An electrical question

[Alareth]

If AC current has no polarity why are plugs in the US polarized?

[GodMark2]

If AC current has no polarity why are plugs in the US polarized?

Because AC has polarity. [/smartass mode]

The two active lines are not equal. One is considered 'Hot'. That line is the one that is driven from +120V to -120V. The other is 'neutral' or 'return' it is actually not driven.

Now, reversing the lines will not make any difference to most circuits, until you remember that the local ground (literally, the dirt itself) exists. The Neutral line has no driving voltage on it, so connecting that line to the local ground will have little efffect, but connecting the Hot line to local ground will cause current to flow. If you happen to be touching one of the lines with one hand, and the ground with the other, you'd feel much better having the neutral line in your hand.

Now, the Neutral line isn't completely voltageless (it builds up a little due to the resistance of the wire between the generator and your hand and the current passing through), but it's much closer than the Hot line. So polarizing the plugs is a safety isue.

[Zombified]

One side is "neutral" which means its near ground and doesn't vary (very much). The "hot" wire does vary above and below both ground and neutral by 120V. Neutral is the high-current return, but ground is the voltage reference.

There are also 3-phase 240 volt lines with one neutral and two hots. The two hots vary by 120V from neutral but have opposite phase, so they are a total of 240V apart (on average; these are AC after all). You can split such a circuit into two 120V circuits by using one hot and the neutral.

Usually for safety reasons if you only switch one wire of a 120V circuit, you want the switch on the hot side.

Also, if I recall correctly, polarized circuits have higher current carrying capacity, so even if you don't care which wire is hot and which neutral, using a polarized plug means your circuit requires/handles the higher current. You can plug an unpolarized plug into a polarized receptacle, but not vice versa.

[phildonnia]

The term "polarized" is a bit misleading; it is perfectly possible for two "polarized" appliances (plugged into different outlets) to have opposite polarizations.

As I understand, non-grounded appliances are required by UL to be double-insulated, with a conductive enclosure between the insulations connected to the neutral. This requirement would make it necessary to insert the plug the right way; hence the plug design.

Polarized plugs do not indicate higher current capacity, although there is a special outlet for appliances needing >20A, which is also compatible with the standard plug.

The neutral is not necessarily "safe", since perverted bastards have been known to put a switch on the neutral side of the load, which would make it hot if and only if the switch were off!

[stormer]

--snip--
There are also 3-phase 240 volt lines with one neutral and two hots. The two hots vary by 120V from neutral but have opposite phase, so they are a total of 240V apart (on average; these are AC after all). You can split such a circuit into two 120V circuits by using one hot and the neutral.
--snip--


Are you sure of this? I looked on the net for this and could only find "dual polarity 120Vac giving 240Vac". So I am a bit confused as 3 phase means something else where I live.

In my country, 3 phase means 3 live wires, with one neutral. The live wires are offset 120 degrees from each other. The difference between any live and the neutral is 240Vac, and the vector differerence between one live and the other two is 415Vac.

[elgarak]

Stormer,

Welcome to the wonderful world of US electric systems. What Zombified described is "240V centertapped single phase", which requires indeed two hots, one neutral and one ground.

This whole zoo of cabling and varied plugs makes shopping for (large) electrical appliances very interesting.

[stormer]

Thanks elagrak. I am used to the 240Vac center tapped single phase - a lot of generators used in boats are wired like this. Makes it easier to sell the genset in 110Vac markets, as well as 240Vac markets. (The frequency is changed by running the genset at 3600 rpm or 3000 rpm, just in case you are wondering)

I was just confused by the "it is 3 phase" statement.

[Alareth]

Thank you for your answers. It's just something that has always bugged me.

[kevin]

Neutral is the high-current return, but ground is the voltage reference.
Not sure what you mean by this, but the ground wire is not actually used in circuits, it's a safety device to provide an alternate path to ground in the case of a short to case situation. If you open up your circuit box (ok, that really isn't recommended) you'll find the grounds and neutrals are actually tied together.

There are also 3-phase 240 volt lines with one neutral and two hots.
That's usually called split phase. This isn't available from the power lines in the US. The US produces 3 phase power that is 120 degrees between phases. Because of this the best you can do with split phase is 208 volts, not 240.

