How to mesure 240V AC with an oscilloscope

[WarSim]

The author is referring to Plug F in the standards list, a CEE 7/4 and commonly called “Schuko plug”.  Still looking up if they use whole house ground fault protection to explains the possible 1.3v offset.

[WarSim]

Looks like the two prongs are hot and neutral which are treated as interchangeable the clips are PE Ground.  So if there is a difference it is either ground fault protection, poor house ground,  noise expressed as an offset on a poor DMM, disappointing protection method on the scope, or something else just as odd. 

Apparently your country is not concerned about the safety issues addressed by standardizing mains polarity.  Apparently your country doesn't have a polarity convention.  Either post could be the hot.  I have not found a code book for your country to find out what your neutral reference is if it is.  Believe it or not some countries depend on the home service losses to ensure line offsets.  Some limits are 2400 unloaded.  This is one of the reasons Fluke DMMs have 3000v protection and limits. 

Only safe way to test phase and offset on your mains is get a 100:1 or 1000:1 three channel differential probe.  Be sure it has cat 3 protection.  A bit expensive. 

If you are just concerned about wave formation only then use a 100:1 or 1000:1 two channel differential probe with AC coupling with cat3 protection.  Less expensive.  Ground clips are connected together not to the socket

If you don't mind the calculating the neutral offset error yourself you can use an AC coupled 100:1 or 1000:1 probe with Cat3 protection.  This will only give you an estimation of wave form and treat both as hot until you figure out which is which by testing.  Ground clip is not connected to the plug, which causes an error but keep your scope from trying to pull your house to any reference.  Least expensive. 

I said 100:1 or 1000:1 cap because it is unlikely you will find a 100:1 with cat3 protection. 

[WarSim]

Oh yes a cheat for the CAT3 protection is use a battery operated scope but I personally do not trust the case isolation of specs of any of them.  Too many spec the physical case but ignore the isolation of the controls and screen. 

Until someone can confirm how you house mains is referenced at lease cat3 protection is needed. 

Even using a transformer method the re-reference the mains is not a good idea if you can not confirm that the transformers isolation limits are greater than the reference offset limits of your mains.  Not to mention you will only be able to measure a wave form estimation due to transformer action. 

[othello]

@WarSim

I'm going to double check what in the Netherlands is common for households. The sockets in the wall are grounded, 3 wires from which 1 is earth (aka ground ?) and the other 2 are hot because polarity doesn't matter.  I use a Fluke DMM to make measurements btw. I'm now going to check see if i can find specification in english to reference to, brb

[Ribster]

Apparently your country is not concerned about the safety issues addressed by standardizing mains polarity.  Apparently your country doesn't have a polarity convention.  Either post could be the hot.  I have not found a code book for your country to find out what your neutral reference is if it is.

I live in the country next to the netherlands and i can confirm that we do not give anything about polarity. The euro plug has a variant with earthwire and a variant without (class II as it is called). You could get fixed polarity with the connector that has an earth pin. Fixed color coding for cablling (blue & brown), it can be easily be reverse connected inside the wallsocket. In Belgium we measure the difference in current going in and out. If there is a leakage to ground, the differential picks it up and shuts off. It's TN/S (Earthing neutral separated).

[Ribster]

Reading your post, i would discourage you to do anything with mains voltage.
If you do want to power something from mains, for your own safety, use a separating transformer (Dutch: Scheidingstransformator).
It's a simple transformer that converts 220V to 220V. So a transformation of 1:1.
This is a safer situation. If you short out the secondary, the consequences will be less dangerous.

Thanks, i read indeed about isolation transformers. Their secondary is still 220V AC but ungrounded ?

The chassis of the transformer is connected to ground. The wires with the voltage difference of 220VAC will never be connected to ground. There is a neutral wire, that is connected to the starpoint of the distribution transformer in your street. The 'hot' wire will be connected to a phase of that distribution transformer. What you do with the isolation transformer, is disconnecting yourself from the distribution transformer in your street. The connection between the primary of the isolation transformer and the secondary is effectively air and isolation. There is no metallic connection (aka galvanically isolated). So when you short the secondary of that isolation transformer, things will not go as bad.

