What's the rationale for 277v not being permissible in dwelling areas?

[zbang]

Some of elevators in NYC were DC. Con Edison discontinued the service in 2007, so it can be done.

It took them something like 50 years to turn off the DC. I remember reading of an engineer who was hired by ConEd to work on the conversion and retired 30+(?) years later with it almost complete.

How long did the other-frequencies to 60Hz conversions take? And Philadelphia, and maybe other cities, still have 2-phase services.

 

[pfalcon]

Regardless of merit, after 100+ years the installed base of wiring, motors, appliances and such would make it economically and practically foolhardy to attempt any major conversion. Half of Japan is on 50 Hz, the other half on 60 Hz. The cost, aggravation, and hazards of attempting to convert are simply too great once established. We still have some odd systems in this country "because they are" and it is cheaper to maintain than to attempt switching over. Some old buildings in Cleveland only converted from DC to AC in the 50's (some of the old switchgear was abaondoned in place and is pretty fascinating).

AC is more efficient in long line tranmission and in high power applications.
DC is safer and friendlier over short distances and low power applications.
An example is lighting, 24VDC provides plenty of lumins, instant on, power savings, color control, and no flicker for a household light. Much better for most small electronic appliances than DC. But not so good for the oven.

 

[Besoeker]

AC is more efficient in long line tranmission and in high power applications.

Or maybe not.
For long transmission lines DC in the form of HVDC is used. At several hundred kVdc. It isn't generated as DC. It gets converted from the AC generated by high voltage inverters at the sending end and high voltage inverters are used at the receiving end to convert it back to AC.
Now, why would you do that if AC long line transmission was more efficient?

 

[iwire]

Or maybe not.
For long transmission lines DC in the form of HVDC is used. At several hundred kVdc. It isn't generated as DC. It gets converted from the AC generated by high voltage inverters at the sending end and high voltage inverters are used at the receiving end to convert it back to AC.
Now, why would you do that if AC long line transmission was more efficient?

OK, why are not all transmission lines DC?

Easy answer, there is never a 'one size' fits all answer, in some cases DC transmission is obviously more efficient but at this point in time AC still is generally more efficient.

I know I am not telling an engineer anything new but every choice we make is a just a trade off trying to find the best solution for the task at hand.

 

[iwire]

It might not entirely surprise you that I totally disagree.

Not so surprised, people are always most comfortable with what they are familiar with. (I include myself in that)

No live electrical part should come in contact with people.

But it happens and that is a fact.

If we want to look at our work that way than over current protection is also unneeded because no conductor should be overloaded.

If a supermarket let that happen they would be under immediate investigation,and the management would probably face criminal proceedings

Certainly it is possible if they were negligent that they would be sued, it would be a almost unheard of occurrence for management to face any criminal charges over this in America. (I am not saying that is good, just the way it is)

Considering that here it is a common trade practice to keep 277 volt circuits out of reach of the general public any place that goes against that practice would be in a worse position to defend themselves.

And 120V isn't exactly a safe voltage anyway.

No, here in the US many more people are killed by 120 volt circuits than any other voltage .......... of course that has a great deal to do with the fact it is everywhere here.

And while 120 is not 'safe' it is in fact 'safer' to humans given the same set of circumstances.

For most residences the ONLY supply coming into the premises is 230Vac single phase. Plugs an sockets are all the same.

So as an engineer you are willing to say one voltage is alleyways the the best to use for the task at hand regardless of the load?

Personally I see no need to bring 230 VAC to a night light, that is much more power than needed.

The live an neutral prongs are disconnected before any conductive part is exposed. When the third pin, the earth (ground) is withdrawn, a shutter closes off access to the live and neutral. Your children or grandchildren can't poke stuff into them.

Here starting with the 2008 NEC we must use receptacles in dwelling units with shutters. It was a good change. :thumbsup:

 

[Besoeker]

OK, why are not all transmission lines DC?

Easy answer, there is never a 'one size' fits all answer, in some cases DC transmission is obviously more efficient but at this point in time AC still is generally more efficient.
I know I am not telling an engineer anything new but every choice we make is a just a trade off trying to find the best solution for the task at hand.

