DC mains

[winston_1]

Easiest way to check polarity is the neon screwdriver. If both electrodes light it is AC. If one electrode lights it is DC. The illuminated electrode shows polarity but I can't remember which way round it is.

[Richard_FM]

Originally Posted by broadgage

Parts of New York still had DC mains until 2007 ! I think Chicago may still have DC service in parts.

DC radio sets invariably had a rectifier so as to permit of use from AC as well.
DC had to be of the correct polarity, if the heaters lit but no sound was produced then supply polarity had to be reversed, easy with non polarised 2 pin plugs.

When determining the polarity of the supply was important two methods were used. "pole testing paper" could be used, this was a white blotting paper type material, supplied in books of small pieces. To use, a strip was wetted and DC of unknown polarity applied via test prods or bared wires. A bright red colour appeared around the negative pole, for a "high voltage" supply of more than about 125 volts, connect a small lamp in series.
The other method used a battery and a small lamp, connected in series to the unknown polarity DC mains. By observing which connection of the battery gave the brighter light, polarity was readily determined.

That's interesting to know!

[G6fylneil]

In the '70s I repaired a "record player" for a fairground ride. It used octal valves but wasn't very old. It came in with external rotary converter to supply it with 240VAC from the rides 110Vdc supply.

[Hartley118]

Originally Posted by G6fylneil

In the '70s I repaired a "record player" for a fairground ride. It used octal valves but wasn't very old. It came in with external rotary converter to supply it with 240VAC from the rides 110Vdc supply.

I have happy teenage memories of fairground dodgem cars in the 1950s/60s with cheerfully loud music from a BSR autochanger and valve amplifier supplied from such a rotary converter. The aspect I particularly remember was the way the record slowed due to the voltage drop in the DC feed to the rotary converter as the cars all started up together. Fairground technology was pretty agricultural back then.

Martin

[kalee20]

Definitely! A valve inverter would've been a better bet... a turntable motor needs only 10W or so, and a little RC oscillator driving a couple of CL33's in push-pull would ve well up to the job.

(I'm thinking of the big steam engine nearby, with front-mounted dynamo, long flat belt with the period click as the joint runs around the pulley, providing the 110V DC... happy days!)

[kellymarie]

Weymouth had DC mains in the town center until the 1960s the last DC customers were the water company who had a pumping station in Stanley Street. The power station was at sunnybank it gave 230/460 volts DC and AC 50cycles consumers had 230 volts on both currents. Our local station was supplemented by a 33Kv feed from the grid by a substation out near Dorchester AKA The frozen north!

[Graham G3ZVT]

I don't believe that my grandparents house was ever supplied with DC, but what it did have was a large metal-clad fuse box with fuses in the live and neutral which I think was intended for DC.

It wasn't ideal when a neutral fuse blew, leaving the socket(s) live, I eventually fixed that by using thick wire and marking them up as neutral links, not to be removed.

All radial wiring, and Wylex sockets (in-line pins) that were unknown in most parts of the country, but fairly common in Manchester.

[emeritus]

The GEC catalogue for 1893 includes a couple of drawings of wiring installations. Fuses are provided at each light switch and in each return conductor for each lighting circuit controlled by each respective switch. . This is stated to allow each circuit to be tested individually by removing the fuse in the return and opening the switch in the line conductor. Presumably that was the original reason for providing fuses in both conductors.

[broadgage]

Originally Posted by emeritus

The GEC catalogue for 1893 includes a couple of drawings of wiring installations. Fuses are provided at each light switch and in each return conductor for each lighting circuit controlled by each respective switch. . This is stated to allow each circuit to be tested individually by removing the fuse in the return and opening the switch in the line conductor. Presumably that was the original reason for providing fuses in both conductors.

I have heard two alternative reasons for fusing BOTH sides of a DC mains supply.
Firstly the supply arrangements might alter, and a supply with an earthed neutral might in future have BOTH poles live.
Secondly, many premises had a 3 phase AC supply for "power" and a DC supply for "lighting". This presented the risk of a short circuit between one phase of the AC supply and the DC neutral, if the AC supply was large such as say 100 amps to a large motor, then the fault current could melt the DC neutral wire before a perhaps 150 amp fuse in the AC circuit operated.

Early AC supplies were often at a frequency too low for lighting, hence the retention of a DC lighting supply.

[Electronpusher0]

Originally Posted by broadgage

Early AC supplies were often at a frequency too low for lighting, hence the retention of a DC lighting supply.

My father used to tell me that when he was a child the lights used to pulse slightly in brightness, I had always assumed that it was due to a slow running dc generator but perhaps it was low frequency AC.
This would have been in the 1920s, the generator was in Duke Street Littlehampton.

Peter

[emeritus]

Some early AC supplies used frequencies higher than 50 Hz. The electrical section of the 1899 edition of Molesworth's Pocket Book of Engineering Formulae says this in the section about AC machines:

" In American practice the frequency generally adopted has bern about 130 per second, whilst in Europe a a frequency of about 70 per second is more common, and there is a tendancy to reduce it. .......
Low frequency increases the cost of the transformers, but increases the available energy. A frequency of 42 per second is about the lowest that can be used in incandescent lamps without perceptible flickering. Forbes proposed a frequency of 16 1/2 as the best for utilising the motors in transmission of power from Niagara. "

The small power station at Lynmouth provided mains of 100V at 100Hz until it was destroyed in the 1952 flood, the last public 100Hz system. Its destruction brought forward the arrangements that were already in hand to replace it by a connection to the national grid at the normal voltage and frequency. I understand that Niagara eventally used 25Hz frequency

[broadgage]

Originally Posted by Electronpusher0

My father used to tell me that when he was a child the lights used to pulse slightly in brightness, I had always assumed that it was due to a slow running dc generator but perhaps it was low frequency ac.
This would have been in the 1920s, the generator was in Duke Street Littlehampton.

Peter

Pulsation on lamps can also be caused by a slow speed reciprocating engine driving a dynamo or an alternator.
With AC the average speed is easy to regulate to produce say 50 cycles, but the speed at any instant is very variable, rising during the power stroke and then falling until the next power stroke. The average voltage may be regulated to whatever figure is desired but most voltage regulators can't keep up with the continual speed variation.
For power generation, high speed reciprocating engines (steam or diesel) are preferable to slow speed, and turbines (steam, water, or gas turbines) are better still.

[broadgage]

For lighting on low frequency supplies, there were several solutions.
1)Retain gas lighting, zero capital cost if already installed.
2) Obtain a dedicated lighting supply at DC or high frequency AC. Either from another supply company, or generated on site, or converted on site.
3) Use lamps of lower voltage, the thicker filaments of which flicker less. 6 volt 36 watt vehicle lamps powered via transformers were sometimes used. Or 50 volt, 300 watt lamps in series to suit the supply voltage. These flickered less than 250 volt 300 watt lamps. The lower voltage lamps tended to be cheaper and more efficient.
For localised task lighting on a machine a bicycle dynamo driven from some moving part of the machine could light a 6 volt 3 watt bulb, almost zero running cost and and more light than a candle with negligible fire risk.

A rather specialist portable lamp used compressed air to drive a miniature alternator via a small turbine, used in coal mines, petrol stores and underwater. These are still made today !