|
[email protected] wrote in message |
|> Radial circuits are the norm in the USA. I've never seen a loop circuit.
|> I understand the UK has these wire with a higher amperage and fuses in
|> each socket for the lower amperage to be supplied.
|
| Almost, the ring main circuit is fused at 30A, the fuses for each
I know they are often called "ring mains", but the correct term
is a "ring circuit". A "ring mains" is what you get in the street
where the supply to homes is connected in a ring, which is very
common in the UK at least.
| appliance are in the plug, not the socket. 3A and 13A are the most
| common fuses in plugs, but other ratings are available up to a maximum
| of 13A. If the fuse was in the socket, then you would have to change
Strictly, the fuse is to protect the appliance _lead_, not the
appliance. If an appliance requires fusing to remain safe, then
it is required to include its own fuse protection. With EU
harmonisation, all appliance leads are required to be able to
clear a fault without damage when protected at 16A as is done in
many other EU countries, so all new appliances would be OK with 13A
fuses anyway in theory. Older appliances with longer leads of small
conductor size could have problems with this though, and
appliances with leads of small conductor size are now restricted
to something like 2 metre length max, so enough current flows to
clear the fault current protective device quickly in the event
of a short circuit.
| in when, for example, you unplugged a kettle, and plugged in a table
| lamp to the same socket. Anything plugged into a standard uk 13A
| socket, e.g. a shaver adapter, has to contain a fuse, as if it did not
| the only protection would be the 30A fuse or breaker in the consumer
| unit, which is far above the capacity of the appliance flex. These 13A
| BS 1363 rectangular pin plugs are the only ones normally used now in
| domestic and office type environments, and are normally wired in a
| ring main. They always have three pins. There was an older standard,
| BS 546, which specified round pin plugs, rated at 2, 5, 15 and 30A.
| These were not internally fused, are seldom seen now, except for the
| 15A, and less often the 5A ones, which are still used for stage
| lighting purposes. I don't know why we wire our 13Asockets in ring
| mains; somebody once told me that it was to reduce the use of copper
| after the second world war, when it was in short supply, but I don't
| know if this is true.
I guess my USA thinking influenced by understanding of some previous
info. In the USA, our receptacles are 15 or 20 amp, and appliances
are supposed to insure they are protected internally of lower levels
of overloads, if applicable. The wire itself can be rated for what
the applicance will use. If an appliance would overuse current then
it is the appliance that should blow its own fuse. The difference
between having the fuse in the plug and in the appliance would relate
to any overload on the wire itself. But the wire doesn't overload,
though it could short out, which would trip the breaker for the circuit.
What I assumed was that the socket was protected by a fuse at the level
suitable for cord short circuits since the loop circuit in the wall was
rated rather high. But with a fuse in the plug, the cord wire is well
protected, anyway.
In your case, a short or overload should blow the plug fuse well before
the house fuse/breaker. Do plugs come with circuit breakers instead of
fuses?
No. The regs describe exactly what type of fuse a plug must have.
There are plugs with RCDs(GFCIs) built in, but they aren't common,
and still have to contain the regulation fuse anyway.
Now the question is, does the circuit in the wall run back to the same
breaker at the end, or does it just come to and end and stop? One would
be a loop and one would be radial.
In the case of a ring circuit, both ends connect to the same 30A
breaker in the panel. When 30A ring circuits were first introduced in
1947, it was also allowed to connect each end to a different 15A
fuse, as it was thought people would convert their existing 15A radial
circuits feeding a 15A round pin sockets into 30A ring circuits
by forming the ring starting from two existing 15A outlets, saving
the need to replace some of the wiring. In practice such conversions
were probably rather rare, and the two 15A fuses scheme has not been
allowed since the early days.
What current levels are used for electric stoves, ovens, hobs, etc?
Traditionally, 30A or 45A circuits are installed for electric hobs
or combined hobs/ovens (is that what you call a stove?) Electric
hobs are very much out of fashion in UK, and electric ovens, unless
very large, can mostly run from a standard 13A socket nowadays because
they are better thermally insulated and very quickly heated with fanned
hot air. As a result, the high current dedicated cooking circuits are
often no longer installed at all.
We have them here up to 60 amps and they run on 240 volts (120 amps
on 120 volts would be rather undesireable).
