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madman
12-16-2008, 09:27 AM
I had a GFI home outside outlet fry when i hooked up a motorhome (bigfoot camper that had a gfi inside it) NOW I was told by a electrician you CANNOT hook up a line with Two GFI in line . It will cause a electrical problem resulting in one frying so to speak. ?? Any info guys? Is this true? Thanx Mike

Lew Hartswick
12-16-2008, 09:36 AM
Without drawing a schematic and some thought I wouldn't risk a
"real" answer, BUT I can sort of imagine there could be a problem.
...lew...

Evan
12-16-2008, 09:37 AM
It won't fry anything Mike. The worst that may happen is false trips. Try it and see if it works.

All the GFI is doing is sensing a current running in the ground circuit compared to neutral. If it is then it opens contacts. You might fry a GFI if the neutral and ground on a circuit are reversed.

kendall
12-16-2008, 12:25 PM
Most often if problems show up it's just false trips.

When I work in the winter (do outdoor work mostly) I often run a cord out to my truck,and hook up a small hair dryer and the battery chargers for my cordless tools. Keeps the truck warm and means I don't have to keep an eye on my batteries, or hunt them down.
I use a three way adapters that have GFI on it, and the hair dryer has integrated GFI, only issue is that sometimes one or the other GFI trips, changing out the three way (have several) often eliminates the problem.
Since it's not always the same GFI that has problems, I think it's more related to the electrical service itself rather than the GFIs.

Ken.

rdfeil
12-16-2008, 11:57 PM
Madman,

Just like Evan said, no problem. There will not even be false trips if the system is wired corectly. All the GFI does is look for current in the ground circuit, if there is it trips. A GFI being fed from another GFI will not have ground current unless there is a problem and then you WANT it to trip. Let'er rip all is well.

Robin

winchman
12-17-2008, 01:45 AM
"All the GFI is doing is sensing a current running in the ground circuit compared to neutral."

"All the GFI does is look for current in the ground circuit, if there is it trips."

Actually, the GFI compares the current in the hot wire to the current in the neutral wire. It trips if it's not the same. Any imbalance indicates current is going to ground by some unintended path = ground fault.

There shouldn't be any current in the ground wire, unless something is amiss.

Roger

gearedloco
12-17-2008, 01:51 AM
Madman,

Just like Evan said, no problem. There will not even be false trips if the system is wired corectly. All the GFI does is look for current in the ground circuit, if there is it trips. A GFI being fed from another GFI will not have ground current unless there is a problem and then you WANT it to trip. Let'er rip all is well.

Robin

Actually, what they sense is a difference in current between the black ("hot") and white ("grounded" A.K.A." "neutral") wire. There is no requirement that the fault current returns through the G.F.I. device. A few moments though should make the reason for this clear, and is left to the reader as an exercise. Hint - If you're standing in a puddle of water and you contact the hot wire, depending on the fault current returning through the devices safety ground is not a great bet.

When I first started as an electronics tech at LRL, now "Berkeley Lab," I went to a safety meeting where the speaker was a Prof. from UCB. He was one of, if not THE, early developer(s) of the GFI device. Interesting talk, lots of graphs, charts, schematics, and photos of students getting zapped with electricity. Very entertaining - don't think anyone could get away with that sort of thing these days, even in the name of "science." This was a number of years before the units started showing up in the local hardware stores.

winchman
12-17-2008, 04:24 AM
Here's a diagram:
http://www.rhtubs.com/GFCI/gfci_circuit.gif

Roger

JoeFin
12-17-2008, 06:27 AM
I've heard the same old wive's tale of 1 GFI pluged into another GFI will cancle the 2 out. Never did hear the actual reason.

As the diagram illustrates the nuetral and the hot current flowing through the coil will cancle each other's EMF - Magnetic Flux out. The coil will not realise a voltage potential.

HSS
12-17-2008, 06:47 AM
Could be that the reason some say not to hook 2 gfci in series is that they have changed gfci's unnecessarily cause they couldn't get the one they were working on to reset? The imbalance would trip both gfci's and they might have thought only one was in the circuit. Of course, if they took the cover off of the recepticle and used their voltmeter, and not just pressed the reset button they would have investigated further back and found their voltage loss. Just a thought......Maybe, maybe not. :rolleyes:

Evan
12-17-2008, 06:50 AM
I've heard the same old wive's tale of 1 GFI pluged into another GFI will cancle the 2 out. Never did hear the actual reason.


