I just realised I don't really understand earth ground!

[IanB]

I don't think ~300 mV is alarming. I have a faulty ground connection in one of my sockets and it measures about 60 V with induced voltages from nearby cables.

The main reason for a good ground rod connection is protection against electric shock; it prevents touchable items in the home floating at voltages significantly above ground.

[ivan747]

I tried testing for continuity, but the beeper in the multimeter would turn on and off like 4 times a second. It wasn't an overload, I'm sure. Must be some residual voltage playing up on me.

Now the only thing that remains is the color mess we have in the service panel. I think tomorrow a real electrician is coming over to check the grounding issue with the heater, but I would like him to check if everything related to electricity is OK because I have lots of concerns. For starters, there's a power inverter wired in a dangerous way, the service box is floating, certain cable's gauge seem too small for the circuit breaker capacity (I saw 60A breakers used with gauge 10 wire, should be 6), split phase 240V outlets might be unfused and with 1 line wired to the power inverter and finally, the color coding (I'm sure that will never get fixed, though  ).

This is electric hell 
I don't even want to touch it, such a mess.

[c4757p]

An electrician came to install a ground lead yesterday, and he bolted it to the service box chassis, but the service box chassis is floating. He says it is grounded because it is embedded into the concrete wall

An electrician? No, an "electrician".

[Things]

I don't think the earth ground is meant to carry full short circuit current if something does short out to a case for example, just enough that it stops mains devices floating like IanB said, say, during a thunderstorm and induced current, but allow enough current to flow to trip a GFI.

Although it probably would have a low enough resistance given a rod deep enough into the ground, it'd only take like 8KOhms resistance to trip a 30mA GFI on 240V, and wet soil is well and truly lower than 8K.

[Monkeh]

I don't think the earth ground is meant to carry full short circuit current if something does short out to a case for example

Yes it bloody well is.

but allow enough current to flow to trip a GFI.

And when you don't have a 'GFI'?

[Things]

but allow enough current to flow to trip a GFI.

And when you don't have a 'GFI'?

Then usually you don't have a ground pin on your sockets at all - very common in the US in older homes.

[Monkeh]

Then usually you don't have a ground pin on your sockets at all - very common in the US in older homes.

Uh, right, okay...

The CPC has nothing to do with the operation of an RCD.

[IanB]

I don't think the earth ground is meant to carry full short circuit current if something does short out to a case for example

Yes it bloody well is.

"Things" is not talking about the earth return in the circuit wiring, he is talking about the impedance of the literal earth beneath your feet. If you stick two metal poles in the ground there won't be a low impedance between them, especially in dry soil.

[Monkeh]

I don't think the earth ground is meant to carry full short circuit current if something does short out to a case for example

Yes it bloody well is.

"Things" is not talking about the earth return in the circuit wiring, he is talking about the impedance of the literal earth beneath your feet. If you stick two metal poles in the ground there won't be a low impedance between them, especially in dry soil.

That's your interpretation.

And yes, I know about earth impedance.

[c4757p]

but allow enough current to flow to trip a GFI.

And when you don't have a 'GFI'?

Then usually you don't have a ground pin on your sockets at all - very common in the US in older homes.

Most US homes don't have GFCIs on most outlets - just ones near water. Most US homes are fully outfitted with grounded sockets.

[c4757p]

"Things" is not talking about the earth return in the circuit wiring, he is talking about the impedance of the literal earth beneath your feet. If you stick two metal poles in the ground there won't be a low impedance between them, especially in dry soil.

Maybe so, but in this specific case, that impedance is being depended upon to carry protective earth currents. Shoving a box in concrete and saying it's earthed means you're depending on the path through the concrete and all the way around to the distribution board to neutralize any faults.

[4to20Milliamps]

Surprising that this discussion keeps lingering on, there's a reason some appliances don't have a ground connection.

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

We have a laboratory here that tests and labels things called underwriter's laboratory, everything has to be listed and labeled.

