Hi,

I'm a bit confused on how I should ground a new home-made device that's connected to mains through a 220v to 15v transformer. I read somewhere that if you connect both grounds of the transformer together to the mains earth, you could have problems later on (e.g. blowing measuring devices like oscilloscopes and multimeters when connecting the ground in the wrong place in the circuit) so I kept both sides of the transformer isolated; only connecting the core of the transformer to the mains earth and to the chassis of the device, but not anywhere in the circuit.

The device works fine as it is, but I just bought the output BNC connectors, and they need to be connected to the chassis -and therefore, to the mains earth-. I made a test connecting the circuit's ground to the chassis, and therefore to the mains' earth, and it seems to also work; the problem is: when I touch the chassis I can feel a small amount of current.

Also, and I don't know if it's important for this case, in my electrical installation all the neutrals of the wall sockets are disconnected -there's no third wire from each socket to earth, only live and neutral-.

So, I'm a bit confused on how I should proceed and a bit worried of doing tests on my own and blowing up something... or even getting electrocuted.

 

Also, and I don't know if it's important for this case, in my electrical installation all the neutrals ?earths of the wall sockets are disconnected -there's no third cable from each socket to earth, only live and neutral-.

So, I'm a bit confused on how I should proceed and a bit worried of doing tests on my own and blowing up something... or even getting electrocuted.

You are confused?

I'm even more confused. Did you mean the earths are disconnected?

If that is the case how did you connect anything to earth?

If the neutrals really are disconnected, what did you connect the low side of the primary winding to on the transformer?

 

This is how I had it with both sides of the transformer isolated:

And this is with the ground of the circuit connected to the core of the transformer, the chassis and the mains earth; which is what the original design I found on the internet instructs to do:

 

Now, at home I have some old wall sockets with just the Load and Neutral wires, and some new sockets with the 3 connectors (L, N and Earth), but all of them have the Earth disconnected; since there are only 2 wires in each wall socket.

What's odd is that: many of the cables I use are of the newer type with 3 wires, and if I use one of those probes to test for the load and neutral wires on the mains, I get current on all 3 -even the earth-. I can't find an explanation for it, other than the current must be somehow leaking or induced to the disconnected 3rd wire that runs along the cable with the Load and Neutral... I just don't know, but according the the probe there's some current going through it.

Anyway, let's forget about that problem. It doesn't seem to affect any other device at home -all work well and you don't feel any current on the chassis, except the new device-.

 

If you want safety on the low voltage side then it is connected per the lower diagram.
Normally anything below 45v is not considered a safety hazard.
But where there is an isolation transformer for say 240/120, then the code allows one side to be connected to earth ground as diag 2.
If you touch a metalic object and you feel a tingle, it usually indicates a ground path and the device itself is ungrounded, and/or the supply is isolated and has leakage .
There should be no potential measured between the outlet ground and neutral.
Max.

 

I hope I can interject a slight but highly related hijack: I'm curious about ground and neutral in my home (in the U.S.).

I believe I have 2 power wires coming in from the outside, each a 120V line, 180° apart so that I can get 240V using both.

Where do all my neutral wires become equal to ground? Just outside my house? I assume there's no ground or neutral wire going back to my utility?

Other than a couple 240V sockets, are all the 120V outlets in my house powered on the same phase? If not, the potential could exist to get 240V across two of the hot black wires. That seems like a problem.

 

If you want safety on the low voltage side then it is connected per the lower diagram.
Normally anything below 45v is not considered a safety hazard.
But where there is an isolation transformer for say 240/120, then the code allows one side to be connected to earth ground as diag 2.
If you touch a metalic object and you feel a tingle, it usually indicates a ground path and the device itself is ungrounded, and/or the supply is isolated and has leakage .
There should be no potential measured between the outlet ground and neutral.
Max.

I just took a multimeter and measured the voltage in a cable with 3 wires connected to a wall socket. I measured the 235v AC between Load and Neutral (more or less what it should be), but also 13.5v AC between Neutral and Earth (when Earth is supposed to be disconnected in the wall socket) and 7v AC between Load and Earth (again, with Earth disconnected)... what's going on?