In a perfect world there would be no interaction between two units on the same circuit. Naturally, nothing is perfect. There is always some ground current flowing because of the resistance in the neutral and induction in the safety ground. There is very often a leakage path, sometimes intentional in the form of a resistor to safety ground in connected equipment through which ground current flows.

The GFCI is watching for a sudden change in the current balance. The sensor circuit (from memory) is a chopper amp that pulses the coil to induce a small signal that is then modulated by changes in the current balance. Because there is always some imbalance that signal is also transmitted for a short distance over the power conductors. Depending on the precise design, sensitivity and chopper frequency of another CGFI on the same circuit the signals may interact and result in trip out of either or both of the units.

winchman
12-17-2008, 07:15 AM
Does a GFCI need power to reset? If two were in series and both tripped, you wouldn't be able to reset the second one if it needs power to reset. That might make someone think it was "fried".

Roger

Evan
12-17-2008, 09:10 AM
They can be reset with power off. It's a strictly mechanical mechanism.

madman
12-17-2008, 10:56 AM
I did have to Replace the one GFI on my Outside House Wall. NOW initially it worked for a while hooked to then Camper NO PROBLEM xcept occasionally I just had to reset the breaker on thr GFI Itself, then all would be good for a Few Hours of Beer Drinking. I would sit in my New Camper (circa 1984) and drink Beer Listen to AC DC and well Drink Beer with a Friend. We would talk about no women can bother us here cause they just wouldnt want to sit in a Camper Period for Hours on end Drinking Beer Like Us. The simple Life indeed. Later on (days Later ) I noticed when inside the Camper NO Power i go outside hit the reset on the GFI and it just is fried. Checked every outlet inside my camper with one of those Plug In Polarity Proper Ground Testers I had layin around. Everything was OK (i had my friend a electrician there for this) I thought there was something amiss in my campers wiring but all ok. NOW when he (THE Electrician) opened the Cabinet and found a GFI inside the camper,he says to meYOU CANT HAVE 2 GFI IN THE SAME CIRCUIT PERIOD???? Well now after the NEW GFI is installed on the House Outside I cannot hook up the Camper to the House at all? Im confused as usual but?????

kendall
12-17-2008, 12:19 PM
some units do need power to reset, the majority don't though.

every 3 socket splitter I've had needed power to reset, and two of the gfis I installed in my garage needed power to reset, the rest didn't.

Ken.

Dawai
12-17-2008, 01:03 PM
Yas..

In construction we plug a gfci into a gfci all the time..
Most companies require a pigtail ground fault in the drop cord.. regardless if it is plugged into a circuit.

Not wiring the building how can you be sure there is a ground fault in the lead in of the receptacle circuit? (*wire in front, feed other receptacles off it)

Most house trailers, they have a ground fault in the bathroom, they feed a outdoor receptacle off it..

One malfunctioning thing.. be it a alarm clock in the camper or a 3phase refrigerator relay that sparks will take it offline.. A meter, you read the power to ground can take them out..

I've been on construction building jobs where the moist morning air takes the temp power ground faults offline.. you have to stand there resetting them cause if you wire past them and somebody dies you are ruined.. so you deal with it like a hot potato..
You can however tell the carpenter "NOT TO USE" the outlet you tape shut.. it is not a ground fault.. THEN.. it is up to him..

A ground fault can save your life. I am sure they have mine. Drop cords get ran over, lil rock holes in the insulation.. they don't get replaced soon enough.. (saftey tip? always start at the power end to roll up a drop cord.. unplug it, then roll it up.. if you learn the parachute knot tie.. they will never be tangled).. younger apprentices untie them one knot at a time thou.. they don't understand..

tdmidget
12-17-2008, 01:48 PM
David is right . I have seen sometimes 3-4 in line. If this set up isn't working something is wired wrong, very common for electricians who think you can't have 2 in line. Get a new electrician.

nitro
12-17-2008, 09:12 PM
I have GFCI in bathroom and wifes hair dryer has one in the plug. No problem ..

madman
12-17-2008, 09:19 PM
I cant figure it out. All checks OK BUT it trips out immediately? New one in Wall?/ Hm>>??

J Tiers
12-17-2008, 09:46 PM
There is such a thing as a bad GFI.... right out of the box....... They ARE made in china, you know, and were bought at the lowest possible price............

