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Old 19th Dec 2016, 3:58 pm   #21
colly0410
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Default Re: Household DC Mains Supplies

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I also remember another house in the locality - a pit-village where the bulbs would periodically all have pulses of dimming (3 or 4 half-second dim periods in a 5-second timeslice). The locals called this 'cogging' and it was when a big conveyor was started to load the coal-train.
I used to live in a Notts pit village (Bestwood Village) where the older locals still moaned at paying for electricity, they used to get it un-metered in with the rent when the pit generated it's own juice up till the 1950's, I presume at DC. The houses went over to AC 240V from EMids electric in 1957-ish. It caused quite a hoo-ah when they had to put shillings in a meter..
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Old 19th Dec 2016, 4:00 pm   #22
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Default Re: Household DC Mains Supplies

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I heard the same many years ago. The explanation given was that a direct current may cause your muscles to 'clench', holding you in contact with the source, whereas an AC supply would 'buzz' you and throw you off. Sounds plausible but I don't really know.
Quite right Andrew, This is what I have always thought was the main reason behind the change from DC to AC voltage.
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Old 19th Dec 2016, 5:03 pm   #23
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Default Re: Household DC Mains Supplies

My family all lived in Usk Road Battersea until the early 70's and my uncle was a regular visitor to that shop. The DC came from a small power station by the river, not from Battersea power station.
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Old 19th Dec 2016, 6:16 pm   #24
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Default Re: Household DC Mains Supplies

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Quite right Andrew, This is what I have always thought was the main reason behind the change from DC to AC voltage.
I always understood that the big drive to switch to AC was because it could be transformed - something which became essential when you had a national generating/distrigbution network rather than local companies delivering power over just a few miles.

Trying to nationally generate and distribute thousands of Megawatts directly at 200-odd volts DC would have needed some seriously-thick copper conductors!

Also, three-phase AC gains you distribution-infrastructure efficiency! The only DC alternative is the 'trick' of centre-tapping the supply so some clients get a +ve DC and others a -ve (which leads to its own problems).

If I had to opine on the issue of AC vs DC then AC definitely wins - though I'd have preferred to distribute and deliver it at something like 400Hz because it means you can use smaller/lighter transformers and also when you need to rectify it the smoothing is so much easier!
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Old 19th Dec 2016, 7:24 pm   #25
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Default Re: Household DC Mains Supplies

Transmission loses is much greater at the higher frequency. Also lower frequency is better for motors. That is why 16 and 25Hz is still used - although not very often now thanks to variable frequency drives.
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Old 19th Dec 2016, 8:45 pm   #26
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Default Re: Household DC Mains Supplies

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I always understood that the big drive to switch to AC was because it could be transformed - something which became essential when you had a national generating/distribution network rather than local companies delivering power over just a few miles.
That is my understanding as well.

It would be interesting to compare the cost of sufficient amounts of copper for a national grid at 200VDC and the option (not available in those days) of superconductors - with all that would be entailed in keeping them cold enough. All in all, far easier to transform AC to many thousands of volts and correspondingly decrease the current-carrying capacity needed.
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Old 19th Dec 2016, 9:16 pm   #27
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Default Re: Household DC Mains Supplies

Yes, until relatively recently, there was no cheap and simple way to transform one DC voltage to another.
Therefore bulk production and long distance transmission HAD to be AC.

Towards the end of the DC era there was almost no production of DC in power stations. AC was generated at relatively high voltage and then stepped down to a lower voltage and rectified to feed existing legacy DC networks.

After about 1930, virtually all new public electrification schemes were at the new standard of 50 cycles AC, 3 phase, 4 wire at 240/415 volts.
DC systems, and AC at other frequencies or at odd voltages remained in use for decades, but the general policy was not to install any new schemes.
Private industrial supplies could be whatever the owner wanted, but increasingly tended to be the same as public supplies so as to utilise standard equipment.

I believe that the Savoy hotel in London used DC into the 1970s ! produced on site, not from public DC mains though. Can anyone confirm this ?
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Old 19th Dec 2016, 9:50 pm   #28
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I am wondering what sockets were used?
 
Old 19th Dec 2016, 10:11 pm   #29
colly0410
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2 prong round pin in our house IIRC, if you plugged the telly in & it didn't work you turned the plug round..
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Old 19th Dec 2016, 10:23 pm   #30
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Default Re: Household DC Mains Supplies

Yes the sockets at my grandfather's brother's house in Brighton were all BS 372 2 pin sockets of the 5 amp and 15 amp rating The 5 amp ones were off of the lighting circuit and were used for the radio and table and standard lamps. There were only two 15 amp ones, they were in the downstairs living rooms, one by each fireplace for portable radiant electric fires!

Trevor.
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Old 20th Dec 2016, 9:35 pm   #31
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Default Re: Household DC Mains Supplies

The standardization process was still going on on the 1970s I read the southern electric board changed the last non standard customers in Bournemouth in 1974. Apparently the man who started the process had retired by then and they had a switching off the last DC customer party!
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Old 22nd Dec 2016, 5:26 am   #32
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Default Re: Household DC Mains Supplies

At least tangentially relevant here is the excellent book “New York Power” (1). It chronicles the history of New York City (NYC) electricity distribution from the beginning with the Edison DC system, through the advent and growth of the AC systems, right up until the final demise of DC distribution in 2007. Quite early on, DC supplies, originally provided by local generating stations, were also provided by so-called “substations” that undertook AC-to-DC conversion using rotary converters or in some cases motor-generator sets. Early AC distribution was at both 60 Hz, for lighting and general purposes, and 25 Hz, largely for supply to the substations, as early rotary converters worked much better at this lower frequency. Also, until circa 1920, AC distribution where there were single-phase loads as well as polyphase was usually two-phase, as it was a quite while before the effects of differential single-phase loads on three-phase systems was well enough understood in a mathematical sense.