Yes, I agree.
Technology moves on too. The sort of voltages being used now for HVDC transmission have become viable owing to developments in semiconductor technology and this, in turn, has had a bearing on the number of HVDC projects in service and currently under construction.
My point was that the assertion made by pfalcon

AC is more efficient in long line transmission and in high power applications

simply does not hold true. Generally, HVDC projects are both high power and long transmission lines. Given the complexity and cost for the conversion/inversion stages, there has to be some benefit over AC or why bother?

 

[Besoeker]

Not so surprised, people are always most comfortable with what they are familiar with. (I include myself in that)

That's a fair point. But I am not disagreeing on the basis of familiarity. I work with quite a range of voltages. We actually make products for the US commercial sectors so I'm reasonably familiar with supplies used there. And you are probably aware that Mrs B is from there with friends, family, and property. So it really isn't a familiarity issue for me. It's just simpler, in my view, to have a single voltage that feeds everything. And it avoids the lost neutral issue (seems to be a common theme here) potentially subjecting equipment rather greater voltages than their rating with the ensuing consequences.

No live electrical part should come in contact with people.

But it happens and that is a fact.
If we want to look at our work that way than over current protection is also unneeded because no conductor should be overloaded.

Not the point I was making. Of course there should be overcurrent protection.
Residual current trips (30mA) are used to detect earth leakage and disconnect the supply should there be current flowing where it shouldn't. Like through a person.

If a supermarket let that happen they would be under immediate investigation,and the management would probably face criminal proceedings

Certainly it is possible if they were negligent that they would be sued, it would be a almost unheard of occurrence for management to face any criminal charges over this in America. (I am not saying that is good, just the way it is)

Safety is being taken increasingly seriously here in UK. As it should. Employers have a duty of care and must take all reasonable precautions to ensure the safety and welfare of employees. Failure to do so can and has resulted in criminal proceedings. As a result, all responsible employers bend over backwards to be seen to be doing the right thing. Sometimes to the detriment of common sense.

No, here in the US many more people are killed by 120 volt circuits than any other voltage .......... of course that has a great deal to do with the fact it is everywhere here.

And while 120 is not 'safe' it is in fact 'safer' to humans given the same set of circumstances.

For the same circumstances, I agree.

So as an engineer you are willing to say one voltage is alleyways the the best to use for the task at hand regardless of the load?

No. And I didn't. But for residential applications I see no need for two different voltages. UK and Europe manages perfectly well on single phase 230V for the vast majority of residences. A very few have 400V three-phase four wire but those are generally very up-market. The three phase is to cope with the total load. It's still 230V phase to neutral.

Personally I see no need to bring 230 VAC to a night light, that is much more power than needed.

It's no more power. And if you already have 230V....why would yo have a need to introduce a different voltage?





Here starting with the 2008 NEC we must use receptacles in dwelling units with shutters. It was a good change. :thumbsup:[/QUOTE]

 

[Twoskinsoneman]

What reason should we when 120 is readily available?

I wouldn't mind seeing a transition if it could be managed. I don't see how it could though.
Sure would save on copper, which must be becoming a precious metal the way the prices are. :happysad:

 

[Open Neutral]

But you have not bothered to explain any reason limiting lighting to 120 volts in dwelling units is creating a hardship.

Two reasons:
a) Lighting is not the issue. I'd love to junk my PoS lawnmower et. al; throw out the gas cans and buy an electric mower & snowblower AND NEVER YANK A ROPE AGAIN.... But the only ones to be had are wimpy toys, because they are limited to 1800 watts. No one will make such that needs special outlets installed; they would not sell. But in the UK, you can get 7200 watts from a plug. I am envious...

b) Copper ain't getting any cheaper; and neither is Reddy Kilowatt. Running washers, windowshakers, etc on 240 cuts losses.

ps: Some old buildings in Cleveland only converted from DC to AC in the 50's (some of the old switchgear was abaondoned in place and is pretty fascinating).

I recall being in a downtown hotel, maybe the Statler-Hilton at E12 & Euclid, in the early 1960's. It still had DC outlets, intermixed with the AC ones.