Some loads will trip GFIs. Computers and other things with line filters...... sure, each one won't pull enough current (3.5mA max) but 2 or 3 might............ almost anything has a computer chip in it now, and maybe a line filter of some sort to reduce EMI......

Even some GFIs can trip others with their continuity checker circuit.

RancherBill
12-17-2008, 10:14 PM
I found this info at another location


Q: Can you install a GfI breaker on you breaker panel and at the same time
install a gfi receptacle on the same circuit. Would this cause any
problems to the circuit?

A: no you cannot. Having two GFIs connected in series would cause one to
counter act the other. This would cause them both to continueosly trip
into the test position.

Q2: I appreciate your response. Why is that? would it that be more like a
back up protection just in case the other one fails.

A2: the reason it doesn't work is because the GFI takes control of the nuetral
side of the circuit and monitors the return current between the nuetral
and ground. When you install a GFI on a circuit that is already GFI
protected it confuses the first GFI and causes it to trip because it acts
as if there is Current on the Ground. As far as additional protection,
thats why the manufactures recommend testing the GFI once a month.http://www.hilo-electric.com/blank?pageid=63&catstart=0&prodstart=0

rdfeil
12-17-2008, 11:27 PM
Ok Guys,
I plead cabbage tooth, can't see what I'm saying :D I agree with the corrections to my earlier statement. GFI indeed does compair the current in the hot wire to the neutral wire and trips if they do not match. It is not required that the fault is to the ground wire, it could be to actual earth or a water pipe or anything else that will allow current flow. I beleve that the inbalance needed to trip is about 1mA (1/1000 of an amp). Please correct me if I'm wrong.

Robin

Dawai
12-18-2008, 12:24 AM
My gfci trips when lighting strikes close by the house.. I calculate it is the surge going back up to heaven traveling through the earthen ground rod and back to the singing wires overhead..

They quit putting lighting arrestors on the top of the power poles down heah.. they/we used to run a bare copper wire down the pole and spiral it underneath the end of the pole.. it was a excellent ground sitting in the damp ground at the bottom fo that pole..

AND our power company.. they quit tying the bare ground to the meter base.. experience showed that they wanted it tied to the home-owners panel so it'd blow up there instead of damaging the meter base.. I tied it to the outside of their meter base after inspected and turned on.. If lighting hits my house.. they are going to replace crap too..

BUT alas.. the ground fault circuitry compares the neutral to the power wire.. any imbalance means the "power is escaping" out through a human or other leak.. and it shuts the power circuit off..
There is no magic.. you could put 100 good ones in a row.. they would not inject, reject, nor read each other.. Nor measure power, nor draw lightning.. just compare the power line to the neutral. I've seen them work where there is no true ground in the panels.. *power plants.. They just compare the in and out and if they don't match, they shut off..

Electricity is like water.. it has to flow in to do the work.. then flow out to continue the working "loop".. it is equal on the lead in and the lead out wires.. really.. the work has the same amount of amperage that the wires do.. Something funny thou.. pull parallel feeders through a junction box.. the shorter wires will heat up and carry more current than the ones only a few inches longer.. ELECTRICITY goes the path of least resistance.. be it copper wires shorter than the others or you to ground..
Something else funny, the neutral.. take 220 and one NEUTRAL wire.. tie them through a device and back to the neutral.. like say, two strings of flourescent lights.. the neutral carries only a very small part of the current.. each phase cancels the other out.. Called a balanced load.. then some hillbilly electrician moves the 220 wires to the same phases in the breaker panel.. and suddenly the single neutral is carrying twice the power of either hot wire.. I did balance lighting loads by using three colors of wire.. denoting phases it was supposed to be tied to.. but I learned a hard lesson.. them maintenance men don't care.. and move the wires around like they are nuts.. or don't know any better..
A control transformer.. did you know you can ground either side?? just make sure the other side is not grounded somewhere else.. and switching? if you ground the neutral side and try to switch the neutrals?? I saw a tenner frame go crazy one night.. they tech had used the neutral wires for the steering contactors to control the motors w.limit switches.. it had rained.. and the switches got wet.. they grounded with the moisture making the "neutral" switches.. turning all the contactors on at once.. they were ratcheting like crazy.. trying to pull in both sides at once..