All modern electrical installations have a three wire grounding type receptacle, the equipment grounding conductor is sized according to the over current protection device<fuse, breaker, whatever.

http://www.stabiloy.com/NR/rdonlyres/19FC4111-742D-47B7-9A28-06DF8AB4023D/0/EquipmentGroundingConductor.pdf

electrical systems have become much more safe over the years, equipment grounding is used to clear faults on the equipment, ground fault protection is used to prevent electrocution for personnel.

http://en.wikipedia.org/wiki/Residual-current_device

[Monkeh]

All modern electrical installations have a three wire grounding type receptacle, the equipment grounding conductor is sized according to the over current protection device<fuse, breaker, whatever.

All conductors are sized to suit the protective device, and it's a bit more involved than 'this carries 20A without getting warm', too.

[IanB]

The problem with using the word phase when talking about AC power is that you can't start calling every transformer with a  tap as having multiple phases , there is only one wave of power moving through the coil from the start of the winding to the end of the winding, that is one phase, no matter how many taps you put on the winding you will still only have one phase moving through the winding till it reaches the end.

You forget that polarity reverses if you swap the measurement points. With the neutral as the center tap on the transformer winding you will get opposite polarities if you measure up to one end of the winding or down to the other end. With a regular sine wave symmetry means that a reversal of polarity corresponds to a 180 degree phase shift. This is illustrated in the diagram below:

[megajocke]

The problem with calling that configuration 2-phase, even though it might make sense, is that this term is already understood to mean something different:
http://en.wikipedia.org/wiki/Two-phase_electric_power

The configuration is therefore called split phase (or single-phase 3-wire):
http://en.wikipedia.org/wiki/Split-phase_electric_power

One important distinction is that 2-phase is a true polyphase system which can create a rotating magnetic field and will deliver non-pulsating total power to a linear symmetric load. Converting between different split-phase systems (for example 2,3 or maybe even 5-phase) can be done using nothing but transformers.

Single phase (whether split or not) can not be converted to polyphase power using only transformers.

[IanB]

I see, so it's a matter of terminology rather than mathematics. In engineering terms two phase power refers back to a system where the two phases were 90 degrees apart rather than 180 degrees with split phase. But awkwardly, the 90 degree configuration does not have the nice symmetrical properties that the split phase or three phase systems have, such as non-pulsating power delivery and the ability to eliminate the neutral wire with balanced loads (the Wikipedia article referenced above says that two phase power pulsates at twice the line frequency).

I am still going to say there are two phases present in a split phase system since there is a phase angle, even if electrical engineers don't want to call it two phase power for historical reasons.

[G7PSK]

Quote from Wikipedia "Three-wire, 120/240 volt single phase power used in the United States and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase. The two live outputs of a 3-wire single phase transformer secondary winding are properly called "legs".

And as show in the wave form diagrams kindly provided by IanB the two halves of the split phase add up to a single phase at twice the voltage.

[IanB]

And as show in the wave form diagrams kindly provided by IanB the two halves of the split phase add up to a single phase at twice the voltage.

Note carefully from my earlier diagram it is not the sum of the phases in a split phase supply that gives a single phase at twice the voltage, it is the difference between them. When you put your voltmeter between L1 and L2 and see 240 V you are measuring the difference between the two 120 V legs.

This is illustrated in the diagram below, with a comparison to the three phase case. In the three phase case the difference between two phases gives 1.732 times the line voltage (208 V in the case of 120 V line voltage). The value 1.732 is of course the square root of 3.

[iamnothim]

epic

[G7PSK]

I understood that the US domestic voltage is 120 and 240 indicating that there is no phase difference between the transformer taps or legs, certainly that is what I have seen when in the US. Also the pole transformers are single phase input with one live wire input and a common neutral earth which is also shared with the LT side of the transformer, on occasions the 120/240 wires are tied onto the neutral/ earth for support. I took some photographs of this in 2001 when I first visited the US I have seen much the same setup in all the parts I have visited. How do you get two phases out with only one phase in.

[IanB]

I understood that the US domestic voltage is 120 and 240 indicating that there is no phase difference between the transformer taps or legs, certainly that is what I have seen when in the US. Also the pole transformers are single phase input with one live wire input and a common neutral earth which is also shared with the LT side of the transformer, on occasions the 120/240 wires are tied onto the neutral/ earth for support. I took some photographs of this in 2001 when I first visited the US I have seen much the same setup in all the parts I have visited.

