|Non-standard BS 1363,
Clock and Walsall plugs and sockets
|1, 5||13A, non-standard socket and plug for use on installations requiring restricted access (medical equipment, computers etc.), or a special type of earthing (see no. 2). Because of the T-shaped earth pin, non-standard and standard BS 1363 devices cannot be mixed. Brand name: MK Electric.|
|2||Both non-standard sockets nos. 1 and 3 have two independent earth terminals. The image shows at left a conventional terminal, linked to fixing screws, and a 'clean earth' terminal at right, linked to the earth pin. The socket can be used for 'clean earth' as well as conventional earth installations. Depending on the installation the green/yellow earth wire link has to be removed; see details below.|
|3, 6||13A, non-standard socket and matching round pin earth plug. Meant for comparable purposes as nos. 1 and 5, but manufactured by Eaton - Cutler Hammer, under brand name MEM. The switched socket has a neon indicator light.|
non-standard socket and plug with pin configuration identical to nos. 3
an 6. Wandsworth is the UK's oldest independent manufacturer of
electrical accessories. It is unclear which company has originally
designed this plus with a round earth pin for applications where a
non-interchangeable facility is necessary.
Note the characteristic typeface of the Wandsworth company name.
|Clean earth socket.
The 16th edition of the IEE Wiring Regulations (BS EN 7671 amendment No.2) require the fixing screws of all mains sockets to be earthed. This is normally achieved by a link that connects a metal insert in the fixing holes of the socket to the earth pin connection.
If a metal back-box is used that is in contact with the building earth, a large amount of current can flow through this path if there is a significant difference in potential between the earth reference bar (ERB) and the building earth at that point. This problem is more likely to occur in an old building where there may be a link between earth and neutral somewhere in the wiring. The fault current can inject large amounts of noise into the equipment that is being fed from the supply. This may interfere with sensitive medical equipment.
The problem can be reduced by eliminating the current path between the ERB and the building earth provided by the earth connection in the mains socket. This can be done by replacing the metal back-box by a plastic one, or by fitting a clean earth socket.
A clean earth socket has two separate and isolated earth connections – one which goes to the earth pin receptacles of the sockets, and the other which goes to the fixing screws (back box). The connection to the earth pin receptacle should run via a separate cable, back to the earth reference bar (ERB). The fixing screws may make contact with the structural earth via the body of the (metal) back-box and so be at a different potential to the ERB. Therefore a separate earth is connected to this ‘dirty’ connection and also returned to the appropriate point on the ERB. Note that in this case the wire link shown in image no. 2 has to be removed.
Source: Edmundson Electrical, Medical Electrical Installation Guidance Notes; Appendix 3: Clean earth sockets.
plugs and sockets for special use
|8, 9||Outlet and plug for AC electric wall clocks, manufactured by MK Electric. The low profile of outlet and plug allows placement hidden behind a sufficiently large clock. Image no. 9 shows the outlet without clock plug (see no. 10).|
|10, 11||Electric clock plug, equipped with a retaining screw to prevent accidental disconnection. Image no 11 shows an enlarged view of the reverse side of the plug, with a BS 1362 2A fuse and three small, flat blade pins (live, neutral and earth).|
|12 - 15
gauge socket and plug. Compared to pin orientation of BS 1363 plugs
each of the
Walsall pins is 90 degrees rotated. Live feed is protected by a 13A BS
1362 fuse. For obvious reasons the plug bears a NOT FOR DOMESTIC USE
Image no. 14 shows a part of a dialysis machine. The Walsall socket was used so that only an approved appliance for the machine (blood pump) could be used. Walsall plugs have also been used for local power supply backup networks and can still be found at some London Underground stations, see images 16a/b below.
Image no. 15 shows the name Walsall on an older (pre-1994) plug [top] and a modern (= image no. 13) model [bottom].
The clock outlet (image nos. 8-11) is generously donated to the museum; see Acknowledgments.
Photo no. 14 (dialysis machine and Walsall socket) has been copied, with permission, from Robmcrorie's Photostream; see http://www.flickr.com/photos/robmcrorie/6539260819 for the original image.
|Photo taken at London Underground Bond
Street station (Jubilee level).
Despite tape on the socket it is clearly visible that it is a Walsall type outlet.
Two possible reasons for choosing non- standard sockets at Tube stations are:
(1) preventing that the public (mis)uses the London Underground electricity network;
(2) the outlet is connected to a centre-tapped 110V circuit; for safety reasons a mandatory practice on construction sites in the UK.
The photo has been taken in 2011 by Alastair Swaffer under difficult conditions in a very busy platform.
Still missing are Britmac plugs and sockets, another derivative of BS 1363. There is (or has been) a type with round and rectangular pins. One of the flat pins is 45 degrees rotated, see image at right. It seems that both live and neutral pins are fused which would allow the use on a centre-tapped to earth 110 - 0 - 110 V network.
If you have a Britmac plug and/or socket on offer, please contact me; see about the collection for the address.