Electricity..
Once it leaks out all over you it is messy.. It causes the water-salt-blood to boil under the skin making big pockets of steam.. when the steam reaches high enough pressure it blows chunks of meat out.. One of my buddies digging with a backhoe I thought was shot.. the big "krack" then the side of his ankle-leg was blowed out.. I was ducking behind the backhoe like I was in Vietnam.. Trying to get him into cover.. He had dug into some power lines and his feet were on the pedals. I figured later that the power surge had came up the boom, then went through his feet on the pedals.. he was a lucky man to be alive.. Those are called "eddy" currents and nobody has explained to me in my 30 some odd years how they flow? seems like bad Karma.. they hit and miss.. can go right by you and knock the next guy down..

Did you know a tesla coil producing a plasma ball on the top? if the air currents drift it over to a wall outlet the 120 can flow up the plasma stream, and if it is flowing across you.. electrocute you.. even thou it appears you are not touching anything hot? OR, they can go into the next room and erase half a Ninja turtles video game?? I still don't understand all I have seen..


{I thought was shot.. the big "krack"}
I was in so much back then I probably had a guilty concience.. *loud noises still make me duck. Once during a shooting in a bar, everyone was looking for me.. except I was under the table looking out..

ONE MORE THING.. than I'll take a pill and fade off to sleep... DID you know Lightning strikes UPWARD??? it flows from the ground to the heaven.. it appears to flow the other way as the current path is established.. I have been hit twice.. mildly.. I am/was lucky and should have driven to Las Vegas immediately.

Evan
12-18-2008, 01:26 AM
A2: the reason it doesn't work is because the GFI takes control of the nuetral
side of the circuit and monitors the return current between the nuetral
and ground. When you install a GFI on a circuit that is already GFI
protected it confuses the first GFI and causes it to trip because it acts
as if there is Current on the Ground. As far as additional protection,
thats why the manufactures recommend testing the GFI once a month.

That statement is meaningless. The only "control" that a GFCI has it to disconnect the power. Up to that point it controls nothing and is only sensing a possible difference voltage caused by unequal current flow in the two conductors.

If you want some proof that not all the current flows back through the neutral put your meter on neutral and ground on about a 20 volt range. With no load you should see very close to zero volts. Then start adding loads such as a 100 watt light bulb and you will see a slight increase in voltage. How much depends on your circuit. Jack up the load with an electric kettle or coffee pot and see what it reads. It is affected by the length of the circuit and the resistance difference between the hot and neutral lines.

Due to slight variations in terminal wire binding, wire thickness, induced fields and crosstalk there will always be some difference and it shows up as a voltage between neutral and ground. Up to five volts is permissible although that isn't good. More than that and you have a problem that must be corrected. It will not only cause the GFCI to trip it will cause misoperation of devices such as computers and other computer operated devices.

J Tiers
12-18-2008, 08:46 AM
If you want some proof that not all the current flows back through the neutral put your meter on neutral and ground on about a 20 volt range. With no load you should see very close to zero volts. Then start adding loads such as a 100 watt light bulb and you will see a slight increase in voltage. How much depends on your circuit. Jack up the load with an electric kettle or coffee pot and see what it reads. It is affected by the length of the circuit and the resistance difference between the hot and neutral lines.

Due to slight variations in terminal wire binding, wire thickness, induced fields and crosstalk there will always be some difference and it shows up as a voltage between neutral and ground. Up to five volts is permissible although that isn't good. More than that and you have a problem that must be corrected. It will not only cause the GFCI to trip it will cause misoperation of devices such as computers and other computer operated devices.

A VOLTAGE on neutral is NOT "proof" of current going elsewhere other than the neutral, or not returning on it. If anything it might be "proof" that some other EXTRA current is flowing in the neutral, although that is unlikely.

The voltage is for the most part simply due to wire resistance, and a similar voltage drop should appear on the hot lead. other effects are very small at normal currents.

it has nothing practical to do with current not returning on the neutral. And, a classical current balance GFCI will not be affected by it. Some types might be, depending on how they work.

While I suppose some minuscule extra current (less than microamperes, in general) flows out through insulation and capacitances due to the slight voltage, the voltage is otherwise merely an indication that there is extra resistance somewhere, due to undersized wires, bad connections, or possibly excess current due (in the US) to loads on the two "sides" of a 120-0-120 circuit generating harmonics which don't cancel.

japcas
12-18-2008, 09:06 AM
I cant figure it out. All checks OK BUT it trips out immediately? New one in Wall?/ Hm>>??

