New speaker issue

[Richard Crowley]

Originally Posted by Paul R Johnson

It just never occurred to me that there was any other system than 3 phase - the notion of a centre tapped transformer just never occurred to me for mains power.

There are actually power schemes with MORE than 3 phases (particularly very high power circuits, and works where they convert between AC and DC (and back again), etc. But 3-phase (120 degree) power is virtually unknown outside industrial sites in North America.

[Greg Miller]

Originally Posted by Richard Crowley

In North America, center-tapped, single-phase 230V is delivered to domestic customers and the center-tap is locally grounded and called "neutral". So typically half the 115V circuits in the house come from one side ("phase") of the transformer secondary winding, and the other half of the circuits are connected to the other side. {snip}
I agree that use of the term "phase" for a simple center-tapped source seems rather pretentious. But that is what electricians have been calling it for several generations.

3 phase power is almost never found in domestic premises (homes, apartments, etc.) But it is commonly provided to larger industrial customers.

Where I've worked, I don't recall hearing anyone referring to to the two 120 volt hot lines as "phases." I've always heard the term "leg." So there are two "legs" of one phase, the two legs being 180º apart.

Of course in an older delta system, where one transformer secondary is center tapped, the two primary terminals of that transformer are connected to two phases of the high-voltage distribution system. But the voltage across the secondary of that transformer is still just one phase and those two 120 volt legs are still 180º apart. So at first glance this might seem to be a discrepancy ... but it really isn't. The primary of that step-down transformer is connected to two [primary] phases, but the secondary voltage is just a single phase (center-tapped) sine wave.

BTW, around here most of the multi-story multi-unit apartment buildings are supplied with 3-phase 120v-208v Y service. In that case, each apartment unit (i.e. single residence unit) may be supplied with only two of the phases (unless there is a high-current 3-phase load, such as a large air-conditioning compressor). If you compare any two phases with respect to neutral you'll find that they're 120º apart, each being 120 volts above neutral. But if you connect a load between any two phases, you will see a single-phase sine wave between the legs with a voltage difference of 208 volts.

[Greg Miller]

Originally Posted by Richard Crowley

But 3-phase (120 degree) power is virtually unknown outside industrial sites in North America.

Perhaps that depends on your definition of "industrial sites." {my dictionary says "industrial: of or pertaining to, or resulting from, industry or productive labor"}

As I stated above, a lot of apartment buildings in this area (typically a few hundred separate apartments in the building) do have three phase coming in. However, any given apartment unit (i.e. occupied by one person or family) might have only two of the three legs, since they have no 3-phase loads.

But many small [non-industrial] businesses have 3-phase service. e.g. movie theatres, coin laundries (laundromats), restaurants, retail stores, supermarkets, banks, etc. I don't think of any of these as being an "industrial site" but I know from my own experience that they do have 3-phase service.

There is one very old business corridor in this town (located on either side of a long alley) where the service is older 3-phase delta. Most of the businesses on that corridor (except for the very small shop fronts) have all three phases coming in, because they have some significant number of 240-volt loads. But of course there's always a problem with load balancing because the 120-volt loads can be connected to only one of the three step-down transformers (i.e. between two of the three high-voltage primary phases).

Most of the newer businesses here have 3-phase Y, with all three legs coming into the premises. The Y connection makes load balancing much easier (since each of the three legs can supply 120v to neutral). The only concession is that higher power loads need to run on 208 instead of 240; however 208 volt equipment has been almost ubiquitous for the last 30 years or more.

[Richard Crowley]

I was imprecise in my language. By "industrial" I was trying to convey the concept of non-domestic.

Certainly most power in North America is DISTRIBUTED as three 120-degree phases. But, it is DELIVERED to domestic end-users as 240V center-tapped "single-phase". Indeed, apartment buildings with more than a few dozen units probably have one or more large transformers that take the 440V (or whatever) 3-phase power and step it down to many 240V feeds which have their center-taps grounded and delivered to the individual unit meters and circuit breaker boxes Virtually no consumer electrical appliances are designed handle the voltages produced by 120 degree phases (208V or 277V)

[Paul R Johnson]

With two different voltages available, presumably from different socket types, is there a constant stream of appliances being damaged? I see American equipment with IEC type connectors, are there rules to stop idiots or the careless blowing things up? People here just see a cable laying in the floor and plug it in. I'd hate to think what we would do with dual voltages in homes!

I'd never realised the differences between us were as great as this.

[Greg Miller]

Almost all residential/consumer equipment operates on 120v, and 240v connectors are significantly different, so it's almost impossible to make the kind of mistake that you envision.

There are two types of connectors in general use for 120v. The 5-15 socket is for use up to 15 amps, the 5-20 socket is for use up to 20 amps. You can find some photos here:
https://en.wikipedia.org/wiki/NEMA_connector .
Note that the 5-20 socket has one T-shaped terminal.

Most residential/consumer equipment draws 15 amps or less, and uses a 5-15 plug with two parallel blades; this plus will fit either a 5-15 or 5-20 socket.

Some slightly larger 120v equipment (e.g. a commercial floor polisher or larger room-size air conditioner) uses a 5-20 plug, with the two current-carrying blades at 90º to each other; this plug will fit only a 5-20 socket.

There is also the question of differentiating between the line (hot) and neutral side of the circuit. The neutral blade is the same thickness as the "hot" blade, but the neutral blade is wider. Such connectors are called "polarized." A polarized plug can be inserted into a polarized receptacle only in the correct orientation. This distinction is required because NEC prohibits switching the neutral connection; only the hot connection may be switched. Thus it's important that the unswitched side of the device is always connected to the neutral side of the circuit.

National Electrical Code permits any given branch circuit (i.e. a circuit that is protected by a given circuit breaker) to carry a continuous load of no more than 80% of the rated size of the circuit breaker. Thus a 20A circuit can legally supply a continuous load of only 16 amps. With nominal voltage of 120v, that's a continuous wattage of 1920 watts.

NEMA 5 connectors are never used for 240v. When you go to the higher voltage, you go to NEMA 6 or larger. There is little chance of confusing anything else with NEMA 5, and it is impossible to insert a 120v NEMA 5 plug into any type of 240 volt socket.

With few exceptions, the only residential/consumer appliances that require the higher voltage are electric clothes dryers and electric cooking ranges. These use NEMA 14 connectors which can't be mistaken for anything else.

Electric water heaters almost always use 240 or 208, but they are hard-wired and do not have any plug/socket connections, so no chance of an error there.

[Paul R Johnson]

What is the most c common type of male connector on appliances - here, we have on domestic kit 3 pin connectors for most connections - with the equipment end the figure of 8 shape that can be reversed so L+N can be connected the wrong way around. This connection has the ground pin of the connector plug usually made from plastic - which is necessary to open up the shutters all our outlets have in homes. This kind of equipment does not have a ground connection, and must not be fitted with one - the case is isolated from the power supply. Three pin connections are done via IEC connectors.
http://static.rapidonline.com/catalo...63584P01WL.jpg
http://uk.farnell.com/productimages/...B/42409699.jpg

More and more professional gear is now being fitted with Powercon type locking connectors
http://uk.farnell.com/productimages/...AV25118-40.jpg

[Richard Crowley]

Originally Posted by Paul R Johnson

What is the most c common type of male connector on appliances?

Virtually everything that draws 15A or less uses one of these mains plugs.
The 2-pin variety are still widely used in "double-insulated" gadgets