True 3 phase is 3 hots and usually a neutral, but not the neutral isn't always needed.

http://en.wikipedia.org/wiki/Three-phase_electric_power

[ktesibios]

The "neutral" conductor, which the NEC calls the "identified circuit conductor" (it and the equipment grounding conductor are the only wires whose color is prescribed- a neutral wire must be either white or "natural" colored plastic and a grounding wire must be either green or bare- wires with any other color insulation are assumed to be hot) is connected to ground at the service entrance. In an ordinary residential entrance panel- your breaker box- a heavy wire runs from a bussbar in the panel to your grounding rod; the neutral conductor coming in connects to this bussbar and so do the neutral and grounding wires for all your branch circuits.

The purpose of grounding one conductor is to help drain off any static charges with respect to ground which might be induced on the wiring coming into your house and to provide some modicum of protection against excessive voltages WRT ground in the event of a lightning strike or a fault from the high voltage distribution line feeding the transformer which supplies you with low-voltage AC to the low-voltage circuit.

The neutral side of a 120VAC outlet will normally be within a few volts of ground potential; there's often a small potential difference between the neutral and ground due to the voltage drop along the neutral conductor due to the current it's carrying. The grounding conductor should be at the same potential as your ground, since the only time it's permissible for the grounding conductor to carry current is in the event of a fault.

The equipment grounding conductor does exactly what it sounds like- it provides a low-resistance connection to the local ground from the frame of an appliance plugged into a grounding outlet.

Because the voltage difference between the neutral and hot conductors is 120VAC and the neutral is connected to ground back at the service entrance, the hot wire will have a voltage of 120VAC with respect to ground.

If there should be a short from the hot side of the AC line to the frame of an appliance, anyone touching the appliance while in contact with an electrical ground would find a potential difference of 120VAC between those two points on his body- and experience a ZAP!

With the appliance frame connected to ground via the equipment grounding conductor, two things happen- the potential difference between appliance and earth ground is limited because the two are bonded together by a low-resistance connection, and a fault current will flow from the hot conductor to the appliance frame and back to the neutral/ground buss via the equipment grounding conductor. If the grounding conductor is good, this current will normally be great enough in magnitude to trip the circuit breaker feeding that branch circuit.

On a NEMA standard 5-15R receptacle- the one used for standard 15A 120V household circuits- the neutral wire is always connected to the wider straight contact, the hot to the narrower straight contact and the grounding wire to the D-shaped contact. On a NEMA 5-20R, which is used for 20A 120V circuits, the female neutral contact is T-shaped to accomodate both the ordinary 15A plug and the 20A plug, which has the neutral blade rotated 90 degrees, but the hot contact is still narrower than the neutral contact.

The point of prescribing which conductor is connected to each contact on the AC recptacle is that in this way the designer of anything which draws power from an AC outlet knows which of the wires coming into that piece of equipment is connected to which AC circuit conductor.

With a 3-pin grounding plug, there's obviously only one way it can be inserted into the recptacle, but with a 2-pin plug it would be possible to insert it in two different ways. Making one plug blade and one receptacle contact wider than the other prevents this.

It would be a pretty dumb appliance designer who allowed a connection between either AC current-carrying conductor and any exposed metal part of the appliance or who failed to insulate one side of the AC circuit as well as the other, but there's still a possibility of an appliance with a 2-wire power cord creating a risk of the user touching a part which is connected to one side of the AC line. An ordinary table lamp is an example.

With the bulb removed, it's much easier to touch the shell contact of the bulb socket than the center contact. With the bulb partially unscrewed, touching the exposed metal part of the bulb's base is the same as touching the shell contact of the socket. A polarized plug on the lamp's AC cord ensures that if it's plugged into a properly wired receptacle, the shell contact will be connected to the neutral side of the line, making it much less likely that someone changing the bulb could come in contact with a point that's hot with respect to ground.

RF bypass capacitors in some home entertainment equipment could also produce leakage current from the hot side of the AC line to the chassis if the AC cord is plugged in backwards.

Now, about 120/240VAC circuits- these are not 3-phase. 120/240V "split-phase" service is the most common configuration for residential use in the USA. In this system, three wires enter your house from the pole transformer outside. The low-voltage winding of this transformer is center-tapped so that the voltage at each end with respect to the center tap is of opposite polarity to the voltage at the other end. IOW, the voltage between one "hot" and the neutral is 120VAC, the voltage between the other hot and neutral is 120VAC and the two are out of phase with each other. The result is that the voltage between one hot and the other hot is 240VAC.

If the current drawn from one hot is equal to the current drawn from the other hot, the current in the neutral wire, which is the algebraic sum of the two, is zero.

