# How do "hot" and "neutral" work in countries with 220 V electrical power?

Here in the states, your general purpose wall receptical has a hot pin that is 120 V away from ground, and a neutral that is at (or very close to) ground. Somewhere in the background is a stepdown transformer from high voltage on the street, and its secondary winding is a 220 V winding with a center tap. The center tap is grounded, perhaps at the pole. The two end taps get connected to two subgroups of hots in the circuit breaker panel. Therefore, if you string wires between recepticals in your house, and plug their ends into the hot pins of the recepticals, you might measure 220 V between the wires, or you might measure close to 0 V.

How does this work in countries where the recepticals all carry 220 V? Is the system similar to the US system, just with twice the voltage everywhere, so breaker panels have 440 V differences inside them?

Pretty much. I’ve also run into switched neutrals, which can be lots of fun if you don’t know it’s there.

A common type of wall sockets - that’s what I understand you to mean? - are the SCHUKO types (from SchutzKontakt - protective contact). They have become the de-facto-standard, though wikipedia says that due to the problems with not being reverse-polarity-protected, they have not been mandated throughout Europe and are wished to be replaced by even safer systems.

Interesting stuff - I didn’t notice this initiative, and all appliances I’ve bought have the big schuko-plug (or like computers, the small plug leading to a transformer).

The difference between the US system and many others is in the use of a household supply transformer. The US system essentially provides a final step where you get two phase power by hanging the centre tapped transformer across one phase of the three phase supply. Other countries simply hang the household across one of more phases of the three phases and do not have a per household transformer.

Thus if you take two sockets in a house on a nominal 240 supply, and measure the voltage across the two actives, you will get either 0v or 415v. Why not 480v? Because on three phase power the phase angle between the two actives is 120 degrees, and sin(120) = 0.866, thus the voltage is 2240v0.866 = 415v. Where the phase angle is 180 degrees (i.e. the US system) the voltage between the phases is 2*sin(180)*110v = 220v.

Otherwise things work generally the same. The question of the method of bonding the neutral to ground is the only difference between systems.

In the U.S. power is transmitted at very high voltages to a substation, where it is dropped down to a much lower voltage (a few thousand volts typically) for distribution. The distribution voltage is then dropped down to 120/240 via a center tapped transformer, which is typically used to provide service for three or four homes.

Some residential service in the U.S. is connected that way Francis Vaughan described. Instead of a single phase split by a center tapped transformer, you get two legs of a three phase system. This means that instead of 120/240 volts you get 120/208, since the two legs are 120 degrees out of phase instead of 180. The reduced 208 instead of 240 means it takes your oven longer to heat up and your clothes dryer longer to dry clothes (assuming you have electric appliances), but otherwise isn’t much of an issue. It isn’t very common, but it is still used in New York City and a few other places in the U.S. (mostly older systems).

In many places around the world, the voltage will be dropped to somewhere around 220 to 240 volts and that will be used as the distribution voltage around the neighborhood. Some places use higher distribution voltages and then use pole mounted transformers to drop the voltage to the final distribution voltage (230 volts) which then goes to several homes. In some places you get two wires out of the three phase system (like the U.S.) and in others you get one hot wire and a grounded neutral. Since the standard voltage is higher than in the U.S. there’s no need for a higher voltage to be provided to power larger appliances. There is therefore no need to have anything like a center tapped transformer.

In the UK the substations that supply domestic users there is a final transformer which has a 3 phase supply.

The final transformer will go down to 415V on 3 phase, and will be connected in star configuration, the 3 phases are joined together at one end (neutral) and the other end is ‘live’(not strictly true but lets keep it this way for easier explanation)

The central point(neutral), in the UK is also grounded to earth, the supply to homes takes a line from one end of one of the outer stars, plus one line connected to the joined end - this gives a supply voltage of between 220-240v depending upon voltage drops on the cable run.

Our ‘live’ is actually the non-grounded end of the supply, but we do have some varying configurations at the star point, it depends upon various situations, sometimes we run both a neutral wire and an earth wire from the same cenral point, and we call this separated neutral earth sometimes we run a single wire from the central point and split them into neutral and earth at the consumer box which we call combined neutral earth.

In some rarer situations we may put in a resistance or perhaps an impedance between the neutral and the earth line - the variations are caused by things such as circuit impednce and the problems of disconnecting a supply when fault conditions occur - sometimes we have to limit the potential fault current.

What all this means is that if you get an electric shock, it will be almost certainly be from a live part of the circuit, through to earth, and we can run protective devices to detect this current flow and shut of the supply pretty damned quick, in less than 20 milliseconds using residual current devices.

The philosophy of our system is, that if you construct a non-earthed system, then if an earth fault occurs, you will not detect it, until a second earth fault also occurs and strange circulating currents can occur, and it is very difficult to ensure a circuit remains completely earth free, so we introduce an earth reference electrode so that we have control over it and any possilbe situations that may occur. This was introduced many years ago, when insulation was nothing like as good as nowadays.

One leg is at 0V and the other at 240 WRT earth ground. It can be a problem if you want to run 120V grey market devices (or industrial demos as I have done) using a step down autotransformer. If you plug it in one way you get 0 and 120V like in the US. If you plug it in the other way, you get 120V and 240V. This can cause caps in EMI filters and varistor in surge arrestors to fail due to the 2X over normal voltage with respect to ground.

There’s no method to make sure the plug goes in only one way, like in the U.S.?

This is like what happened to us years ago in a small shop that made control panels. We built a large one, in a walk-in cabinet, with everything internal running on 120 V, and we tested it by patching 120 V into its internal mains. There was a 440 to 120 stepdown transformer on top to power it in its intended application. Somehow it got wired backwards and distributed 1760 volts throughout, and once installed the whole thing lit up like a Christmas tree, but not for long. In retrospect, a little double checking and an extra test step seem wise.

There is in the UK, as the plugs have three pins.

Not that I have seen in Germany.

In some 230 V countries such as the UK yes, in some such as Germany no. (at least where one-phase AC is concerned. Connectors for 3-phase AC (400 V) are designed to fit only one way, of course, as three-phase motors would reverse direction if you swapped two phases.

Connecting appliances with the neutral doubling as protective earth has been deprecated for decades - either the appliance’s casing is connected to a separate protective earth wire, or the appliance meets standards for insulation of all circuits from the casing, in which case no protective earth is needed.
I assume the above is also the case in all developed countries that use polarized AC plugs, and that the polarized plugs are a legacy standard from when a separate protective earth wire was not mandated.

Older residences in the US may have NEMA 10-30 or 10-50 receptacles for electric ovens and clothes dryers, which necessitates that neutral is connected to the chassis of the appliance. It’s tolerated for existing residences, but cannot be used for new construction and might in fact need to be updated if other work has to be done on the circuit. New installations have to use 14-30 or 14-50 receptacles which is 4-wire earthed.

That seems really odd, since it’s the stuff from decades ago that do not have the uneven prongs. Furthermore, I’ve plugged in a plug backwards into an extension cord before, and it caused a bunch of problems.

I always regarded 3-prongs as old fashioned, as all the newer stuff I get is always two pronged.

I’m not disagreeing with you, since I’m sure you know what you are talking about. But I find it curious.