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UK/USA/EU terminology NEC BS7671 etc

Z

Z

Jan 1, 1970
0
I'm trying to understand some of the posts here from the USA.
Is a GFCI a breaker that monitors the current on the neutral and if it
drops due to current on the earth possibly going through a human caused
the breaker to trip - in UK terms a Residual current device RCD?

Is a branch circuit a circuit for sockets which daisy chains a cable
from the mains to all the sockets in turn but does not return to the
mains after the last socket - in UK terms a radial circuit?

In the USA are cables sizes quoted in millimetres squared ever? Are the
accessories and switch gear roughly called the same things?

Is the National Electrical Code available online at all, free? The IEE
wiring regulations - British Standard 7671 in Britain isn't.
Is the NEC used in countries other than USA?
 
D

Den

Jan 1, 1970
0
A Brit in Calif writes ....

* A GFCI is a RCD.
* Ring circuits aren't used in domestic applications, circuits are radial
* Dunno about the NEC question.

* And (sob!) a 20A circuit can only carry just over 2kW ... while waiting
for my 1.5kW kettle to boil in the morning (to make tea with imported M&S
tea bags) I fondly remember the speed with which my 3kW 240V electric kettle
boiled in Berkshire!

D
 
J

John G

Jan 1, 1970
0
message
[email protected] wrote in message

Almost, the ring main circuit is fused at 30A, the fuses for each
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
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.

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.

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).

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.

Several things to note; all three phases are symetrical about the
earth point, we never have one with a higher Voltage on it. Our 230V
is that Voltage from earth, it is not a centre tapped system with 115V
on either side. We never have delta systems with one corner grounded.
The Voltage is standardised throughout the country, you will not get
anything other than 400/230V from the public supply, unless you are
operating something *very* large, like a railway, or a
steelworks.

Stephens description is pretty much how it is in Australia
also.
But our nominal is/was 240/415 volts and as we are far from
Europe are not as tied to their Harmonisation but I guess it
will come and we will accept 230/400 volts.(not very
significant any way)
We have a lot of pole mounted transformers so I don't think
they supply as many homes as the substation arrangement but
the result is much the same.
 
A

Andrew Gabriel

Jan 1, 1970
0
| [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).
 
B

Beachcomber

Jan 1, 1970
0
.... This resulted in lights designed for 277 (or 347)
volts. And all of this is really because Edison wouldn't accept AC (if he
had, then 3 phase would have made sense and the voltages would not have
the 2:1 ratio). Also, Edison was a bit paranoid about electrocution,
perhaps due to his mistaken belief that the neutral wire, when grounded,
would provide protection, and didn't realize its failure was because that
wasn't really a very good design.


Some history here... Universal 3-phase for residential service was
briefly considered in the USA, but rejected. The REA (Rural
Electrification Agency) looked at various systems that could bring
power to towns and isolated farmhouses in the 1930's and decided that
it liked the economy of single (spilt) phase 110/220 service with a
center tapped neutral and two hot legs. One of the major
considerations was the ability to power farm machinery and it was
determined that a major portion of motorized farm loads could be done
with motors less than or equal to 10 horsepower, which were readily
available in single phase models. For motorized loads over 10 HP, the
(single phase) repulsion start - induction run motor was available.

What became the REA standard for farms and rural areas then became the
standard for the US overall.

It is true that back in the 1880s, Edison preferred DC, but after
Westinghouse demonstrated his AC system (developed by Tesla) in
Chicago at the Worlds Fair in 1893, the overwhelming technical
superiority and economics of long distance AC transmission quickly
became the only practical system from the standpoint of future
installations. DC distribution from that point onward, was limited to
railroads and streetcars, (and a few large cities provided DC for
elevator service in their central business districts).

Edison's long lasting contributions (in addition to inventions) were,
the selection of a distribution voltage of 110 volts and the 3 wire
circuit with a neutral, designed to effect a savings in copper and a
reduction in voltage drop. The 3 wire system first appeared on
Edison's DC central stations (120 VDC dynamos in series) but an
equivalent circuit was developed for AC with center-tapped
transformers.

It appears that none of the early systems, AC or DC were grounded.
The importance of grounding for safety was not recognized. Grounding
was a complex subject and early efforts appear to indicate that the
motivation was primarily for lightning (equipment) protection vs.
human safety.

Beachcomber
 
B

Beachcomber

Jan 1, 1970
0
Still, I think even more savings could have been had by just dropping
the neutral and going with straight 220 volt. It's those Edison lights
that made that not practical. That and their exposed metal base which
would have been a shock hazard on a 220 volt system with the ground tap
in the center at the transformer instead of one end. But if a safe
light for 220 volt 2 pole had been adopted, the wire cost would have
been 2/3 as much. OTOH, you'd need 2 pole switches, and still some
kind of ground.

I think the neutral-less circuit would be the best, but historically,
I can't see an easy way to get to it.