[othello]

Apparently your country is not concerned about the safety issues addressed by standardizing mains polarity.  Apparently your country doesn't have a polarity convention.  Either post could be the hot.  I have not found a code book for your country to find out what your neutral reference is if it is.

I live in the country next to the netherlands and i can confirm that we do not give anything about polarity. The euro plug has a variant with earthwire and a variant without (class II as it is called). You could get fixed polarity with the connector that has an earth pin. Fixed color coding for cablling (blue & brown), it can be easily be reverse connected inside the wallsocket. In Belgium we measure the difference in current going in and out. If there is a leakage to ground, the differential picks it up and shuts off. It's TN/S (Earthing neutral separated).

Thank you, i believe your explaination will help to clarify the situation in the Netherlands for WarSim and others. So i hope now it is more clear why i mention 3 wires, of those 2 are hot (polarity doesn't matter) and 3rd wire is earth. So based on this information and using a 100X probe to display the waveform on the DSO it is a matter of connecting such probe to 1 of the hotwires and ground the probe to the earth wire ?

PS: again, i'm not going to attempt it, just want to learn more about this like the how / why / what / where stuff

@WarSim / Others

Not sure what you mean with standardizing polarity for mains AC ? I mean AC doesn't have polarity as far as i know. So how can it be standardized like if it was DC as in - and + ?

[WarSim]

Reading your post, i would discourage you to do anything with mains voltage.
If you do want to power something from mains, for your own safety, use a separating transformer (Dutch: Scheidingstransformator).
It's a simple transformer that converts 220V to 220V. So a transformation of 1:1.
This is a safer situation. If you short out the secondary, the consequences will be less dangerous.

Thanks, i read indeed about isolation transformers. Their secondary is still 220V AC but ungrounded ?

The chassis of the transformer is connected to ground. The wires with the voltage difference of 220VAC will never be connected to ground. There is a neutral wire, that is connected to the starpoint of the distribution transformer in your street. The 'hot' wire will be connected to a phase of that distribution transformer. What you do with the isolation transformer, is disconnecting yourself from the distribution transformer in your street. The connection between the primary of the isolation transformer and the secondary is effectively air and isolation. There is no metallic connection (aka galvanically isolated). So when you short the secondary of that isolation transformer, things will not go as bad.

Ok so disembarkation referencing so if they limit tailing to 500m then offset should be below the lower safety limit of 1250v.  If you are assured of a shorter tailing the isolation limit can be lower. 

Here we used distribution and disembarkation referencing and base our safety limits on the second failure point.  In short we use 600v as our safety, because our tailing (drop + distribution) is limited to less that 250m. Also the reason why any main termination rated transformer in North America is rated for 600v isolation as a minimum. 

[othello]

Finally found something which clearly makes a difference between what might be common in the US or in Europe !

http://www.eetimes.com/document.asp?doc_id=1272972

For example:
"
Myth: A computer treats the neutral wire differently from the hot wire.

International office product safety regulations (including IEC 950 and UL 1950) prohibit these wires from being treated differently. Each is considered a safety hazard under the regulations and is required to be disconnected from and have appropriate safety spacings from logic circuits. Furthermore, inspection of the wiring schematics of any computer equipment clearly shows that the input hot and neutral wires are connected to the same circuits in the same ways and are interchangeable. Belief in this myth is very uncommon in Europe since anyone can see that a European plug can be reversed.
"

That is what i mean with the 2 hot wires, they're both hot / dangerous and only the 3rd wire is earth.
Not sure how it is in the US but i see people talk about mains AC and mention HOT and NEUTRAL and GROUND (EARTH), so this makes it confusing since AC has no polarity.
So the question is, what is this NEUTRAL wire ? Why call it NEUTRAL at all when it comes to mains AC ?

PS: I'm reading / learning more about this but any info is always appreciated guys !