Hey Madman, did you wire up to the right set of terminals on your new gfci receptacle? One set of terminals is for incoming power for the receptacle. The other set of terminals is to feed other receptacles on down the line and they will be protected then also. It should be marked but I wouldn't take it for granted that they are. It should say in the installation instructions. They won't work right if wired wrong, although I don't know if they won't work at all or just not trip in a ground fault situation.

Evan
12-18-2008, 11:00 AM
it has nothing practical to do with current not returning on the neutral.

Bull. If there is a ground path somewhere it will much more likely be to neutral than to hot in a piece of gear and if there is a potential on the neutral then it will take the ground path. I fought with these problems for 23 years and monitored many a circuit for these sorts of problems using a Dranetz Analyzer. The courses I took are eligible for credit hours toward an engineering degree. I have enough of them to take almost two degrees. Over the years I have spent about two full years of 7.5 hour working days in the classroom.

When I told the power company there was a problem they went and fixed it instead of doubting I knew what was going on. Excess neutral voltage drop is by far the most common problem on a branch circuit. You will be hard pressed to find a branch circuit that has connected equipment that doesn't have ground current flowing. Further, If one side of the split phase 230 has a neutral voltage drop it unbalances the 230 by the amount of the neutral drop in proportion to the current draw.

Also, in a computer the neutral has a significant stray capacitance to ground. If it is elevated above ground anything that causes sudden changes in the line voltage can induce ground currents in the chassis of the machine and crash the box.

topct
12-18-2008, 12:02 PM
Why not just use the one GFI on the outside of the house? Anything plugged into it will be protected. You do not need another one in your camper.

J Tiers
12-18-2008, 11:25 PM
Bull. If there is a ground path somewhere it will much more likely be to neutral than to hot in a piece of gear and if there is a potential on the neutral then it will take the ground path. I fought with these problems for 23 years and monitored many a circuit for these sorts of problems using a Dranetz Analyzer. The courses I took are eligible for credit hours toward an engineering degree. I have enough of them to take almost two degrees. Over the years I have spent about two full years of 7.5 hour working days in the classroom.

Sorry, Evan, it's not bull..... you are just wrong.

The existence of voltage drop in the neutral is NOT proof of a ground fault, more likely the reverse.....

Sure, IF there is a ground fault, then it would split current between them, but naturally that would REDUCE voltage..... due to multiple current paths.

But if the drop is higher, it is more likely that there is NO fault..... or it would be lower again.

Most people do not realize that the neutral and hot must have the exact same insulation types. There is no distinction or difference just because the neutral is "nominally" at a lower potential to ground.

What you are saying is that IF there is a neutral-to-ground fault, it will pass more current if neutral is at a higher voltage..... That's fine, but there is already a fault then.

That is a lot different from saying that the existence of the voltage PROVES there is a fault.

The assertion that a neutral to ground fault is less likely to be discovered is correct, unless there is a GFCI on the circuit. That will find it fast, but NOT due to a higher voltage.....

Of course a high neutral voltage is probably already an indication of some problem, either excess current, long branch circuit, harmonic currents, or bad connections. None of those need have anything to do with a neutral-to-ground fault.

BTW.... I am NOT interested in how many courses you took.




Also, in a computer the neutral has a significant stray capacitance to ground. If it is elevated above ground anything that causes sudden changes in the line voltage can induce ground currents in the chassis of the machine and crash the box.

I believe you will note that I mentioned capacitance to ground..... And at power frequencies the effect is minimal if even noticeable, due to the very low neutral voltages we are talking about. The capacitance is NOT ALLOWED to pass more than about 3.5mA EVEN AT FULL MAINS VOLTAGE. This is measured by the UL and CE "touch current" test. UL uses a goofy set of calculations, but it ends up about the same as CE.

For neutral, capacitive currents will be lower in proportion to the amount the neutral voltage is lower than full mains voltage. Your 5 volts gets you about 150 microamps for a max capacitance from neutral to ground. That is about 35 times less than the trip current of a 5 mA GFI.


If one side of the split phase 230 has a neutral voltage drop it unbalances the 230 by the amount of the neutral drop in proportion to the current draw.