Many of the Edison urban DC power stations were converted to substations with rotary equipment. From about the mid-1960s, solid state rectifiers, installed in unattended street vaults, started replacing the substations, the last of which was retired in 1977. These solid state rectifiers worked with 60 Hz AC, so their advent also marked the start of the scaling back of the 25 Hz AC network. There was some 25 Hz-to/from-60 kHz conversion capacity. Strangely, there was minimal use of mercury arc rectifiers for AC-to-DC conversion, the transition being direct from rotary to solid state.

This book is a better reference source than the possibly better known “AC/DC: The Savage Tale of the First Standards War” (2), although as always, the more sources one has on a topic, the better. Many elements of the NYC story are likely applicable to other cities that had DC distribution systems that were eventually superseded by AC systems.

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(1) Joseph J. Cunningham; New York Power; CreateSpace Independent Publishing Platform; 2013; ISBN 13:978148-4826515.

(2) Tom McNichol; AC/DC; The Savage Tale of the First Standards War; Jossey-Bass; 2006; ISBN-13: 978-0-7879-8267-6.
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Old 22nd Dec 2016, 8:48 am   #33
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Default Re: Household DC Mains Supplies

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I heard the same many years ago. The explanation given was that a direct current may cause your muscles to 'clench', holding you in contact with the source, whereas an AC supply would 'buzz' you and throw you off. Sounds plausible but I don't really know.
I worked in a high voltage test laboratory as an apprentice and we were told that if you were in any doubt as to whether an uninsulated wire was live or not then you should touch it with the back of the fingers so that no matter if it was ac or dc your hand muscles would reflex away from the wire not towards or around it. Sounds very sensible to me, if rather casual from an H&S perspective, I guess apprentices were expendable then !!
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Old 22nd Dec 2016, 2:10 pm   #34
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Default Re: Household DC Mains Supplies

Depending on the nature of the load you can get some serious arcing across the contacts of a DC switch at 200 volts. Could this also be a contributor to the changeover?
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Old 22nd Dec 2016, 3:44 pm   #35
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Default Re: Household DC Mains Supplies

I can remember in the 1960s on London underground hearing the puffs of compressed air that were used to blow the arcing out on the motor tap switching systems they once had. You could only hear that sound on the older brake dust smothered trains. Later trains had resistive braking.
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Old 23rd Dec 2016, 6:22 pm   #36
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Default Re: Household DC Mains Supplies

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Depending on the nature of the load you can get some serious arcing across the contacts of a DC switch at 200 volts. Could this also be a contributor to the changeover?
At one time domestic switches for things like fuseboxes, lighting and power sockets were sometimes described as "QMB" - Quick Make-Break - meaning they used an overcentre spring-toggle arrangement to ensure a snap-action of the contacts and reduce the risk of arcing when switching a possibly-inductive DC load. Typically they had two fixed contacts and a U-shaped metal 'saddle' that fitted between them, so providing two significant and separate air-gaps across which any arc had to jump when separating.

These days because everything's AC most domestic light- and power-switches use a simple cam to separate the contacts, and trust to the voltage/current-reversal of AC to nullify any arc.
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Old 24th Dec 2016, 9:52 pm   #37
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Default Re: Household DC Mains Supplies

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I worked in a high voltage test laboratory as an apprentice and we were told that if you were in any doubt as to whether an uninsulated wire was live or not then you should touch it with the back of the fingers so that no matter if it was ac or dc your hand muscles would reflex away from the wire not towards or around it. Sounds very sensible to me, if rather casual from an H&S perspective, I guess apprentices were expendable then !!
I always employ the same technique, almost unconsciously, even if I'm confident that something isn't live.
It's also useful for painessly discharging static; for example, the local shopping centre always charges me up for some reason, so I rap the escalator handrail with my knuckles before mounting it.
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Old 25th Dec 2016, 1:19 am   #38
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Default Re: Household DC Mains Supplies

I use a powered wheelchair which generates lots of static in the supermarket (plastic floor + plastic wheels + metal frame + polyester fleece clothes = Van de Graaff generator) so I use that technique to discharge static onto metal fridges. Otherwise it discharges onto conductive food items.
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Old 25th Dec 2016, 1:45 am   #39
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Default Re: Household DC Mains Supplies

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I always employ the [outside of hand] technique, almost unconsciously, even if I'm confident that something isn't live.
It's also useful for painessly discharging static; for example, the local shopping centre always charges me up for some reason, so I rap the escalator handrail with my knuckles before mounting it.
Snap! One particular store in my home city is particularly bad for this, so I always give the handrail a quick touch with the outside of my hand before getting on the moving stairway.

I was shown the actual technique of using the outside of your hand, so your natural reflex action tends to draw you away from something that may be electrified rather than into it, a long time ago .....
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Old 25th Dec 2016, 8:45 am   #40
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Synchrodyne: Was it a typo or did NYC really have some DC distribution until 2007?
If true I assume it was for industrial or transportation & therefore off-topic, but I was curious.
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