Think i found it explained, i will paste the relevant part so other newbies can understand this thing better:

"...
The reason that one of the power wires is named "neutral" is because it is connected directly to the building ground connection at the circuit breaker panel. Therefore it is connected directly to the grounding (third) wire. In essence, then, two of the three wires at the wall receptacle are actually grounded wires, one being used for power flow, and the other connected only to exposed metal parts on the equipment. The power wire that is grounded is called the "neutral" wire because it is not dangerous with respect to exposed metal parts or plumbing. The "hot" wire gets its name because it is dangerous.

The grounding of the neutral wire is not related to the operation of electrical equipment but is required for reasons of safety. To reduce the chance of electrical shock, it is important to provide a means to automatically shut down the electrical circuit if an exposed metal part becomes accidentally connected to a hot conductor or circuit. This is accomplished using the 3-wire system by an ingenious technique:

Every electrical circuit is protected by a circuit breaker. The purpose of the circuit breaker is to prevent the building wiring from overheating as a result of excessive user loads being plugged in. However, in the 3-wire system, the circuit breaker provides another critical safety function. If a hot wire or circuit were to become accidentally connected to an exposed metal part on a piece of equipment, then a shock hazard would exist. However, if the exposed parts are connected to the grounding wire, then the hot wire becomes connected to the grounding wire. This would not cause anything unusual to happen except for the fact that the second power wire, the neutral wire, is also connected to the grounding wire at the circuit breaker panel. Therefore, for this safety hazard the grounding wire essentially becomes connected as a load. The low resistance of the grounding wire causes it to draw a very large amount of current when it becomes inadvertently connected as a load, which in turn causes the circuit breaker feeding the hot wire to trip. Therefore, the 3-wire system operates in a manner which transforms a safety hazard into an over current condition, causing the safety hazard to be automatically cleared by the circuit breaker. The circuit breaker is used as both an over current protector and a shock hazard protector.
...
"

[Richard Crowley]

It is called "neutral" because it is the "low" side of the power.  At least here in North America, back in the circuit-breaker distribution box, each branch circuit white-wire "neutral" is actually hard-wired to green-wire safety ground.  And then typlcally connected locally to physical ground/earth.  A copper water pipe, or an actual ground-rod driven into the crust of the planet.

The typical mains service to a home or small office is 240V center-tapped and the center-tap is grounded.  So you have two 120V "phases", each of which is referenced to ground.  The grounded side of the branch circuit is called "neutral" because it is designed to be nominally at ground-potential.  120V branch circuits take their power from one or the other "phase".  High-power loads will use BOTH "phases" for the full 240V experience.  But that 240V is center tapped and grounded.

It is interesting to see how mains power distribution evolved in different countries.  I was the engineer for a musical group that performed on the Queen Mary just after it was opened as a "floating hotel".  The entire steel hull of the ship was both "green-wire" safety-ground, AND "white-wire" branch circuit neutral because the power distribution was single wire only, and they just used the metal hull as the return path.  The guitar players had an interesting time with those goofy 2-way power switches that would put a small capacitor to one side or the other of the mains plug (back before the plugs were polarized, even the un-grounded 2-prong ones.)

[vk6zgo]

Finally found something which clearly makes a difference between what might be common in the US or in Europe !

http://www.eetimes.com/document.asp?doc_id=1272972

For example:
"
Myth: A computer treats the neutral wire differently from the hot wire.

International office product safety regulations (including IEC 950 and UL 1950) prohibit these wires from being treated differently. Each is considered a safety hazard under the regulations and is required to be disconnected from and have appropriate safety spacings from logic circuits. Furthermore, inspection of the wiring schematics of any computer equipment clearly shows that the input hot and neutral wires are connected to the same circuits in the same ways and are interchangeable. Belief in this myth is very uncommon in Europe since anyone can see that a European plug can be reversed.
"

That is what i mean with the 2 hot wires, they're both hot / dangerous and only the 3rd wire is earth.
Not sure how it is in the US but i see people talk about mains AC and mention HOT and NEUTRAL and GROUND (EARTH), so this makes it confusing since AC has no polarity.
So the question is, what is this NEUTRAL wire ? Why call it NEUTRAL at all when it comes to mains AC ?