If "split phase" means US type wiring, it only shows up for neutral referenced equipment. For a 230 only unit, the neutral drop won't show up.

darryl
12-19-2008, 03:55 AM
In any properly wired system, the ground wires won't be carrying any current except in case of fault. They will be the only wires you would normally use to reference voltages from hot or neutral wires. In other words, if you want to know how much voltage is actually reaching an outlet in question, one meter lead would have to go to the ground wire. That voltage would of course drop when current is being drawn through a load because of the resistance of the hot wire side of the system that's delivering the power. The load is going to see even less voltage because there's also going to be a voltage drop in the neutral side of the system where the load current returns. Measuring voltage on the neutral wire referenced to the ground wire will show what the voltage drop on the neutral side is. If there's a voltage existing between a neutral and a ground, that should be the voltage drop due only to the return current flowing in the neutral. So far nothing is amiss, even though there may be a voltage reading between neutral and ground.

In a GFI, a hot and a neutral wire pass through the sense coil. If nothing is amiss, the magnetic field induced by current in the hot wire will exactly cancel the field induced by current in the neutral wire. The result is that no magnetic field is induced in the torroid, even though many amps may be flowing into a load. The secondary winding on the torroid will have very many turns because it will have to generate a significant voltage even if only a few milliamps difference in current flows in the hot and neutral wires. This suggests that the sensing circuit will have some fairly high impedances, and will be sensitive to extraneous signals. If you've ever seen the electromagnetic 'hash' that can exist on power lines, you won't be surprised that a GFI unit could seem to malfunction for no appanent reason. It could well be that high current loads, and particularly inductive loads which can generate voltage spikes, could cause the units to trip out. I'm sure they are designed to be as resistant as possible to these things, but nothing is perfect.

If we look back at the ground wire system, it will be acting as an antenna, so there will be lots of various signals being carried on it. Whether or not that makes a difference to voltage readings will depend on the meter in use. I would suspect that most meters won't respond in error to this, but some might. Bring a radio near a wire in your wall and see if it brings in better or worse reception. That represents a sensitive system responding to signals being emitted by the wiring- all the wires, not just the ground wires. It's a wonder in this day and age with all the electronic crap we use that anything works as well as it does.

clutch
12-19-2008, 04:58 AM
The ground wire doesn't mater at all. In older homes with two wire systems, gfci's are often used to repace a two wire recept. Put the recept up stream, chain the down stream recepts from it and you are good to go.

The only problem is that your gfci tester will not work since it normally leaks a bit of current from hot to ground to test and you don't have a ground.

If you are chaining gfci's and have a problem, likely the gfci that is tripping is finding a legitimate problem. Find and fix that.

Clutch

Evan
12-19-2008, 05:52 AM
That is a lot different from saying that the existence of the voltage PROVES there is a fault.



The existence of voltage drop in the neutral is NOT proof of a ground fault, more likely the reverse.....


Read it again Jerry, that isn't what I said. See the word IF?

It does prove there is a fault. What I said is that if the voltage exceeds 5 volts there is a fault in the neutral wiring. That particular number is based on the allowable voltage drop in a 250ft run of 14 gauge wiring at 12.5 amps. You can't directly measure the voltage drop on the hot as you don't have a reference but you can measure the neutral at the point of load. If the neutral has a voltage that exceeds the maximum the chance of a ground fault current that exceeds a safe limit is increased.



I believe you will note that I mentioned capacitance to ground..... And at power frequencies the effect is minimal if even noticeable, due to the very low neutral voltages we are talking about. The capacitance is NOT ALLOWED to pass more than about 3.5mA EVEN AT FULL MAINS VOLTAGE. This is measured by the UL and CE "touch current" test. UL uses a goofy set of calculations, but it ends up about the same as CE.


What frequencies are you talking about? I am talking about anything that causes sudden changes in the line voltage . You, as usual, do not actually respond to what I post. Transients are not limited to 60hz nor are they limited to "full mains voltage".

The problem with your replies is that you do not address my points. You are only adressing your own points.

J Tiers
12-19-2008, 08:07 AM
Evan, hold to ONE argument, and don't try to re-define it after the fact.



It does prove there is a fault. What I said is that if the voltage exceeds 5 volts there is a fault in the neutral wiring. That particular number is based on the allowable voltage drop in a 250ft run of 14 gauge wiring at 12.5 amps. You can't directly measure the voltage drop on the hot as you don't have a reference but you can measure the neutral at the point of load.

