AC Ground Problem

[Krankshaft]

Well before I get into my AC outlet grounding problem I think I should start from the beginning.

A few weeks back I was using a PC power supply to power an HD Radio I repaired since it required 5 amps at 12 volts and my bench supply couldn't handle it.

So I touch the PC power supply casing while it was running and I get a little shock oddly enough I get that same shock when touching the metal antenna leaving the radio.

It wasn't much it was ever so slight so slight I touched it again to make sure it wasn't my imagination. So I assume huh (we all know what that stands for) the problem was in the radio or PSU and put it aside for awhile.

Today I was soldering with my iron (it has a grounded plug of course) and while the iron was in contact with the solder I had my hand on a soldered joint further down the board and I felt a little shock again. While the iron was in contact. The unit I was working on didn't have any charged caps and was battery powered. So the source wasn't the equipment.

At this point I knew something was seriously wrong and having a discharge from your iron with semiconductors in circuit is just asking for trouble.

So I get out my multimeter and measure from hot to neutral at the outlet and I get 122 volts normal (I'm in the US).

So I then measure from hot to ground and I get 116 Volts.

The measurement is the same for all outlets on that circuit.

A 6 volt difference could this explain my issue?

Is there some kind of resistance in the ground wire causing this?

I checked another outlet on another circuit and the hot and neutral reading 122 Volts was the same as hot and ground 122 Volts which is how its supposed to be.

My work area is in a finished basement where I have had problems with corroding contacts on phone jacks and the like. So is it possible that the screw on connectors or the bare copper ground wire on the outlets are corroded causing this ground problem?

I've worked with wiring like this before but before I tear into every outlet in the circuit (oh man I'm feeling tired already) trying to track down this issue I would like some opinions first.

Thanks for any help you can provide.

[PCBONEZ]

Re: AC Ground Problem

I think your conclusion about a bad connected creating a poor ground is correct.

If you know how the wires are routed you can check from the problem socket towards the power box until you find one with the correct voltages. The problem should be in that one or the one you checked just before it.

.

[starfury1]

Re: AC Ground Problem

All disclaimer's apply on this legal and otherwise...
do at your own risk etc

well Id have to go reread the posts and links on how the USA system works. I am "NOT" from the USA

From memory your Neutral "I think" is tied to True Earth so therefore should be at the same potential (ideally)
but probably close to true Earth so you should only measure a small voltage (2~5 volts or so) difference between Neutral and Earth.
(i dont think you did a Neutral Earth)
This is what you likely to find in our mains system

BUT not totally sure on the above!!!


Another thought is;

I dont recall the full theory of it off hand in regards to earth and psu's,
some psu are death traps in this regard
A SMPSU will direct rectify the mains which results in a floating common
(that old "live chassis" you see on the back televisions sometimes I am talking about here)

people did get tingles from the ant socket or cable shield

Like I said not sure of the theory so perhaps someone here has it down pat and can explain it a lot better then I.

I would myself do the hard yards and rule out an Mains Earth connection problem as Bonez said, first off regardless....at lest you will know its safe that way (the mains earthing that is). and it will most likely solve the problem as he said
sound like a good possibility to me too. (So yeah zero ohms or very close to true earth) since you mention corrosion you probably do have a high resistance contact/connector somewhere on the earth line

switch off the mains while checking power points etc.
(stating the obvious here)

IF I got it wrong PLEASE corrected it somebody

HTH Cheers

[davmax]

Re: AC Ground Problem

So I get out my multimeter and measure from hot to neutral at the outlet and I get 122 volts normal (I'm in the US).

So I then measure from hot to ground and I get 116 Volts.

The measurement is the same for all outlets on that circuit.

A 6 volt difference could this explain my issue?

Yes it is quite common to havea difference voltage between earth and neutral.

Often the voltage difference is the other way round. ie Neutral to active is less.

It all depends where earth and neutral connect together. The proper situation is that all neutrals, including the one from the power pole connect together at the same point. If they do not then difference voltages can occur. this difference occurs due voltage drop caused by the current flow in the neutral return to earth. Obviously wire resistance and contact resistance determine the voltage value (V=IR). Clearly if there is no neutral current there will be no voltage drop (no difference between earth and neutral). The most common problem occurs with long neutral cable runs in large buildings where there can be quite a large voltage developed across the neutral return.