In a three-phase circuit, which is relatively rare in residences but common in commercial buildings, there are 3 hot conductors and one neutral. The voltages on the three hot conductors are 120 degrees out of phase with each other. If the voltage between one hot and the neutral is 120VAC, the voltage between any two hots is 120V * sqrt(3), or 207.84VAC. This is called a 120/208V Y(or star)-connected circuit.

As with the split-phase circuit, if the loads connected between each hot and the neutral are equal, the neutral current is zero.

This provides quite a bit of flexibility- you can have ordinary 120VAC circuits for small appliances, 208VAC or 120/208V 3-wire circuits for appliances needing more power and 208V 3-phase circuits for running induction motors, all from the same electrical service.

These wikipedia articles might make all this clearer:

http://en.wikipedia.org/wiki/Electrical_outlet (there sure are a lot of different kinds of outlet in the world)

http://en.wikipedia.org/wiki/Split_phase (diagram of the 120/240V configuration)

http://en.wikipedia.org/wiki/Three_phase (no diagrams but the math is there)

[kevin]

Thank you for your answers. It's just something that has always bugged me.

There is a difference between the power itself being polarized and the connectors being polarized. Recent (last 20 years?) outlets in the US are called polarized, with one slot being larger than the other. This is to ensure the plug is inserted a particular way. This can be important on some devices, especially those with built in circuit breakers.


Hot -------> Device ------> Neutral

Current will flow in the direction of the arrows. If you're building a safety device that halts the current when a problem happens you want it to be on the hot side of the circuit.

If you put a switch on the neutral, under normal conditions the device will not operate because current can no longer flow


Hot -------> Device ---/ ---> Neutral

But if a problem occurs, say the device shorts to the case


Hot -------> Device ---/ ---> Neutral
|
Person
|
Ground
current will flow through the device, through the person to ground. That would be bad.

If you put your switch on the hot side, then this problem won't occur

Hot ----/ ---> Device ------> Neutral
|
Person
|
Ground

Now a fault in the device can't still electrocute someone. But in order to ensure this works properly you have to make sure the plug can't be reversed. The only way to guarantee that with a two prong plug is to make the plug polarized so it can only be plugged in one way.

[Zombified]

I stand corrected on terminology - most of my knowledge of electricity is low voltage stuff, and I'm not surprised when I don't get high(er) voltage stuff quite right.

(Though another forum I was involved in had a participant who worked for a power company and considered anything below 400V "low".)

[ktesibios]

The National Electrical Code draws the "high voltage"/"low voltage" dividing line at 600V, so a 277Y480V circuit in an industrial plant would be considered "low voltage". That's something I would definitely not want to get bitten by.

The IEC puts the dividing line at 1000VAC or 1500VDC. Yikes!

OTOH, the Fluke DMM I use at work shows an "HV" warning in its display when measuring any voltage greater than 40V AC or DC.

I suppose it depends on what you're accustomed to working on.

And I shoulda mentioned that "polarized" refers to the connector and not the current.

[stormer]

In an ordinary residential entrance panel- your breaker box- a heavy wire runs from a bussbar in the panel to your grounding rod; the neutral conductor coming in connects to this bussbar and so do the neutral and grounding wires for all your branch circuits.



Could you just confirm the statement above, that the Neutral and the Ground are connected together in the breaker box for wiring done in the US?

Reason is because it is never done here. The Ground goes to a ground spike, and the Live and Neutral go back to the Mains supply. (UK style plugs and sockets, this is a Commonwealth country)

We no longer use Earth Leakage Circuit Breakers (ELCB) but now only use Residual Current Devices (RCD, or Residual Current Circuit Breaker, RCCB). Could this be the reason why?

[davefoc]

The safety grounds and the neutrals are absolutely tied together in the circuit breaker panel in American electrical systems as far as I know. I was an electrical engineer and have done a moderate amount of house wiring work. I have never seen any other arrangement.

As to the issue ELCB and RCB's. I had to look these terms up. I was only familiar with the American acronym GFI or GFCI. According to wikipedia that is what RCB's are. ELCB's seem to be a device that I was unfamiliar with that would shut a circuit off if an excessive ground current was detected. A GFI works by monitoring the current on the neutral and hot line. If they aren't the same within a preset limit the GFI trips. The idea is that if current is leaving the hot side and not going through the neutral then it is going somewhere else and that somewhere else is possibly a person getting electrocuted.

As to the difference in grounding technique being the result of RCB's versus GFI's. Apparently not, as they seem to be a different name for the same thing. Also, note that it is not necessary to connect the ground of a GFI outlet to ground for the GFI outlet to function properly. The decision to trip is based just on the current difference between the hot and the neutral line and a ground connection is not required for that.