 

Just made another test in another socket and now I get 0.1v AC from Neutral to Earth, and 0,2v AC from Load to Earth.

Shouldn't it be 0v on both measurements and on both sockets?

Where are those 13.5v and 7v coming from in the first socket?

 

I hope I can interject a slight but highly related hijack: I'm curious about ground and neutral in my home (in the U.S.).

I believe I have 2 power wires coming in from the outside, each a 120V line, 180° apart so that I can get 240V using both.

Where do all my neutral wires become equal to ground? Just outside my house? I assume there's no ground or neutral wire going back to my utility?

Other than a couple 240V sockets, are all the 120V outlets in my house powered on the same phase? If not, the potential could exist to get 240V across two of the hot black wires. That seems like a problem.

Wayne, we have discussed US wiring many times here.

A forum search will reveal many pretty diagrams. Sgt Wookie did a very good one a few years back.

You have what is known as split phase and the two phases are 180° apart. You house neutral is connected to a local ground at your distribution box (code requirement). This is supplied by the centre point of your local transformer secondary. The ends of the secondary supply the split phases.
There is are indeed 240V between them and this is used for large demand fixed devices such as air con or water heating.

I think if you want more you wshould really start your own thread, since adam is still discussing.

 

Many call it 2 phases but actually it is single phase centre tapped, as well as the neutral (centre Tap) being grounded at the service entrance, it is also grounded at the 1ph supply transformer.
N.A. predominantly use this system, most of the rest of the world use one phase of a star secondary transformer and the star point neutral is grounded so the supply is just 240v and neutral, no 120v or C.T.

Max.

Just made another test in another socket and now I get 0.1v AC from Neutral to Earth, and 0,2v AC from Load to Earth.

Shouldn't it be 0v on both measurements and on both sockets?

Where are those 13.5v and 7v coming from in the first socket?

You are apparently not located in N.A. but your reading should be the same between ground and neutral, there should be no noticeable difference in potential between ground and neutral, do you have a GFI or ground fault interrupter with a ground rod set in place?
Max.

 

A forum search will reveal many pretty diagrams. Sgt Wookie did a very good one a few years back.

OK, you nudged me enough to do some searching. I didn't find any diagrams but I did indeed get my answers:
"In a normal household electrical system, L1 is used for half of the circuits, and L2 is used for the other half. Hopefully, this results in a fairly well-balanced load overall." SgtWookie
This arrangement makes perfect sense but does allow for two outlets to potentially have 240V between their hot wires. I was surprised to learn this.

Anyway, apologies to the OP for the hijack. I do think that understanding just what neutral and ground really are will be helpful to him though.

 

This arrangement makes perfect sense but does allow for two outlets to potentially have 240V between their hot wires.

It is possible for the same dual outlet to have L1 on one the other on L2, this is done in kitchen outlets in an attempt to spread the load between L1/L2, as most appliances in a Kitchen are often high wattage and a number can be on at the same time.
If you look close on a typical outlet, there is a link that can be cut bridging the two outlets in order to achieve this.
Max.

 

You are apparently not located in N.A. but your reading should be the same between ground and neutral, there should be no noticeable difference in potential between ground and neutral, do you have a GFI or ground fault interrupter with a ground rod set in place?
Max.

I think I found out where those 13.5v between the Neutral and Earth come from. I noticed that each cable that I connect to a socket (a cable with 3 wires, which is supposed to have the Earth disconnected) leaks between 0.1v and 0.9v to the Earth cable. I also found that some devices leak much more; like the computer, which for some reason leaks up to 7v to the Earth wire.

Now, since I have a lot of cables and devices connected to the same extension, which is connected to just one socket, I guess that the voltages from the leakages are adding up and reaching those 13.5v.

Is this a reasonable assumption?

In any case, I think I sort of minimized the problem by separating the cables as much as possible from the extension where the leakage seemed to be building up, and now it's down everywhere to just under 1v.

In answer to your question: I'm not in NA, I'm in Spain; and I'm not sure if what I have is a GFI (it has another name in Spanish), it's located where the supply comes into the house and it shuts everything down when there's a short circuit anywhere. I think it needs 15Amps to go off, so nothing happened with the small leakages I felt in the chassis.