My whole push for such a system is to get a single voltage, and one that
is adequate for heavy residential loads like air conditioning and stoves.
If that had been 220 volts (if I were picking it, I'd go a bit higher),
then single phase could do it with two 110 volt poles, and three phase
could do it with three 127 volt poles. As long as nothing was designed
to use a neutral wire, then it would be the one voltage all around.

Alternatively, I'd like to see universal 3 phase or 2 legs of 3 phase.
That way the voltage level would be consistent.

Today, I would not want 3 phase power unless it were practical for me
to derive 240 volts from it, since many things like electric stoves are
not designed for 208 volt systems (most models are not, but some are).
What I mean is full performance. Most of those that are designed for
just 240 volts do say they work at 208 volts, but with a reduced level
of wattage, which I translate into taking longer to bring water to a
boil.

Phil:

What you say makes sense but there are a lot of competing interests
here that might have problems with what you might not have mentioned.

First of all, Edison made thousands of lightbulbs at all different
sorts of voltages. If you ever get a chance to visit the Edison
Museum in Ft. Myers Florida, you can see these on display. Some of
the coolest are the incandescent "searchlight" bulbs that operate on
thousands of volts. The Edison system involved a search for the ideal
filament optimized by resistance for what Edison should be the
standard residential/commercial distribution voltage of 110 V.
however. European incandescent lights, of the same physical size and
wattage rating often run much hotter (because of the 220-240 volts).
The resistance problem is not trivial at the higher voltage because,
lets say R is more or less the same, the power dissapated is I squared
R and that can be a lot of power at the higher voltage. Touch a 60
watt bulb in the US, and you will pull your fingers back and say that
is hot. Touch a 50 watt Phillips bulb on a Euro 240 volt circuit and
you could get 2nd degree burns. ( I know from personal experience,
I've done this in visits to Europe).

Back to the neutral problem... You mentioned that it would be great to
have a system without a neutral. From the point of view of installing
ovens, air conditioners, or dryers, this might be desirable, but from
the point of view of the electric utility servicing your property,
maybe not so.

If there is a fault in the power line serving your residence, whereby
lets say a primary operating at 34.6 kV comes in contact with the
secondary feeders to your home (either 230 or 120/240). If you have a
grounded neutral system, chances are good that the fuse on the primary
will blow and you will live. However, even if the fuse on the
primary does not blow, because of the grounding, the highest voltage
that just fried your radio, TV, and CD player is likely to be very
much more that 240 volts, but also very much less than 34.6 kV.
Grounding and specifically a grounded-neutral is a very important
safety feature for the utilities.

Likewise, for lighting protection, transients and surges, utilities
have found grounded systems (and all the associated equipment such as
arrestors, and reclosers) to be the best defense against equipment
damage. Grounded systems also protect customers from bearing the full
brunt of a lighting strike to exterior equipment (poles, overhead
lines, substations).

Beachcomber
 
A

Andrew Gabriel

Jan 1, 1970
0
| 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.

So what kind of fault would happen on the lead (wire, cord, etc), that
would not be protected by the fuse in the appliance itself, and would
not trip the breaker or blow the fuse of the house circuit itself?

The fuse in the appliance itself can't protect the lead.
However, I think I'm not understanding what you're asking.
Fuses in plugs are uncommon in the USA. But I do recall seeing them
in Christmas tree light strings many years ago. These were parallel
strings where each individual light was at full voltage. Apparently
it was considered that these wires were more subject to damage and
ease of spreading a fire from a dried out tree was high.


|> 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.

So what is in fashion instead of electric hobs? The combined hob/oven?

Natural Gas hobs, or combined hob/oven where the oven is electric
and the hob is gas. So the electric load is often now only the oven,
and unless it's large, it can run from a standard 13A socket outlet.
|> 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.

The NEC now requires a minimum of 2 circuits of 20 amps with 20 amp
receptacles in the kitchen countertop. But with only 120 volts, you
are looking at 2400 watts maximum. This is one of the reasons I plan
to have 240 volt receptacles in the kitchen of the house I will be
building in a few years. There aren't that many appliances that work
on 240 volts now (I have seen some larger microwave ovens that do),
but maybe I can find some I could import from the UK as long as they
are not fussy about how the grounding is done (center tapped here)
and the 60 Hz. I would have to change the plug.

They don't care about how the grounding is done. Products sold in the
EU have to be suitable for use anywhere in the EU, and there's a mixture
of supply types, some with live/neutral polarity unknown at socket
outlets, and some where neither terminal is near ground potential.
|> How many homes would be on a single substation?
|
| Several hundred -- usually quite a few residential roads.

So if each home has a capacity of 100 amps, and the diversity factor is
say 10, you're still looking at thousands of amps normal capacity in the
substation, which would translate to tens of thousands of amps of fault
capacity. You'd be depending more on the wire resistance than in the
transformer impedance to limit the fault current.

On average, the supply cable from the substation is probably
longer than in the US. I think my nearest substation is perhaps
400 yards (2 streets) away. Of course, some people will be right
next to it.
|> What would be the total current rating for that substation secondary?
|
| 1MVA is the rating of one I know.