One thing I noticed browsing US electrical department stores was the
lack of high powered portable appliances. Someone already mentioned
the lack of a 3kW kettle. I was looking to see how much my Microwave
cost in the US, but could only find the cheaper version of my Sharp
model without the browning element. Then it dawned on me that they
couldn't sell a 2.7kW (IIRC) appliance in the US as you couldn't plug
it in to a standard outlet on a kitchen worktop.
| For industrial use at higher current ratings, the most common are the
| connectors which were known as BS 4343, but which are now BS EN 60309.
| These are the colour coded plugs which have 3,4 or 5 pins recessed
| within a circular plastic surround. Versions here are rated at 16,
| 32, 63 and 125A, in the USA, they are rated at 20, 30, 60 and 100 or
| 120A, I can't remember which, and I don't know the standard number for
| them. They don't seem to be as widely used over there as they are
| here.
You might be referring to the IEC pin-and-sleeve connectors. Those
go all the way up to 690 amp versions (yes, six hundred and ninety).
| British electrical distribution is very different to that in the USA.
| Over here the most common voltage is 11kV, but other Voltages, 6k6V,
| 22kV and 33kV for example are also used. We use relatively large
| three phase transformers in substations, not the small things stuck up
| poles that you have over there. The output from the substation is a
| three phase four wire star connected system. The neutral is connected
| to the star point, which is earthed (grounded).
Questions:
How many homes would be on a single substation?
Several hundred -- usually quite a few residential roads.
What would be the total current rating for that substation secondary?
1MVA is the rating of one I know.
What would be the fault current if there was a solid short circuit?
The max permitted at the suppliers terminals in your home is 18kA
single phase and 25kA 3-phase. Normally it's nowhere near this,
and most people have a main cutout of a type which can only handle
up to 10kA, and that's still plenty of spare margin. It is required
that the PSCC (Prospective Short Circuit Current) at any premisies
be taken into account when designing the installation. A case where
you can run into problems is in an apartment block with integral
substation; the apartment nearest the transformer might need to have
some excess supply cable snaked around to increase the supply
impedance slightly. I've seen a photo of an incident where this was
not taken into account, and all the protective devices failed to
clear the fault current, resulting in the wire exploding out of the
wall.
I prefer the small transformers for fewer homes, as it keeps the fault
current levels low.
One thing I notice in the US verses the UK is poor regulation -- turn
on a higher current appliance and the lights dim. That's sufficiently
rare in the UK that people usually think there's a fault if they see it
happening. However, it's not clear to me where this voltage loss happens
in US systems. A 2kW load is a noticable proportion of a 20kW pole mount
transformer's capacity, but a quite insignificant proportion of a 1MW
pad mount transformer, so transformer regulation could be an issue.
Additionally, higher current for same load could create a very much more
significant voltage drop in conductors, but I haven't checked to see how
you size your conductors compared to ours, and if this is a significant
factor. Another issue is that we in the UK tend to have dedicated
lighting circuits, so they won't be competeing on the final branch
circuit with any high current loads. That's 3 possible factors, but I
have no idea their comparitive contributions to the overall observable
affect in the US.
But in dense urban areas, where transformer space
is at a premium, there is often what is called "network service" which
is a substation with a larger 3 phase transformer bank (sometimes more
than one in parallel). The voltage is 208/120 in the star configuration.
Customers are supplied with either three phase, or if they only need
single phase, they are supplied with 2 of the three phase lines.
| The nominal Voltage is 400 from phase to phase, and 230 from phase to
| neutral. Until recently it was 415/240, but nothing has really
| changed, it was a European harmonisation thing. All four wires are
| run along each street, in towns and cities normally underground, and
| all wires are taken into buildings requiring a three phase supply.
| Houses are normally supplied with only a single phase and neutral, the
| phases normally being used in rotation in each building along the
| street, so there is probably 400V between your mains sockets, and
| those of the house next door. A typical substation would typically
| feed several hundred houses, while large buildings such as factories
| would normally have their own.
Can you get just 2 phases if you have a need for 400 volt single phase
but not three phase? Is there equipment like circuit breakers and panels
designed for 2 poles like that, or would you just have to use 3 phase
stuff with one dead line?
In the UK you can get just 2 phases. There is no switchgear for two
phases -- people either use two sets of single phase switchgear, or
use 3-phase with one not connected (the latter being essential if
you have any appliances needing more than one phase, but I can't
think of any such which don't need all 3 phases).