An unusual BS546 plug

[winston_1]

Originally Posted by duncanlowe

I do wonder if they are still in use in another part of the world. For sure in South Africa round pin plugs of the old UK style are still the norm.

India, Nepal, Pakistan, and Sri Lanka use 5 and 15 amp versions but never seen a 30 amp one. Likewise Malaysia and Singapore use 15 amp version for supplying fixed devices, usually air con or electric showers (showers there only need to be 3KW due to the hot climate), but again never seen a 30 amp one. I would expect to find them in street markets if they existed but never have.

On the subject of big plugs France has some unusual ones for cookers. A poke around LeRoy Merlin (bit like B and Q) electrical section is interesting.

[Stuart R]

I remember seeing larger-than-normal round-pin sockets in our local hospital (built in the early 1970s). Just one or two at skirting board level in each ward. Portable X-Ray machine, possibly?

SR

[G6ONEDave]

I think Lewden did a 30A round pin connector for outdoor use that was a similar size. The difference being that the Lewden version was in a metal body with a screw on fixing ring.

Dave

[Lucien Nunes]

It's still made, but it's not to BS546 dimensions; smaller, nearer 15A in scale.

The existence of a 30A type to BS546 raises the question of what purpose a 30A light-duty plug serves in 220-250V localities.

In a domestic environment there are few movable appliances over 3kW that would benefit from a plug and socket i.e. discounting plumbed-in or ducted water and air heaters. Whether a freestanding domestic cooker is movable has been somewhat moot but in any case in the UK we have always had a convention of hard-wiring cookers into the installation. There is some logic in enforcing a high-reliability earth connection where leakage currents are likely to be high as with sheathed heating elements.

For industrial applications where there might have been more likelihood of needing to move equipment with consumption over 15A, since 1930 there was the option of BS196 industrial plugs and sockets offering a much more robust construction than BS546, which included a 30A size. These started to become obsolete with the introduction of BS4343 in 1968, which as EN60309 is now the universal solution for 32A at 230V. Certain industries invented their own types with specific features, e.g. the Healee used in film lighting (15, 25, 45A) where like domino connectors the plug and inline socket are both flat to minimise trip hazards on a stage floor.

So the 30A BS546 seems to fall through a hole in the middle; too big for a wash boiler, not wanted on a cooker, too flimsy for a welder, too lumpy for a film light...

It's interesting to compare the situation in other countries where things turned out slightly differently. In Europe there has been more of a habit of plugging cookers into sockets and also significant domestic use of 3-phase supplies. This has led to such plugs as the Perilex 3P+N+E which exists in 16A and 25A variants. With 3-phase you can get up to 11kW from just the 16A size so that is the more commonly-seen. These are sometimes sneakily used as 32A 230V single-phase by making two pins line and two neutral, the problem being that there is no rule on which line pin becomes the second neutral. There have been others in the same role, e.g. Russia had a specific single-phase 'stove plug' with 3 flat pins rated at 25A.

In the USA, high-load appliances are often 240V (as most premises have 120-0-120) and therefore not limited in power to the 1800W available from a standard 15A NEMA 5-15 outlet. However, their tumble dryers have typically gone beyond the UK's 3kW domestic white-goods limit and needed in excess of 15A even at 240V. Therefore amongst the dozens of NEMA plug designs, the one that came to be called a 'dryer plug' was the 3-prong 30A, 240V NEMA 10-30 which is conceptually equivalent to the BS546 30A. Except for the nagging detail that residential 240V runs between the two lines, so with a 3-prong plug there is no neutral. Manufacturers were in the habit of returning small 120V loads such as the timer motor to protective earth, which was oficially permitted until quite recently. Nowadays the 4-prong NEMA 14-30 is mandatory with separate N & PE, with the side-effect that it can also be used as a source of 120V, 30A. Again there is a distinction between domestic and industrial applications, with twist-lock type L6-30 finding use in the latter.

[G6Tanuki]

The 'big' metal-clad round-pin plugs/sockets I remember were made by Clang, later Niphan. They were used extensively by the Military [which is where I came across them, connecting trailer-mounted generators to radio-trucks] and also on the Railways - both in static trackside/workshop applications and for the interconnects between locomotives and carriages.

[broadgage]

Originally Posted by emeritus

I was aware that BS 546 included a 30A plug, but this is the first one I have seen a photo of. Perhaps they were needed for 3kW appliances on 110V mains?

Wandsworth also made the eponymous plugs and sockets, where the earth pin was slotted, and a transverse bar in the slot operated an internal switch in the socket when the plug was inserted. My Aunt's circa 1930-built council house on the Becontree estate in Essex had them until the house was rewired in the mid-1960's

Possibly for 3kw at 110/120 volt, but heavy loading appliances were not common on 110/120 volt systems.
Most systems at those lower voltages were sized primarily for lighting, and the odd low power appliance such as
Radio set.
Desk fan.
clothes iron, 600 to 750 wats.
Slow boiling electric kettle often 600 watts.
Small heaters seldom exceeding 1 kw.
Vacuum cleaner of a few hundred watts.
A single electric boiling ring of a few hundred watts.
Large scale space heating, water heating or cooking was by gas or coal.

A lot of 110/120 volt systems had a service fuse of only 20 amps for a small house or 30 amps for a large house.
Sub-circuits were typically 5 amps for lights, limited to about 10 lamps per circuit.
5 amps for several 2 amp socket outlets.
5 amps for a single 5 amp socket.
10 amps for two 5 amp socket outlets, often unofficially extended to three or more outlets.