PS: I'm reading / learning more about this but any info is always appreciated guys !

Think i found it explained, i will paste the relevant part so other newbies can understand this thing better:

"...
The reason that one of the power wires is named "neutral" is because it is connected directly to the building ground connection at the circuit breaker panel. Therefore it is connected directly to the grounding (third) wire. In essence, then, two of the three wires at the wall receptacle are actually grounded wires, one being used for power flow, and the other connected only to exposed metal parts on the equipment. The power wire that is grounded is called the "neutral" wire because it is not dangerous with respect to exposed metal parts or plumbing. The "hot" wire gets its name because it is dangerous.

The grounding of the neutral wire is not related to the operation of electrical equipment but is required for reasons of safety. To reduce the chance of electrical shock, it is important to provide a means to automatically shut down the electrical circuit if an exposed metal part becomes accidentally connected to a hot conductor or circuit. This is accomplished using the 3-wire system by an ingenious technique:

Every electrical circuit is protected by a circuit breaker. The purpose of the circuit breaker is to prevent the building wiring from overheating as a result of excessive user loads being plugged in. However, in the 3-wire system, the circuit breaker provides another critical safety function. If a hot wire or circuit were to become accidentally connected to an exposed metal part on a piece of equipment, then a shock hazard would exist. However, if the exposed parts are connected to the grounding wire, then the hot wire becomes connected to the grounding wire. This would not cause anything unusual to happen except for the fact that the second power wire, the neutral wire, is also connected to the grounding wire at the circuit breaker panel. Therefore, for this safety hazard the grounding wire essentially becomes connected as a load. The low resistance of the grounding wire causes it to draw a very large amount of current when it becomes inadvertently connected as a load, which in turn causes the circuit breaker feeding the hot wire to trip. Therefore, the 3-wire system operates in a manner which transforms a safety hazard into an over current condition, causing the safety hazard to be automatically cleared by the circuit breaker. The circuit breaker is used as both an over current protector and a shock hazard protector.
...
"

Electricity authorities are deeply distrustful folk,& assume that either,some sockets will be wired with Live & Neutral reversed,or that you may be in a country where the plug can be reversed.

Accordingly,they require the same level of insulation for both conductors,so that if what would normally be the Neutral connection inside the appliance,with little or no potential to the Earth,& hence Earth/Neutral connection back at the house supply entry point,now becomes the Live connection,there will be no safety or operational problem.

This can be very important,as the following anecdote shows:-

Back in the early 1970s,I was involved with the installation of a Closed Circuit TV  system.
The pan & tilt heads for the cameras were unusual,in that they operated from the Mains,using appropriately rated cable to make the connections between the control room & each such device.

A test box was made up which could be plugged into a nearby GPO,& switched the Live mains leg to each motor in turn.
We had to stand on a ladder & adjust stops which operated microswitches ---these prevented you "panning" or "tilting" your camera lens into a wall or other obstacle.

All went well,till I was adjusting one of these stops,received a nasty shock,fell off the ladder,& ended up on the floor.
On investigation,we found that the manufacturer had insulated the "Live" wiring well,but hadn't worried about the "Neutral".

I,of course,had "hit the jackpot"----a GPO  with reversed connections!

That said,as far as the power outlet is concerned,with a correctly wired normal 220v single phase supply,the only "hot" socket pin is the live or "Active" pin.

[WarSim]

The neutral wire is called neutral because the reference on that wire has a neutral reference.  AKA the one attached to a distributed ground.

You plug has two hots because either on could be connected to hot or neutral.  One wire is hot one wire is neutral you just don't know which.

Any myth that states the polarity of a single phase modern appliance is false.  Current code everywhere I have looked prohibits neutral return trough user access able surfaces.  One exception is light bulb sockets.  The hot is suppose to be the deepest pad but many lamps ignore this. 

Lol how exactly did you achieve PE ground on a ship.  . Now on a ship with hardened ring power that system actually is two hot wires. 

[othello]

This stuff gets more interesting by the hour :-) Despite that it's not easy to understand for the untrained !