So, going back to the device that I need to ground... should I connect the ground from the circuit to the core of the transformer, the chassis and the earth... even though the earth is really disconnected and apparently leaking quite a lot?

 

I believe Spain has the same system as the UK?
It would seem you have a standard breaker, if you had a GFI or equivalent, it would have tripped by now if there was a leakage from any equipment live to ground.
It is a job to advise you without knowing the integrity of the wiring you have.
If there is a problem with the quality of your earth ground and any high resistance on the earth ground conductor between your outlets and the earth ground of your system earth at the panel, then it really requires checking otherwise you could compound the problem,
But in the normal course of things, you would earth ground the transformer frame, and if necessary re-establish your circuit by earthing the common of the transformer secondary, this means though that you cannot also earth any resultant DC powered from this secondary otherwise you blow the rectifier..
Max.

 

Yes, I think we have the same system here as in the UK. The only difference seems to be that at home in the UK we had all the cables and plugs for home devices with an earth connection and all the home sockets were properly earth grounded. That's not the case in Spain: there's no earth wire from the panel to the sockets (even though new sockets and power cables now come with an optional connection for the earth). This was the way they used to do home installations years ago -only with live and neutral and without an earth-.

I just tried the device again with the common of the secondary connected to the (dummy) earth -which is connected to the transformer core and chassis, and this time I didn't feel any leakage when I touched the chassis. Of course, this time I connected the main cable to a socket with an apparently low leakage to ground (less than 1v).

The problem is, I'm a bit worried about what's going to happen when I connect the device somewhere where the leakage has built up again... and also about the first issue that took me to keep both sides of the transformer isolated: so that I can connect the oscilloscope's ground anywhere in the circuit without provoking a short circuit through earth.

By the way, I think that all the computers I had probably also leaked through the chassis/earth; I remember that same tingling sensation sometimes when touching the chassis of all my previous PCs.

 

No. Max there are significant differences between Spain and the UK.

The European Union has 'harmonised' voltages by adopting a standard 230volts, -10 +15 to acomodate those countries like Spain using 220 and those like the UK using 240+.

http://www.justlanded.com/english/Spain/Articles/Property/Utilities-in-Spain

The main difference, however is the provision of earthing. As luck would have it a recent blog by a local UK/Spanish electrician

http://tonysparksinspain.blogspot.co.uk/

In the UK the electricity supplier usually provides ther earthing.
In Spain (as in NA) the consumer provides the earthing.

 

just trawling around the forum I found lots more on earthing, grounding and split phase.

post#19
http://forum.allaboutcircuits.com/showthread.php?t=8325&highlight=earthing

post#11
http://forum.allaboutcircuits.com/showthread.php?t=30939&highlight=earthing

http://forum.allaboutcircuits.com/showthread.php?t=544&highlight=split+phase

 

In the UK the electricity supplier usually provides ther earthing.
In Spain (as in NA) the consumer provides the earthing.

My Early electrical training was in the UK, at that time the service company did not provide an earth ground, and if they did, the installation company could not use it, although the originating source had a neutral grounded, a local ground had to be verified by a ground resistance test back to the source ground point, usually the transformer star point, the local ground point was often the metallic water supply, with the advent of non-metallic supply a ground rod had to be set and a ELT (Earth leakage Trip) had to be installed.
Also the earth ground was not allowed to be re-referenced to the neutral at the service panel as it is in N.A.
In NA the consumer does not provide the earth ground, the service company does, but it is re-referenced to earth via water supply or ground rod at the service panel.
Max.

 

In the U.K. I was even instructed to wire certain older residential area equipment up with 3 phase due to a particular load (?Load?)!.
Never heard of in N.A.!
Max.

 

There's one issue more that might be related... after some usage, the transformer began to hum; actually, more than a hum it's a slight vibration that you can even feel by touching it, and even hear the whole chassis shacking when it's not in a stable position. It didn't it when I first tried it, I think it began after I connected a regulator the wrong way and think I provoked a short circuit.

Is it normal for small 6VA 15v transformer to vibrate like this?... could it have something to do with the current I felt when I touched the chassis?