentially _infer_ that current is traveling to ground because there's unequal current flow through the two power wires. (Kirchoff's laws to the rescue). The _assumption_ (a conservative one) is that the current is flowing through a human. Humans are notoriously ill-suited for use as electrical conductors.
Quite familiar with Kirchoff. I aced an electrical engineering exam question once, when I had the sense to apply Kirchoff to the problem. I was apparently the only one in the class to do so.
Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time average the current measurement over several cycles?
The currents should always be exactly opposite, no matter what the reactance is. Remember, you're comparing current, not phase relationship. Also, current in a series circuit with have the same phase relationship at any point.
James, On Monday 21 March 2005 16:24, James Knott wrote:
Randall R Schulz wrote: ...
I don't know about where you are, but in North America, a split phase 240/120V system is used, where the center tap of the line transformer is grounded and becomes the neutral. The two "hot" lines are 180 degrees apart, and have 240V between them. It is possible to have leakage to the opposite phase.
Any single branch circuit in a residential environment (except for those used for things like electric stoves) is single phase.
...
Hmmm... I wonder, how do GFCIs deal with reactive loads? Do they time average the current measurement over several cycles?
The currents should always be exactly opposite, no matter what the reactance is. Remember, you're comparing current, not phase relationship. Also, current in a series circuit with have the same phase relationship at any point.
Yeah. Carlos already reminded me of that. Randall Schulz
Randall R Schulz wrote:
James,
On Monday 21 March 2005 16:24, James Knott wrote:
Randall R Schulz wrote: ...
I don't know about where you are, but in North America, a split phase 240/120V system is used, where the center tap of the line transformer is grounded and becomes the neutral. The two "hot" lines are 180 degrees apart, and have 240V between them. It is possible to have leakage to the opposite phase.
Any single branch circuit in a residential environment (except for those used for things like electric stoves) is single phase.
Split receptacles are common, where each half of a duplex outlet, is connected to one of the sides of the 240V feed. The other half is connected to the other side, resulting in a difference of 240V within the box. These outlets are common in kitchens and workshops.
James, On Monday 21 March 2005 17:27, James Knott wrote:
Randall R Schulz wrote:
James,
On Monday 21 March 2005 16:24, James Knott wrote:
Randall R Schulz wrote: ...
I don't know about where you are, but in North America, a split phase 240/120V system is used, where the center tap of the line transformer is grounded and becomes the neutral. The two "hot" lines are 180 degrees apart, and have 240V between them. It is possible to have leakage to the opposite phase.
Any single branch circuit in a residential environment (except for those used for things like electric stoves) is single phase.
Split receptacles are common, where each half of a duplex outlet, is connected to one of the sides of the 240V feed. The other half is connected to the other side, resulting in a difference of 240V within the box. These outlets are common in kitchens and workshops.
I wasn't aware of that, but the GFCI concept is still valid, since no one uses these kind of outlets to operate 240V appliances. Any given device is still operating off of a single phase. I suppose a person could hold two appliances plugged into each outlet of such a split-phase duplex outlet box and both of those appliances could have faulty insulation and there could thus come to exist a current path from one phase to the other. How likely is that? Not very, but this _is_ the sort of situation to which Murphy's law applies (the failure mode was not excluded by the essential nature of the device or design, thus it must be expected to happen, eventually). Nonetheless, a GFCI in either or both of the outlets in the duplex pair would trip when that happened, right? The only thing that would persistently and unconditionally (and inappropriately) trip it was if you ran a single 240V appliance off the the pair of split-phase outlets. That would require jury-rigged wiring. In that case, the creative soul who tried to exploit the split-phase duplex outlet would deserve not to be able to turn his appliance on. RRS
The Monday 2005-03-21 at 17:49 -0800, Randall R Schulz wrote:
I suppose a person could hold two appliances plugged into each outlet of such a split-phase duplex outlet box and both of those appliances could have faulty insulation and there could thus come to exist a current path from one phase to the other. How likely is that? Not very, but this _is_ the sort of situation to which Murphy's law applies (the failure mode was not excluded by the essential nature of the device or design, thus it must be expected to happen, eventually).
Nonetheless, a GFCI in either or both of the outlets in the duplex pair would trip when that happened, right? The only thing that would persistently and unconditionally (and inappropriately) trip it was if you ran a single 240V appliance off the the pair of split-phase outlets. That would require jury-rigged wiring. In that case, the creative soul who tried to exploit the split-phase duplex outlet would deserve not to be able to turn his appliance on.
However, if you use a single GFCI, or differential switch, designed for three wires (two live, one neutral); ie built with the three wires coiled on the same iron nucleus, it will trigger in any combination of ground fault you can imagine. It comes from kirchoff law: the sum of all currents entering the house must be zero. If it isn't, it is because some current is going some other way. Or body. I was not aware of how your residential system was designed, with two voltages, 180 degrees. Just for the record, I'll mention that here we have, on homes, a single phase system, 220 or 240 depending on country. It derives in fact from one of the 3 phases of a tri-phase transformers (120 degrees, 380 Volts), which can be used for residential elevators and relatively small industries. This is of course, off topic. Lets imagine that we travel and need to know how to connect our Linux computers on each country O;-) -- Cheers, Carlos Robinson
participants (6)
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Ben Higginbottom -
Carlos E. R. -
Danny Sauer -
James Knott -
John Sowden -
Randall R Schulz