Sometimes US electrical arrangements can look pretty hokey, can't they?

How do you get two phases out with only one phase in?

It's because the phase of an AC voltage is inverted (or shifted 180 degrees) if you reverse the connections. With a centre-tapped transformer secondary as seen in the US you have your common neutral/ground reference in the middle. Then if you go "up" one leg you get a phase angle of 0 say, and when you go "down" the other leg you have reversed direction giving you a voltage that is 180 degrees out of phase. You can see this in my picture in post #63 where the red wave between L2 and N is "upside down" compared to the black wave between L1 and N.

If you wanted two voltages that were exactly in phase you would have to take one between L1 and N and the second between N and L2 (going "down" a leg each time). But this arrangement would not give you a common neutral and would not provide a single grounding point. To get two outputs with the same phase and a common neutral you would have to do something like this:



You could no longer connect between L1 and L2 to get twice the voltage, but now you could put L1 and L2 in parallel to get twice the current. This is not how things are usually arranged in power distribution transformers, but it is sometimes seen in low voltage applications.

[G7PSK]

I have a site transformer which gives 110 volts on the secondary with an earthed center tap so that you have 55 V + 55 V I will put it on the scope and have a look but I am pretty certain it is only a single phase output.
Even if the US transformers give 2 phase out puts they are only used as single phase of either 120 or 240 surely where would 2 phase be in actual use.

[johnwa]

I see, so it's a matter of terminology rather than mathematics. In engineering terms two phase power refers back to a system where the two phases were 90 degrees apart rather than 180 degrees with split phase. But awkwardly, the 90 degree configuration does not have the nice symmetrical properties that the split phase or three phase systems have, such as non-pulsating power delivery and the ability to eliminate the neutral wire with balanced loads (the Wikipedia article referenced above says that two phase power pulsates at twice the line frequency).

I don't think this is quite right about the non-pulsating power supply. I believe a two phase supply at 90 degrees will give a continuous flow of power (into a suitably balanced load), since sin^2(x) + cos^2(x)=1. This is the property that allows conversion between polyphase systems with transformers that megajocke referred to.

A true polyphase system can deliver power continuously, while a single or split phase supply gives power intermittently in pulses.  Therefore, in order to synthesize three phase power from single (or split), some form of energy storage is required - big capacitors in a VFD, or the inertia of a rotary phase converter.

You are right about the neutral current not cancelling in a two phase system IanB. Three phase is the lowest number that gives continuous power and no neutral current.

[G7PSK]

I have realised that there is an anomaly created by the scope connections if you put the earth connection on the center tap and the probes on the two end taps you have inversion of on or other of the halves, if the scope earth is connected to say the bottom leg and the probes to the next two legs the waveforms are in sync. I am not sure why this perhaps someone with more scope experience can throw some light on this and explain the correct way to connect in such cases.
I am attaching some photos (not very good exposures are slow).

[megajocke]

I see, so it's a matter of terminology rather than mathematics. In engineering terms two phase power refers back to a system where the two phases were 90 degrees apart rather than 180 degrees with split phase. But awkwardly, the 90 degree configuration does not have the nice symmetrical properties that the split phase or three phase systems have, such as non-pulsating power delivery and the ability to eliminate the neutral wire with balanced loads (the Wikipedia article referenced above says that two phase power pulsates at twice the line frequency).

I am still going to say there are two phases present in a split phase system since there is a phase angle, even if electrical engineers don't want to call it two phase power for historical reasons.

Yes, it's most definetely only a matter of terminology.

Actually, that article on Wikipedia says "Power transfer in a three-phase system with balanced loads is constant, whereas it pulsates at twice the line frequency in single-phase systems.", so that statement is actually not about two-phase (90 degree) systems. Two-phase has nonpulsating power as can be seen from sin^2(x) + cos^2(x)=1 as johnwa wrote.

If you want to be mathematically consistent, it might make sense to call split-phase (the center tapped transformer) 2-phase just as you want, and rename the traditional configuration known as 2-phase to "4-phase without neutral".