The case in the example above has the active to neutral higher that active to earth. This indicates that the earth circuit at that point is not taken back to the common earth point, it is tapped into the neutral return line at a point that is about 6 volts above the common point.

To summarise. The magnitude of voltage is determined by the product of neutral current and circuit resistance and the +- relationship to earth depends on the point of connection.

[PCBONEZ]

Re: AC Ground Problem

If you are not in the US I don't know,,, but here the codes require the neutral and ground be connected together inside the breaker box. (Inside the power box the bus bar (connection strip) for the white wires is connected to the bus bar for the ground wires. In fact some boxes only have one bus bar for both ground and neutral. )

Given that if there is no problem in the wiring they should both electrically be at earth ground potential.

Note: That is the ONLY place the white neutral wire and ground should be connected together.

This is deliberate. The intent is to keep the neutral at ground potential so the 'hot wire' doesn't drift higher that it's rated above actual earth ground due to the effects of some load on the lines.

In other words they want to keep 115v at '115v TO GROUND', not just 115v to neutral.

They ensure this condition with the codes and by requiring every main power box have the neutral grounded inside the box. - This keeps the neutral for the entire grid (no matter how far from the power plant) at ground potential.

.

[PCBONEZ]

Re: AC Ground Problem

In essence, the neutral and the ground wire are both earth grounds in building wiring here AS LONG AS the neutral is properly connected to and in the power box.
.
It's a redundancy/safety thing.
.
For example: Your toaster (or whatever) won't drift up to 150v from ground because the neutral drifted up to 35v from ground.
.

[PCBONEZ]

Re: AC Ground Problem

Here.
This should help you 220v guys understand how we are wired over here.
It's not single phase like there, it's called split-phase.

Not everywhere is wired like this but it's the most common for homes or non-industrial buildings.

.

[starfury1]

Re: AC Ground Problem

Thanks Bonez nice pic wish Id have had that before

yeah mate got that from the other long post started by pentiums pwr meter and then I had to correct the post started by KBcowboy

Cause I shot my big mouth off without realizing you guys use a "split phase" setup
So I miss understood his reference to red black / positive negative Active and the meaning of positive and negative generally speaking is what I got on to.

oddly enough I did find a ref to it being using in the SWER (Single Wire Earth Retun) system here but I don't work for a power company so no real Idea if it is done in reality here

Difference of course was 220V split phase and still one half to one house type thing so not using it as you guys do with both Actives to house.

I think its called MENS system (Multiple Earth Neutral System) thats what we use
Bonding the neutral and Earth together at the house mains box.

I don't know for sure on the below bit

I sort of remember its some what newish
(like it came in here as a requirement 30 odd years ago)
I don't know for sure if before this, its was just an earth required at the house and that could be a water pipe..now I think you must have an Earth stake as well regardless of water pipe
I am not sure either but I guess possibly the earth and neutral weren't tied together at the house..I suspect this was the case

Now it must meet MENS requirements as far as I am aware

I was fairly sure from memory the Earth & Neutral tie together like ours and Yep EARTH is SAFETY WIRE has no real purpose in life but to hopefully protect your arse.

http://www.epanorama.net/documents/g...al_wiring.html

http://en.wikipedia.org/wiki/Ground_and_neutral

http://www.epanorama.net/phpBB2/viewtopic.php?t=1821

cheers

[Logistics]

Re: AC Ground Problem

It sounds like you may have just had something go haywire due to corrosion, but have you purchased any new products lately? Perhaps something you have plugged into a socket somewhere has decided to go bad or a new product you have is defective.

[Krankshaft]

Re: AC Ground Problem

Thanks for all the input guys I just tested my theory the mysterious jolts from the radio's antenna vanished when I plugged it into another circuit that functioned properly.

Voltage measurement on good circuit:

Hot + Neutral was the same as Hot + Ground

And to think a neon tester said everything was fine that is why I don't trust those testers with "Idiot Lights" .

I added new breakers to the box years back and yes the ground and neutrals come to the same bus.

As for grounding my system has 2 grounds one wire is strapped to the cold water pipe and the other wire is attached to a stake driven into the ground outside.

I guess its for redundancy?

I am going to check for any obvious corrosion first in all the junction boxes on that circuit if that fails I will disconnect the circuit from the service box and measure for resistance between wires to see if there is a short somewhere.

Its funny the wiring is Romex and it's barely 20 years old and it's giving problems.