Split over three phases that would be over 1380 amps of capacity.
Given a typicaly large capacity transformer impedance of 5% to 7%
you could be looking at fault currents of 20000 to 27000 amps, not
considering the distribution wire resistance (which will lower it).
That sounds a lot like the "network service" areas in the US.

I'm not well up on the substation transformers, but I believe they
have some feature which increases the supply impedance during gross
fault currents. I've no idea how it works.
| 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.

They they must be running some long or thin wires to get fault currents
down to those levels.



| 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.

The dimming is typically due to the lower available current, either due
to longer or thinner wires, or lower capacity transformers. It's not
regulation that is the issue, since voltage regulation is done mainly
in the generation (exciter control) and transmission (tap changers).

By regulation here, I was refering to transformer output voltage
variation with load.
Conductor sizes could, in theory, be similar. Both the UK and the US have
240 volt systems. It's just that the US puts the ground tap in the center
of the transformer secondary and also runs a wire from it as the neutral
(hence a 3 wire system). So for a given kW usage, we're looking at about
the same amps total. But with more devices running from 120 volts, they
tend to be current hogs on startup.

I think that's misleading. For a given kW load plugged into a 120V
outlet, I think you will need 4 times the crossectional conductor
size to reduce your cable voltage drop percentage and power loss to
the same as ours.
Recent (the last 20 to 40 years) home construction here does put lights
on separate circuits from receptacles and dedicated appliances. Still,
I see quite a whopper dimming of lights when I start the garbage disposal
grinding motor. That's something I plan to try to get a 240 volt version
in the future. I'm also considering isolating things that do that on
their own transformer.

In US hotels, I often find the bathroom socket and light on the same
circuit. The hairdrier often has a low power mode achieved with a
diode rectifier. This causes the bathroom light to flicker rather badly
at 60Hz...
 
A

Andrew Gabriel

Jan 1, 1970
0
| In article <[email protected]>,
| [email protected] writes:
|>
|>| 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.
|>
|> So what kind of fault would happen on the lead (wire, cord, etc), that
|> would not be protected by the fuse in the appliance itself, and would
|> not trip the breaker or blow the fuse of the house circuit itself?
|
| The fuse in the appliance itself can't protect the lead.
| However, I think I'm not understanding what you're asking.

There are two different kinds of reasons for a fuse to blow. One is that
the load is pulling a little more thgan it should, which would gradually
heat up the wire, possibly melting insulation. The lead itself will not
cause this kind of problem. So a fuse in an appliance would protect the
lead from an overload. The other problem is damage to the lead causing
a short circuit. The breaker for the circuit in the wall will trip on
this kind of problem.

OK, we call these two overload currents and fault currents respectively.
If an appliance which uses a socket outlet has any likely failure mode
which would otherwise be unsafe if it was not protected by a fuse, then
it has to include a appropriate fuse within itself. I'm not sure but it
may be that this only applies if the protection required is less than 16A,
which is I believe the highest current protection used anywhere in Europe
on standard domestic outlets. The fuse in the plug is to protect the lead
from overload or fault currents. The next upstream protection would
otherwise be 30A/32A for the ring circuit protection in the UK.
|> So what is in fashion instead of electric hobs? The combined hob/oven?
|
| Natural Gas hobs, or combined hob/oven where the oven is electric
| and the hob is gas. So the electric load is often now only the oven,
| and unless it's large, it can run from a standard 13A socket outlet.

Among people who prefer all electric, would they still have the same
thing but in electric version? Or is gas so preferred that you can't
even find people that prefer electric?

You can buy expensive ceramic and halogen hobs for people who want a
hob which looks like one of those, but most people find gas much better
to cook on and cheaper. Cheap electric hobs are used in low-end rented
rooms.
|> The NEC now requires a minimum of 2 circuits of 20 amps with 20 amp
|> receptacles in the kitchen countertop. But with only 120 volts, you
|> are looking at 2400 watts maximum. This is one of the reasons I plan
|> to have 240 volt receptacles in the kitchen of the house I will be
|> building in a few years. There aren't that many appliances that work
|> on 240 volts now (I have seen some larger microwave ovens that do),
|> but maybe I can find some I could import from the UK as long as they
|> are not fussy about how the grounding is done (center tapped here)
|> and the 60 Hz. I would have to change the plug.
|
| They don't care about how the grounding is done. Products sold in the
| EU have to be suitable for use anywhere in the EU, and there's a mixture
| of supply types, some with live/neutral polarity unknown at socket
| outlets, and some where neither terminal is near ground potential.

So does this mean the chassis is not connected to any wire, and a fault
between a live wire and the chassis can go undetected (since it would not
fault to ground) until a human becomes the path?

No, the chassis of all microwaves is grounded. I just meant that the
microwave won't be assuming that neutral will be at similar potential
to ground, nor which is live and neutral. I would expect it would work
without problems on a 240V centre-grounded supply.
 
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