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Averagesupernova
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In the USA the neutral is NEVER, EVER, under any circumstance interrupted. Switching the neutral is prohibited. Sophie, I think this is not the case in the UK.
Averagesupernova said:In the USA the neutral is NEVER, EVER, under any circumstance interrupted. Switching the neutral is prohibited. Sophie, I think this is not the case in the UK.
it is my opinion that between the NEC and fairly rigorous experience a well qualified electrician should be at least as capable of designing a functional and efficient electrical installation from the utility transformer right down to the lamps as any degree-holding engineer. Sadly, electrical engineers are too often painfully ignorant of things like building codes, practical uses and applications, and other aspects of physically constructing a building that electricians must be proficient with on a day-to-day basis.
And they ususally don't like it when an electrician demonstrates this either, contrary to some of the sentiments above.
"Hubris begets Humility , after much pain." old jim
krater said:Asn, you're very close to right about that. However, if you've ever dealt with a cabinet heater thermostat, a 120v motor starter coil in which the overload relay is typically located after the coil (the neutral side), or a four pole break transfer switch, you will find that on some occasions the (utility or system) neutral does in fact get broken. It's not common practice, and as I said as a general rule you're right, but you need to be careful if you're going to invoke NEVER, EVER in the electrical industry.
sophiecentaur said:Presumably the breaker on the incoming supply to a US home is just Double Pole? What happens with fusing ?
I can't see why a neutral would EVER be shared in a residence in the UK since doing so will guarantee the sum of the currents of the 2 hots that are associated with said neutral. The only place sharing a neutral works is multi-phase or split-phase and then has plenty of potential to be done wrong.sophiecentaur said:'Shared Neutral' is avoided, even within a single household, where only one phase is present so circuits can be truly isolated.
The US domestic system is more complicated and, IMO, more risky as a result. Dual voltages in one hhousehold seems a bad idea, to me.
Averagesupernova said:Utility disconnects that I have seen that are a double throw for auxiliary power do not break the neutral.
Averagesupernova said:I can't see why a neutral would EVER be shared in a residence in the UK since doing so will guarantee the sum of the currents of the 2 hots that are associated with said neutral. The only place sharing a neutral works is multi-phase or split-phase and then has plenty of potential to be done wrong.
I can't agree. :)
jim hardy said:Observe that National Electrical Code is published not by the academic IEEE, but by the National Fire Prevention Association.
brenfox said:As an after thought (this is where i will get my technical *** whipped!). According to Kirchoff, current entering a node will leave a node in a closed circuit. So whatever current enters a load will leave this load via the neutral in a line to neutral system. The potential voltage will be dropped across the load, so in theory the current will run along the neutral with no volts pushing it?? How does this work?
psparky said:If you run two different phases and share a neutral [sic], ...However, I believe this shared neutral circuit is now "illegal" in USA...
psparky said:If you run two different phases and share a nuetral, technically this works and the nuetral never gets overloaded because the phases are out of phase by 180 degrees.
sophiecentaur said:What bothers me is the appliance lead is not protected for a fault on the appliance because it is upstream of the appliance and can be supplied with far too much current from upstream.
Averagesupernova said:One thing to note here is this: A purely inductive load that draws 15 amps on one side and a purely capacitive load that draws 15 amps on the other side (can't imagine this in the real world) will put 30 amps on the neutral. This was discussed a while back here on PF. Can't recall what thread. A search will show it up I am sure. Jim Hardy was involved in the discussion as well as myself. It had never occurred to me up to that point that is was indeed possible to overload a neutral. But, it most certainly is.
Not "totally" but very often. I am not at all surprised at your statements about the protection provided by a downstream device. I am more surprised that those 80 electricians were surprised. But that protection can only sense and react to what's going on downstream of itself but how can it deal with the following? A common scenario is a damaged power cord - perhaps with a heavy box resting on it for days and days or maybe jammed under a closed door. Any intelligent circuit protection inside the appliance can hardly know about that. Eventually, the wear or pressure can cause a partial short across the conductors and let 20A flow through conductors rated at 5A. There is no protection without an upstream 5A fuse. However smart the protection device happens to be, it cannot disconnect at the socket and that's the only way of eliminating the effects of the fault.krater said:Under the right conditions an overcurrent device is totally capable of protecting conductors upstream of the device.
Let that sink in for a second. Yes I realize it goes completely against the basics of circuit design. Nevertheless it is true.
Etc.
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sophiecentaur said:Not "totally
Things are changing. Almost the majority of outlets these days are to supply low voltage equipment and the Wall Wart is very common. That is a good solution UK or US to the protection problem - as long as the low cost imports can be relied on to follow the rules. How long before we can expect a 12V distribution system in all homes?
sophiecentaur said:A point of information: what is the current available at a typical outlet in a living room? How many outlets on a typical circuit?
I didn't mean 12V for everything. I meant an extra circuit for all those little things that work off wall warts and USB outlets. They are pretty much in the the majority - not to mention the 12V lighting rail that's so popular. I wouldn't suggest a 12V, 2.5kW kettle! We had an LV supply on all the Physics Lab benches when I was at School. Massive great black box on the wall with a Captain Nemo style wheel and switches.psparky said:Probably not soon? You would need 10 times the copper in each house.
At 12 volts, a #12 (4mm2) wire would now need to be #2/0 (70mm2) to power a good microwave!
That would drive the cost of copper up as well.
But who knows...
Averagesupernova said:15 amps. Code requires 2 20 amp counter-top circuits in the kitchen. A 20 amp circuit is required for a bathroom outlet also. Washing machine will require a dedicated 20 amp circuit. Code does not typically dictate how many outlets are on one circuit but there are some obvious exceptions as the above implies. Every 120 volt circuit in the house now requires an arc fault breaker excluding kitchen, bathroom, garage, dedicated circuits like washing machine and furnace. Any living room, bedroom, den/parlor and I believe hallway will require arc faults. Ground fault protection is required in any unfinished area such as garages and basements. Bathrooms also require it. Kitchens counter-tops within a given distance of the sink require it too. 32 amps at 240 volts equals an available 7680 watts at each outlet in the UK. In the US typically 1800 watts available. Is it any wonder the UK has a fuse in each plug?