The problem with your replies is that you do not address my points. You are only adressing your own points.

Bull crap. Your "points" slip around like wet Jell-o on teflon.



If you want some proof that not all the current flows back through the neutral put your meter on neutral and ground on about a 20 volt range. With no load you should see very close to zero volts. Then start adding loads such as a 100 watt light bulb and you will see a slight increase in voltage.

Due to slight variations in terminal wire binding, wire thickness, induced fields and crosstalk there will always be some difference and it shows up as a voltage between neutral and ground. Up to five volts is permissible although that isn't good. More than that and you have a problem that must be corrected. It will not only cause the GFCI to trip it will cause misoperation of devices such as computers and other computer operated devices.

So remind me again how "If you want some proof that not all the current flows back through the neutral...... ", and "It will not only cause the GFCI to trip...." is NOT saying that voltage on the neutral proves the existence of significant current flowing that does not return on the neutral.

Ev, old pal, your arguments are a moving target, and your original statements as quoted are wrong, and may cause people problems if they believe you.

Sorry, voltage on the neutral does not prove current isn't flowing back on it....

And you CAN measure voltage drop on the hot wire, it just takes a longer meter lead.;)

There isn't any difference on the neutral. Measuring relative to the local grounding wire ASSUMES that the grounding system has no current or voltage drops on it. That MAY NOT BE THE CASE, although it should be.

believe whatever you want, but please do not spew misleading and wrong safety information.

Evan
12-19-2008, 08:37 AM
So remind me again how "If you want some proof that not all the current flows back through the neutral...... ", and "It will not only cause the GFCI to trip...." is NOT saying that voltage on the neutral proves the existence of significant current flowing that does not return on the neutral.




Time for some continuing education Jerry.

I recommend you read the entire article.



Applying what we've learned

As many power monitor setups do not use L-G connections along with L-N and N-G connections, you must ferret out true common-mode events. With sole usage of N-G connections to detect common-mode events, you may develop a tendency to ignore low amplitude N-G transients. After all, if recorded transients are always present at a given level and you can determine no adverse effects, why waste monitor memory recording those events? Increasing the monitor threshold to avoid capturing lower level and commonly occurring N-G events may leave the true common-mode events undetected.

We can sum up our experiences as follows:


N-G voltages less than 3V and developed at power frequencies seldom cause adverse effects.

Low level N-G transients less than 25V peak and caused by load cycling usually do not cause adverse effects. However, the potential for adverse effect will increase as frequency content and amplitude increases.

Higher frequency, true common-mode events can cause adverse effects, but you may not be able to detect or correctly identify their presence.

Measuring N-G voltages with a multimeter is a valid procedure, and the measurements you make may help identify wiring problems that cause excessive voltage drop. Remember, high levels of N-G voltage invariably arise from grounding/bonding problems.

Shaughnessy is vice president, PowerCET Corp., Santa Clara, Calif.

http://ecmweb.com/power_quality/electric_clearing_neutraltoground_voltage/

topct
12-19-2008, 11:09 AM
The ground wire doesn't mater at all. In older homes with two wire systems, gfci's are often used to repace a two wire recept. Put the recept up stream, chain the down stream recepts from it and you are good to go.

The only problem is that your gfci tester will not work since it normally leaks a bit of current from hot to ground to test and you don't have a ground.

If you are chaining gfci's and have a problem, likely the gfci that is tripping is finding a legitimate problem. Find and fix that.

Clutch

Yes, all it takes are a hot and neutral wire to be pinched together. No broken insulator, just crushed tightly in a crowded box. Normally not a problem unless there is a GFI involved.

darryl
12-19-2008, 06:05 PM
To the OPs original question, no there should not be any problem using GFIs in series, since they don't interfere with each other and any one of them could or would trip in response to an unbalance. (leakage, ground fault, whatever). If they were all identical and tripped exactly at the same time, they would all trip out in response to a problem. I believe they require power to hold in the on position, so to reset, the first in line would be the first one to reset, etc. If any one was to trip out, all others past that would automatically trip out as they lost power, so the same reset procedure would be used. If they don't need power to hold on, then they need power to trip out- in that case every one past a tripped one would automatically come back on when the tripped unit was reset. There is no conflict, just some understanding needed if a fault has caused one or more to trip.

