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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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30th Jan 2018, 5:18 pm | #41 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
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Re: Earthing of phone lines
I believe that earth return was avoided because the return current would tend to pass along the railway lines rather than through the general mass of earth, and this might interfere with track circuits used for signalling purposes.
Sometimes the supply was derived from an isolating transformer, in other cases it was either phase and neutral or two phases from the public supply. |
30th Jan 2018, 10:58 pm | #42 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
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Re: Earthing of phone lines
Single cut ( rail signal terminology = circuit where supply is broken in a relay with a contact in one, usually supply leg). Double cut places a relay contact on both legs.
Earthing on rail signaling circuits presents a fault, as, in single cut circuits, any earth fault on a signal wire could S/C the relay contact to break the circuit, causing a fault condition to misoperate a signal or equipment... Hence the introduction and preference for double cut circuits. For those of us familiar with circuit diagrams of relay contacts- rail drawings are as speaking in a foreign tongue. ( From memory)Current system shows a relay operate path ( e.g.) as a straight line with a make contact depicted as a "v" above the line, and a break as an inverted v below the line. This was adopted to prevent mistakes as the older practice was to show make contacts as above but not connected to the line, but break contacts as above, but connected to the line. The newer practice was adopted after a lot of drawing office mistakes. in parts of the country where traction current can be delivered over the tracks, signaling sections were ( I don't know if this is still the practice) isolated by using large inductors and AC signaling. Each rail was connected to the next length using a large inductor so that 50 Hz would pass, but the inductance blocked the HF signals. |
31st Jan 2018, 1:21 am | #43 |
Dekatron
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Re: Earthing of phone lines
I did read somewhere that the reason why 3rd and 4th rail traction supply had to be used for London's District and Metropolitan Lines' electrification circa 1904 was due to the Post Office's concern about leakage currents affecting the telegraphs. The Post Office didn't seem to have been bothered by tube lines that had opened a decade or so earlier that used only third rail electrification, but I suppose that, unlike the District & Metropolitan railways, tube lines run in cast iron tunnels that would have provided effective shielding. The Central Line tunnels pass within a few hundred yards of what used to be the Post Office's central telegraph office near St. Pauls seemingly without causing problems, and the Central Line didn't change to 3rd and 4th rail system until the 1930's.
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31st Jan 2018, 11:20 pm | #44 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
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Re: Earthing of phone lines
3rd rail is a rail close to a running rail and constantly powered at 650vDC. The fourth rail is a rail in the middle of the running rails ( referred to as the "four foot"). I've heard several tales of when this rail is live. But the consensus of opinion on all NR courses I've been on was to treat as live, as it depended on trains/UG in section.
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31st Jan 2018, 11:43 pm | #45 | |
Dekatron
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Re: Earthing of phone lines
Quote:
Something to do with protection perhaps? Sorry to veer off topic a tad.
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1st Feb 2018, 12:56 am | #46 |
Nonode
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Re: Earthing of phone lines
NO- third rail ( one adjacent to a running rail, but mounted on insulators) is live WRT earth at 650v DC. NR safety courses advised us to treat the fourth rail, mounted midway between running rails as live at all times at a hazardous voltage.
Something else public is not aware of is that on the likes of the West Coast mainline, there are things called "red bonds", where a high voltage can exist on earth. And the line voltage on overhead is constantly uprated using autotransformers to keep the Overhead at 25KV. |
1st Feb 2018, 9:47 am | #47 |
Moderator
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Re: Earthing of phone lines
Back on topic please.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
1st Feb 2018, 10:30 am | #48 | ||
Heptode
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Re: Earthing of phone lines
Quote:
Ian . |
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1st Feb 2018, 3:46 pm | #49 | |
Dekatron
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Re: Earthing of phone lines
Quote:
Incidentally, these telegraph earth-return circuits to the PO you mentioned earlier in post #37... Were they a straight-forward key/sounder system or did they use a 'silent' Fullerphone-type arrangement where signals could not be detected in the earth-loop (and so intercepted)?
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1st Feb 2018, 8:18 pm | #50 |
Heptode
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Re: Earthing of phone lines
Re the 'earth return' telegraph circuits - I'll have to some 'digging' through diagrams. Luckily I have an original set of diagrams for the GPO's telegraph and telephone equipment in use in circa 1886 - so that should give a clue.
I have one of those exchanges but mine came from a more remote location - https://www.google.co.uk/maps/place/...9!4d-2.0695406 |
1st Feb 2018, 8:36 pm | #51 |
Dekatron
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Re: Earthing of phone lines
If it's that age it won't be Fullergraph stuff as I believe that was invented in 1915 during WWI. I have a book: 'Instruction in Army Telegraphy and Telephony' (1914) and I imagine some of the circuits within will be like the ones you are about to look for.
The book mentions that earth returns and the advantages for telegraph ccts, but only on short telephone ccts, 'especially in the country, well away from other wires carrying electric currents; where they are often used for military work...'
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1st Feb 2018, 10:58 pm | #52 | |
Heptode
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Re: Earthing of phone lines
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1st Feb 2018, 11:51 pm | #53 | |
Nonode
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Re: Earthing of phone lines
Quote:
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2nd Feb 2018, 1:40 am | #54 |
Nonode
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Re: Earthing of phone lines
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2nd Feb 2018, 9:43 am | #55 |
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Re: Earthing of phone lines
Sorry no. There are plenty of railway forums where you can discuss that subject.
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2nd Feb 2018, 12:46 pm | #56 | |
Heptode
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Re: Earthing of phone lines
Quote:
One of the 10 line exchanges was used on Foula & Papa Stour (in the Shetland islands) plus Soay and other Inner Hebridean Islands before they were replaced with similar exchanges to the one on Canna. These were known a 20 line 'Island Automatic eXchange No 5' (IAX5). However Soay was never converted. I have the mainland end of the junctions from Canna, Muck, Rhum and Soay recovered from Mallaig in 1997. These exchanges were developed by Ian McPherson of THQ Scotland - the exchanges being built by PO Factories. Somewhere I have a folder of notes and diagrams that Ian McPherson wrote and had ended up at Inverness before coming my way. There is an article in the IPOEE Journal describing the 10 line exchange. The workings of the IAX5 appeared in the THG Journal around 2000. Ian J |
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