So if some one touches by accident the neutral wire then nothing will happen ? I assume because the neutral is grounded to the earth (grounding) wire ?
In the US the plugs can only fit in 1 specific way if this hot vs neutral is important for household appliances for example ?

[IanB]

Lol how exactly did you achieve PE ground on a ship.  . Now on a ship with hardened ring power that system actually is two hot wires.

Well if you are inside the ship then the ship's hull is the only neutral potential that exists, surely? So it doesn't really matter how much current you return through the hull, the potential difference between you and the ship is always going to be zero. There is no external ground to reference the voltage against. If you bond everything securely to the hull then I would assume nothing can float to any unreasonable voltages.

[WarSim]

Lol how exactly did you achieve PE ground on a ship.  . Now on a ship with hardened ring power that system actually is two hot wires.

Well if you are inside the ship then the ship's hull is the only neutral potential that exists, surely? So it doesn't really matter how much current you return through the hull, the potential difference between you and the ship is always going to be zero. There is no external ground to reference the voltage against. If you bond everything securely to the hull then I would assume nothing can float to any unreasonable voltages.

I wasn't disagreeing with the hull being the neutral on a ship with a non-hardened power distribution.  What I though was amusing is you referred to it as a ground.  Yes I would have found it amusing if you called it neutral and water also. 

[IanB]

I wasn't disagreeing with the hull being the neutral on a ship with a non-hardened power distribution.  What I though was amusing is you referred to it as a ground.  Yes I would have found it amusing if you called it neutral and water also. 

Ah yes, "ground", I see. I wasn't the original poster, that was Richard Crowley, but apparently I didn't pick up on the joke. Oh well... 

[vk6zgo]

This stuff gets more interesting by the hour :-) Despite that it's not easy to understand for the untrained !

So if some one touches by accident the neutral wire then nothing will happen ? I assume because the neutral is grounded to the earth (grounding) wire ?
In the US the plugs can only fit in 1 specific way if this hot vs neutral is important for household appliances for example ?

It shouldn't if all appliances were fitted with double pole switches.
A lot of older stuff & also cheap newer appliances only have single pole switches.

Another thing is that,if you open the Neutral line of a lamp circuit,motor,transformer primary or similar,you can still get a shock from the Neutral side of the device's circuitry.
This really only of importance to people working on the equipment,but is nevertheless, the reason Australian rules forbid fuses in the Neutral line of devices.

Even with strictly regulated  pinouts,you can get caught out.

The dear old People's Republic sometimes make toasters with the "off" switch in the Neutral line.
.
In these cases,even with no toast in the unit,there is always Live 240v available to zap you.
A proper design uses a double pole switch & disconnects both sides.

The term "polarity"does not really apply to the Live/Neutral relationship.

An AC waveform reverses polarity on each half cycle,so that if you could sample the voltage of Live with respect to Neutral at the peak of each half cycle of a 220v RMS waveform ,it would give readings of  +311v,-311v,+ 311v,-311v & so on (I started on the +ve half cycle for convenience).

You can see that whatever half cycle you are on,a device connected between Live & Neutral will work.
If it is connected between Earth & Neutral,it will not!

[mzzj]

Looks like the two prongs are hot and neutral which are treated as interchangeable the clips are PE Ground.  So if there is a difference it is either ground fault protection, poor house ground,  noise expressed as an offset on a poor DMM, disappointing protection method on the scope, or something else just as odd. 

Apparently your country is not concerned about the safety issues addressed by standardizing mains polarity.  Apparently your country doesn't have a polarity convention.  Either post could be the hot.  I have not found a code book for your country to find out what your neutral reference is if it is.  Believe it or not some countries depend on the home service losses to ensure line offsets.  Some limits are 2400 unloaded.  This is one of the reasons Fluke DMMs have 3000v protection and limits. 