I am not completely sure that there is a substantive difference between UK safety grounding and US safety grounding from your description. In the US the ground of the panel is tied to a grounding rod I believe and this as I understand is what happens to the safety ground in the UK. I didn't think there is a neutral in the UK power distribution system the way there is in the US system so there is no neutral to tie to the safetyground in the electrical panel. Aren't both the conductors in the UK power distribution system hot?

One thing that I might mention is that there are several ways that a neutral wire ends up with a significant voltage on it. Among the ones I can think of are.

1. Somebody screwed up. The hot and neutrals are reversed.

2. There is device connected to the power line that is feeding current into the neutral line creating a voltage because of the IR drop of the neutral line back to the panel.

3. A practice that I personally don't like, but that seems to have been used in older installations, at least, is to use a common neutral line for two opposite hots. This saves wire but when the circuit breaker is switched off for one of the hots and not the other there can still be current running through the neutral.

And what can make possibilities 2 and 3 particularly dangerous is if the neutral line has been disconnected back to the panel. Now the full voltage can be fed to the neutral circuit through a device (like a lamp) that is plugged in and from the neutral circuit to you and through you to ground.

[stormer]

Aren't both the conductors in the UK power distribution system hot?


No, they aren't. A meter between Ground and Neutral measures 0V

Well, I cannot really comment on the UK actually, since I am not there. This country is a former British colony so the electrical system is "UK style".

So the US has 110Vac single phase, 220Vac split single phase, and 120/208Vac 3-phase. Wow.

[davefoc]

So the US has 110Vac single phase, 220Vac split single phase, and 120/208Vac 3-phase. Wow.

The home situation is quite a bit simpler.

A standard service for a home is split phase 220 VAC. Almost all the outlets and lights are wired with a neutral and one of the two hot legs to produce nominally 110 VAC at the fixture or outlet. The actual voltage is generally about 117 VAC. A few locations in a home are wired with both hot legs to produce 220VAC. These locations are for high current consumption appliances and include the air conditioner, stove (if it's electric) and the dryer. A standard outlet is never wired for 220VAC. If the 220VAC is connected to an outlet the outlet is a special outlet that a standard plug won't connect to.

Three phase power is provided to industrial locations, mostly to run motors and other high power devices. Even in an industrial location all the standard outlets and the light fixtures will be wired for 110 VAC.

One thing that does seem to be true is that UK standard outlets can source about twice as much power as US outlets. Standard American circuit breakers are either 15 or 20 amps. I think this might be true for the UK also except that the voltage is twice as high.

[bruto]

Could you just confirm the statement above, that the Neutral and the Ground are connected together in the breaker box for wiring done in the US?

Reason is because it is never done here. The Ground goes to a ground spike, and the Live and Neutral go back to the Mains supply. (UK style plugs and sockets, this is a Commonwealth country)

We no longer use Earth Leakage Circuit Breakers (ELCB) but now only use Residual Current Devices (RCD, or Residual Current Circuit Breaker, RCCB). Could this be the reason why?

It is definitely done here. Codes call for neutral and ground to be separate from the breaker box on out, but they are joined there, so that neutral is always at ground potential.

Oops. I see Davefoc caught it first.

[Just thinking]

Because AC has polarity. [/smartass mode]

The two active lines are not equal. One is considered 'Hot'. That line is the one that is driven from +120V to -120V. The other is 'neutral' or 'return' it is actually not driven.

Ooops ... although you are correct for the "Hot" and "Return" descriptions, I believe the voltage is not +120V to -120V but rather +170V to -170V (http://physics.bu.edu/~duffy/PY106/ACcircuits.html) peak to peak --- remember AC values are given in RMS, unlike DC. This is not too critical of an oversight if one understands this beforehand, but is quite incorrect when sine functions of voltage are shown going from +120V to -120V. This explains why certain types of power supplies can deliver higher DC voltages than one might initially assume.

[GodMark2]

Ooops ... although you are correct for the "Hot" and "Return" descriptions, I believe the voltage is not +120V to -120V but rather +170V to -170V (http://physics.bu.edu/~duffy/PY106/ACcircuits.html) peak to peak --- remember AC values are given in RMS, unlike DC. This is not too critical of an oversight if one understands this beforehand, but is quite incorrect when sine functions of voltage are shown going from +120V to -120V. This explains why certain types of power supplies can deliver higher DC voltages than one might initially assume.

Doh! That's quite correct. I had that sentence worded differently but sense making not did it promote as such, rewriting it required.