Discuss Another socket puzzle: Why the extra terminal? in the Electrical Forum area at ElectriciansForums.net

Lucien Nunes

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Here's an old accessory made to BS1363. You can connect it to a ring and plug 13A plugs into it. What is the fourth terminal for?

I've blanked off some text moulded into the bakelite so as not to give too many hints at first. Nothing else is hidden.

4 terminal socket.jpg
 
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Lucien Nunes

Lucien Nunes

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Yes. What for?
 

Spoon

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Lucien Nunes

Lucien Nunes

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Fused output for e.g. a round pin lighting socket
Surely the point of BS1363 was that all appliances had the same plug, eliminating other kinds of socket? A lighting point controlled by a wall switch, fed by the lighting circuit, might reasonably use a round pin socket to avoid other appliances being connected. But this is off the ring, so why not use more 13A sockets?
 
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Lucien Nunes

Lucien Nunes

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You need to stop biting your fingernails
I ripped chunks off both thumbnails ages ago using them as emergency screwdrivers, but my hands have taken such a beating with working round the clock for months that all my finger tips are basically worn through the skin and the nails worn and split. I was even contemplating wearing gloves which I hate!
 
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Lucien Nunes

Lucien Nunes

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Front. So what's the terminal for?

20190208_193405.jpg
 

SJD

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So maybe a fused output for something you wouldn't need to plug/unplug, but is hard wired in - e.g. a doorbell?
 

SJD

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Aah, I was thinking "fireside" socket but no idea why it would need an extra terminal, unless for something to be hard wired in.
 
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Lucien Nunes

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Fused supply for the 5 Amp sockets, get a grip
They're 2A, and the fuse is built-in.
 
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Lucien Nunes

Lucien Nunes

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Why would you want to cater for 2A sockets when installing a ring?
 

SJD

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My comment was in reply to Lucien's question just above as to what you would use the 2A sockets for!
 

Pete999

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Table or standard lamps as was the norm in those days, 50s 60s 70s maybe long before people decided to change careers
 
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Lucien Nunes

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The fourth terminal is indeed an output sharing the onboard fuse protecting the 2A sockets. It's intended for the rarely-seen companion unit containing a further four 2A sockets. The combination appears on the ring as one double point (one 13A socket and one unswitched FCU) but gives a total of seven sockets in two accessories. Heater, clock, lamp, TV, radiogram, other lamp, train set, spare, or whatever.

Never caught on, presumably as it defeated the idea of all plugs being identical and hence interchangeable.

20190208_193556.jpg
 

littlespark

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Interesting piece of history there... never seen something like that before.
My gran had an adaptor from 13a plug to 2x15a + 1x 5a, so they could still use their old appliances without changing the plug.
 
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Lucien Nunes

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So it doesn't fit in a standard single k.o. box?
No, it's a custom size.

Kept meaning to show you this
Is that Bath - Off - Sink for a dual element immersion? Yes please, we don't have any of those. Will happily pay postage! Pity the red indicator lens is missing but we can find something suitable.

A side note about the fireside socket fuse. The original design shown here takes a BS646 fuse so the largest you could fit was 5A, which was correct for multiple 2A sockets on one circuit. But the later style in a standard 2G plate took a BS1362 fuse so a 13A fuse could be fitted leaving the sockets poorly protected. But in practice, unless the wrong appliances were connected to 2A plugs, due to a lack of 13A sockets, it was unlikely to cause serious hazard.
 

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