I don't get the idea that the insulation on hot and neutral wires has to be the same type- what am I missing here? I also don't see a problem with hot and neutral pinched together where the insulation is unbroken- what? If the insulation is not broken there is no short- why would this tend to trip a GFI? One thing I can see is if one of the wires passes too closely to the relay and a high current draw induces a magnetic field that interferes with the mechanical operation of the relay. I've never seen this happen, but I suppose it could. This would be a problem with how the wires are dressed within the box containing the GFI. Obviously, if pinched wires eventually separate the insulation and allow a short or partial short to occur, then that's a realistic thing to note.

topct
12-19-2008, 07:00 PM
I don't get the idea that the insulation on hot and neutral wires has to be the same type- what am I missing here? I also don't see a problem with hot and neutral pinched together where the insulation is unbroken- what? If the insulation is not broken there is no short- why would this tend to trip a GFI? One thing I can see is if one of the wires passes too closely to the relay and a high current draw induces a magnetic field that interferes with the mechanical operation of the relay. I've never seen this happen, but I suppose it could. This would be a problem with how the wires are dressed within the box containing the GFI. Obviously, if pinched wires eventually separate the insulation and allow a short or partial short to occur, then that's a realistic thing to note.

When the insulation is crushed to a point (not a direct short) to allow a sufficient amount of the field around a hot wire to also induce a slight amount of current to neutral a GFI will trip.

I did not believe it either. But when found and the wires seperated, the GFI in question did not cause any more problems.

Evan
12-19-2008, 07:12 PM
When the insulation is crushed to a point (not a direct short) to allow a sufficient amount of the field around a hot wire to also induce a slight amount of current to neutral a GFI will trip.



Think about a clamp on ampmeter. A single turn is all it takes to induce a measurable current.

J Tiers
12-19-2008, 09:41 PM
Evan.... have you changed the subject YET AGAIN?

Either that or you just don't understand.

You suggested a multimeter to "prove" that not all the hot lead current is returning on the neutral, by finding a voltage drop on the neutral. We must presume you mean SIGNIFICANT current, or you would not bother mentioning it.

This is BOGUS, because a voltage drop vs ground does NOT show EITHER the non-existence NOR the existence of any particular amount of current in the neutral wire, or any other particular wire. The information simply is not given by a voltage alone, with no other data.

That voltage suggests there may be bad connections in neutral. They can give such a voltage.

A current in ground, NOT NECESSARILY FROM THAT BRANCH CIRCUIT, could give a voltage to ground.

A very high bonding resistance from neutral to ground might give such a voltage.

There are other things which are NOT GFI-tripping events which might cause the voltage.

You can even have such a voltage with NO current in any of the relevant wires, and perfect, to-code connections between them. A large external magnetic field, from some unrelated piece of equipment can do it, and there won't be a current if there is no cross connection... it will simply be an open-circuit voltage. I have seen this.

The ONLY way to determine if current is returning in the proper neutral is to measure it. A GFI does this.

If it were so simple as you suggest, the GFI would just measure the neutral voltage......... but they don't. They don't because that gives no information about return current path.

While I admire your agumentative attempt to secure the "high ground" by suggesting "continuing education"........ it won't work.

Nice try, no cigar.

Your cited reference has only the most tenuous and peripheral connection specifically with your issue of current returning or not returning on the neutral in normal conditions in a house or shop, not a big data processing installation etc. Essentially, it is noise, for our purposes.

The portion you highlighted say essentially "you can find excessive voltage drop with a multimeter". Big surprise there, you bet...:rolleyes:

The bit about bonding and grounding says essentially "a large drop means there is a bad connection somewhere". (or some other place in the wiring might have an issue). Again, no surprise.

No part of that article suggests that normal wiring issues of current leakage off the neutral are "PROVABLE" (as you claimed) with a voltage measurement.

Sorry, Evan, you simply are wrong about it.

I don't propose to waste any more time on this. Please learn some basic electrical concepts.

If you can, in your own words, without external links, explain the mechanism by which you personally think that this voltage measurement proves the existence of significant return current on ground or anywhere other than the neutral, then we can talk.

I am not going to bother reading or effectively arguing with, a bunch of links.

Let's have YOUR PERSONAL analysis, as YOU are the one who made the assertion.

Then we can see if we agree if your analysis makes sense, or not.

This is ON Topic, IF we can relate it to finding safety hazards in the shop with simple measurements.

I claim it is limited to finding bad connections, and even then provides only SOME of the data needed.