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Lucien Nunes · 2nd Jul 2022, 4:19 pm

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.

2nd Jul 2022, 4:19 pm	#24
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: An unusual BS546 plug 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.