Heavy loading heating and cooking appliances were GENERALLY restricted to 200/250 volt circuits. Often two versions were manufactured, one for 200/210/220 volt circuits and one for 230/240/250 volt circuits. No 110/120/127 volt versions of most heavy loading appliances.

[AndiiT]

Hi, I have spotted 30 Amp sockets in St James Hospital, Leeds, at least I did ten years ago when my grandson was born.
As has already been mentioned, I suspect that they were for use with portable X-Ray machines.

Andrew

[Dave Moll]

Originally Posted by broadgage

Possibly for 3kw at 110/120 volt.

Except that the plug is clearly marked "250V".

[broadgage]

Originally Posted by Dave Moll

Except that the plug is clearly marked "250V".

Yes but I presume that means "up to 250 volts" and not "only for use on 250 volts"

[emeritus]

The GEC Heating and Cooking Appliances catalogue for 1911 has two electric ovens (with hotplates), rated at 2.5kW and 3.4kW respectively. The catalogue says that all appliances were available for voltages between 100V and 250V, voltage to be specified when ordering, and that the lower-voltage versions cost extra. Given their size and weight, and that GEC's highest rated plug for domestic use at the time was 10A (*), the cookers would probably have been connected via fixed wiring. This would almost certainly have been the case for the 3.4kW oven, which was offered in two versions, one with its switches on the side of the oven, the other on a discrete wall-mounted control panel.

(*) 25A and 50A plugs were offered for theatrical and industrial use.

[Tractorfan]

Hi,
The hospital I worked in, many moons ago, also had large metal clad sockets on the wards for portable X-ray apparatus.
Also, curiously, some X-ray units were equipped with a BS1363 gauge plug of very heavy duty and without a fuse. Presumably for use where a special 30 amp outlet wasn't available. I often used to wonder how the standard 13 amp sockets coped with X-ray usage.
Cheers, Pete.

[Dave Moll]

See also post no. 37 in this thread.

[broadgage]

Originally Posted by Tractorfan

Hi,
The hospital I worked in, many moons ago, also had large metal clad sockets on the wards for portable X-ray apparatus.
Also, curiously, some X-ray units were equipped with a BS1363 gauge plug of very heavy duty and without a fuse. Presumably for use where a special 30 amp outlet wasn't available. I often used to wonder how the standard 13 amp sockets coped with X-ray usage.
Cheers, Pete.

Standard 13 amp sockets were fine with the 20 amps or more used by a portable X ray machine. The overload was very brief indeed, less than one second whilst the X ray "picture" was taken. Indeed the overload was so brief that a standard plug fuse often survived, but not reliably.

[Dave Moll]

I assume, however, it needed to be plugged into a ring main with 32A breaker rather than a 16A spur, as the latter presumably would be tripped by the brief overload.

I should say that this wouldn't apply with old-fashioned fuses rather than breakers.

[Lucien Nunes]

Good point, there is no value in uprating the plug fuse to a brass slug only to be caught out by the upstream protection. A socket served by a 13A fused spur would be a potential headache but these would not normally be found in commercial premises.

The conventional OCPD rating for radial circuits serving socket outlets is 20A even if only one point is served, which should be fine for an X-ray. The distinction between 32A ring and 20A radial is starting to get blurred by the increasing use of 32A radials wired in 4.0mm².

A 20A MCB to either BS3871 or EN60898 must hold in on its thermal trip for 1s at 2.55*In (51A) and the magnetic trip must not operate below 3*In (60A) on a type B, but commercial / industrial socket outlet circuits are often type C or 3 with a yet higher magnetic threshold of 5*In.

[sentinel040]

Earlier this year my partner had a stay in St Thomas's hospital in central London and the room she was in had one of these sockets close to the door.

Ian

[broadgage]

I recall seeing these decades ago in catalogues.
A switched 15 amp round pin socket but unusually with a cartridge fuse holder mounted on the face plate. Fuse was a 15 or 20 amp cartridge fuse as used in consumer units. Blue for 15 amps and yellow for 20 amps.

Engraved with the wording "air conditioner" The 20 amp fuse was to allow for the starting current of a large air con unit with a running current not exceeding 15 amps.

30 amp or larger fuses would not fit the fuse carrier thereby deterring over fusing.

Primarily for export I suspect, perhaps to places with small and lightweight mains connectors not judged suitable for long hour use at a full 15 amps.

I recently found one in the wild ! Still in use for an air conditioner that had succumbed to the heat wave. Re-used it for a modern air conditioner.

[kellymarie]

When I was in Bromtom hospital years ago I remember seeing large presumably 30 amp sockets clearly marked x Ray only! Interestingly at the time the hospital was being rewired so not sure if those sockets survived much after that time. Weymouth hospital has the 13 amp x Ray sockets again with a warning to not plug anything else in

[McMurdo]

Wandsworth did and AFAIK still do have a special hospitals divison, bedhead panels, nurse call systems, intrinsically safe light switches for operating theatres etc

[pmmunro]

Originally Posted by McMurdo

Wandsworth did and AFAIK still do have a special hospitals divison, bedhead panels, nurse call systems, intrinsically safe light switches for operating theatres etc

Sorry to be pedantic Kevin, because I know what you mean and I'm sure you fully understand the concepts.

The switches must have been flamability protected (Exd or Exe probably) because the intrinsically safe technique (Exp) limits energy in the protected area to levels incapable of igniting a hazardous atmosphere and therefore would not be suitable for lighting.

PMM