I still have 1950s era BX in the house and it hasn't given me any trouble.

Maybe that's because the sheathing acts as a ground and is less susceptible to failure?

I guess I'll need to run one of my big orange extension cords for my equipment until I can sort this whole mess out.

[davmax]

Re: AC Ground Problem

It appears that nobody read my post or did not understand. Krankshaft if you have two earth points make sure that there is no voltage difference between them.
All neutrals being tied to earth is the same method whether split phase or not. Whilst in theory this means neutral and earth are the same voltage however when neutral is carrying current it can be elevated above ground at an output socket due to poor connection to the neutral buss or common earth point, also high current in the buss. I use the word buss because in some installs there is one major conductor to which all neutrals connect, more often found in larger buildings. Remember this voltage difference only occurs due to current flow in neutral.

[starfury1]

Re: AC Ground Problem

Well actually Davmax I did but it was a little while before I got back to this and the reply was more to Bonez.

So regardless the Earth & Neutrals are tied together in the USA as here.
(the point I wasn't 100% sure of in my first post, if it was the same as to how its done)

In regard to Neutral Voltage to Earth (measured)

The current flow causes a "voltage drop" across the wire due to the resistance of the wire and termination points resistances.

"Voltage drop" is dependent on the amount of current flow in that neutral wire and from what point its measured from Neutral to earth (ditto earth point).
(Ohms Law)

Ideally Neutral wire and Earth points should be zero ohms resistance but the reality is it will have some resistance
The lower it is the better. (between 2 points..or all points for that matter)

So a higher resistance contact wont really show till you pass some decent current through it....
A multimeter on a resistance or ohms range will show continuity but not true resistance accurately due to the tiny current used.
(and the low orders of resistance we need to reduce the voltage drop)

bottom line
misc currents in neutral line will develop voltage drops due to wire termination point resistances and true earth.
Ditto earth wire and contact resistance of termination points .

Agree, all earth & neutrals should be tied at one point to reduce this and how this is done is depend on the wiring standards that apply for the power grid system in your area.

Now, Hope I got it right there Davmax


Krankshaft

the Wire or Romex you use and how your points are terminated is a little different from our setup from what I have seen on the net of your wiring

I know at one time there was a swing to using Aluminium wire which caused a lot of problems but since you say its barley 20 years old think that stuff had long seen its day in the USA
(and somewhat rightly so from what I remember of what I read about it)

Our stuff is a little different

So just for those like me who don't know I found this on what you are talking about.

http://www.faqs.org/faqs/electrical-...section-5.html

you will find a few photos in this link about half way down the page.


Anyway sounds like you got it nailed to a problem with the wiring, now the hard part of finding it grrr I know what you mean, walls roofs under the floor

Hope its does turn out to be an easy one for you anyway.

Cheers

[PCBONEZ]

Re: AC Ground Problem

I understood you davmax the point of the drawing was to clarify how that happens.

I guess it didn't take. ???

If the neutral has a loose/bad connection somewhere (slight resistance) and the ground doesn't then the neutral will 1) (if there is a load on that line) show a voltage between itself and ground, and 2) (load or not) the voltage hot to neutral will be a little less than the voltage between hot and ground.

Focus on what happens with resistance in the return/neutral between the test point and the power box (or other earth ground) and it will make sense.

Some electricians here in the US also add backup grounds from various points in the distribution to a water pipe or whatever. It used to be standard but I think it's now optional. A lot of houses have plastic pipes now days. Also there is a risk of a home owner replacing a section of pipe with plastic and unknowingly removing the ground path. So, as I understand it grounds to pipes are optional but the earth ground at the box is required.
-
The earth ground at the power box may not look like much but they are actually quite big. They are typically and 8 to 10 foot copper (or copper clad) rods driven into the ground. - [Driving those buggers in is not fun. I had to do it when I added a sub-box to one of my houses.]

I had to fix some aluminum wire in this place - YESTERDAY. - Wife blew out a socket using a heater and a *(^&$@%&* hair dryer at the same time and some remodeler connected aluminum wire to copper wire with twist-lock connectors with no anti-oxidation compound in the cap. (dumb, the wife too.).
- This place was built mid 70's I think. I've lived in about 20 houses in my life and this is the first time I've even seen aluminum wiring in a house.
The bad thing is if you attach it to copper wire dry it oxidizes, builds resistance, and the caps melt and may flame in the wall before the breaker trips.

.