I have no idea about Netherlands but at least here in Finland it is common in older buildings that grounded outlets are wired with "pseudo-grounding" (in lack of better english terminology)
With old "pseudo-grounding" the neutral and ground wire are connected already at the outlet and share same wire to  the distribution/fuse box.  This causes all sorts of trouble, including potential differences between outlet ground connections in different branches because of voltage drop on shared ground/neutral wire! (also a massive safety hazard if the house wiring becomes faulty with loose connection somewhere on the shared neutral/ground wire)

Most of the Europe doesn't have polarity convention, or if there is such it is just based on "old habits". IE here in Finland we would usually wire the "EUROPEAN "SCHUKO" CEE7" with neutral on the left side even that it really doesn't matter since the plug is non-polarized!

[WarSim]

I think the point is being lost here.  The point in single phase home wiring is not to know which wire is neutral. It is to know that one of the wires is referenced and where.  If you need to measure it in someway in relation to PE Ground.  Because most scopes are PE referenced in some way, this is important.  Not to mention the operator is likely PE referenced some how.  Unless of course they are wearing an isolation suit.

As stated if referencing is from the feed you need at least 3000v isolation  if referenced at disembarkation with a long tailing 1250v isolation.  Referenced at disembarkation with short tailing 600v isolation.  Referenced at distribution 600v isolation.  A fully floating long mains feed is a dangerous thing.  And this is the important thing about code.  No mater what safety processes are used. 

The point was to make sure the author was safe not to judge another countries wiring conventions.   

[WarSim]

Huva
I have Finish heritage and many relatives there, so it is good to see Finland represent.  .   Although my family has been out of country for three generations and now considered "outsiders" it's nice. 

[mzzj]

As stated if referencing is from the feed you need at least 3000v isolation  if referenced at disembarkation with a long tailing 1250v isolation.  Referenced at disembarkation with short tailing 600v isolation.  Referenced at distribution 600v isolation.  A fully floating long mains feed is a dangerous thing.  And this is the important thing about code.  No mater what safety processes are used. 

I am not sure what are you referring to with long tailings and isolations?

[WarSim]

Isolation is how much voltage difference will be prevented from arcing over.  Usually given in Category classifications such as cat 1 - 4 protection. 
Tailing is the service wire that connects to the mains receptical.  Specifically the length of wire used from the reference point to the load.  Reference point being where the AC cct is referenced to a global potential.  Which is PE ground in most cases.  Sea basic for ships.  Derived neutral for delta etc.   

[mzzj]

Isolation is how much voltage difference will be prevented from arcing over.  Usually given in Category classifications such as cat 1 - 4 protection. 
Tailing is the service wire that connects to the mains receptical.  Specifically the length of wire used from the reference point to the load.  Reference point being where the AC cct is referenced to a global potential.  Which is PE ground in most cases.  Sea basic for ships.  Derived neutral for delta etc.   

"Ok so disembarkation referencing so if they limit tailing to 500m then offset should be below the lower safety limit of 1250v.  If you are assured of a shorter tailing the isolation limit can be lower.

Here we used distribution and disembarkation referencing and base our safety limits on the second failure point.  In short we use 600v as our safety, because our tailing (drop + distribution) is limited to less that 250m. Also the reason why any main termination rated transformer in North America is rated for 600v isolation as a minimum.  "

umm, sounds like a'murican or boat standards. 

Any sort of appliance transformer here(europe) is rated for 2.5kV and Safety isolation transformers(including "wall-wart" type transformers) have to be rated for 4kV insulation.

[WarSim]

 Quote
Any sort of appliance transformer here(europe) is rated for 2.5kV and Safety isolation transformers(including "wall-wart" type transformers) have to be rated for 4kV insulation.
/quote

Yes that would be about what is required with your power delivery system.  Here the minimum is 600v because of how this power system is setup here. 

Also the key why people here are saying you need a 10:1 probe assuming our system.  Also the reason I started with my replies pushing for 100:1 and the 2.5kv isolation. 

I hope this puts the x10 and x100 discussion to rest. 

[Ribster]

Just use a differential probe and be done with it. Damn .



This is how it's done. TN/S. The earthing and neutral are separated at the transformer side and being brought out.
This is wat everybody means and refers to neutral / hot. Hot being Lx and neutral being N.

Use a differential probe so you aren't referenced to anything, no problems. Dave has some video's using differential probes.