Remembering the PDL 40A Interrupted phase tap-on plug

The final version of the 40A (likely discontinued in the early 1990s) – distinctively coloured white and red

Recently while watching the YouTube channel of UK Electrician John Ward I came across a most interesting clip where an eager viewer from New Zealand has posted in a considerable collection of electrical bits and bobs. Myself originally being from New Zealand it was amusing to watch. Among the collection is a most interesting combination antenna & power socket, which certainly, I had not ever seen before.

One item our enthusiastic mailer of electrical articles has not included, but has made the host aware of, is the subject of this article: The long-discontinued PDL 40A – the de-facto symbol of Kiwi electrical innovation and nostalgia.

Wired with a 4 core piece of red-white-blue flex, an uncommon practice. In this case red is used for phase, and white for phase return

The key difference between these plugs and a regular tap-on is that the phase pin on the rear socket is not connected to the plug side, therefore, using a 4 core cable, the socket on the back can be switched via some kind of control device on the end of the lead.

The underside of a “red-white” 40A. The screw in the centre holds the phase pin to keep it isolated from the rear connector

Typical uses were:

  • Float switches for water pumps
  • Timer switches for lighting or heating devices
  • In engineering environments it is common to find them with a loop of wire attached to the phase pins for attaching inductive clamp meters
  • Anything else you can think of that has to switch a single appliance, without the desire to expend effort fitting a socket to that device
A 40A retrofitted with the back of newer “slim” 40 plug, this particular one wired with a loop for use with clamps meters

While they were designed for use with 4 core cable – ‘Kiwi ingenuity’ is actually another form of the phrase ‘Hook or by crook’ and not surprisingly I have not ever seen one wired like this (that wasn’t wired by myself). Typically 3-core cable is used, then the earth wire gets re-purposed as the phase return, and the switching device has to do without earth. In the case where an earth is connected to the switching device, it’s because the neutral has been done away with, or some other solution is devised that doesn’t involve purchasing a length of  4-core cable.

An early “all-black” 40A found in a typical use-case, a wall thermostat

I find myself wondering if the practice of using these plugs with 3 core cable may have contributed to PDL’s decision to discontinue it. Certainly in the case of earlier versions of the plug which aren’t easily identifiable as interrupted phase versions, subsequently wired with 3 core cable in some unknown likely dangerous arrangement i.e. earth connected to the phase pin – that cable could be mistakenly re-wired onto a metal chassis appliance likely leading to a fatal electric shock.

As expected – the earth wire becomes phase, and red becomes phase return. Our thermostat doesn’t happen to need an earth. A death sentence in the case where this cord is cut from the thermostat and re-used on another appliance by an unsuspecting individual
The top of an older “all-black” 40A
The black 40A. Correctly re-wired (for demonstration) with 4-core brown-black-grey flex

The Australians have got their own version of this – made, of course, by Clipsal.

Clipsal 461SUA – Still manufactured at the time of writing

For anyone wanting this kind of plug, at least these are still made, and certainly, by the time I started wiring stuff it was the only one purchasable. I can say from experience it’s just not the same as using a 40A. While not quite of the same quality – It could be argued that the Clipsal is better, because both the line and neutral are “interrupted”, for the almost inconceivable scenario where an RCD is doing the switching perhaps? Making full use of this does require a rather unwieldy length of 5-core flex, which by the time we get to 1.5mm is pretty big stuff, typical for full load 10 amp applications.

The fact that we’re using one of these plugs at all indicates that we’re not exactly flush for time or money; and in practice I doubt anyone has ever bothered with two pole switching, typically bridging the neutral inside the plug, instead stuffing a couple of lengths of figure eight Christmas tree wire into it, getting us the minimum requisite four conductors.

The innovation of the PDL 40A is frequently complimented by comparatively innovative ways of avoiding purchasing 4-core flex

In this day and age 40As are exceptionally difficult to come by. They were unheard of in domestic environments, and uncommon in industrial / commercial environments too. I got a taste of its rarity when entering an electrical wholesaler with one about 15 years ago, to ask where I could get another: “Whoa!” said the guy behind the counter – “Haven’t seen one of those for a while!” Apparently that day when a 40A was carried into their store was a special one.

The few that still exist are very precious and typically hoarded by obsessive people like myself, a very unusual item to be in possession of indeed considering that I now live in the UK. I can boast a very large collection of 1 (and a broken black one), which is about as many as I’ll ever have.

Will I ever find a use for it? Even if I moved back to New Zealand, probably not.

Posted in Bits and pieces

12 thoughts on “Remembering the PDL 40A Interrupted phase tap-on plug

  1. Thanks for posting this Matt.
    I had an old 40A plug until recently, when my apprentice lost the bloody thing.
    I used it to test the start and run currents of various things like single phase compressors.
    It is nice to know that the Clipsal one is still available, which bought me to your site.
    Have a good one,
    Mike Trump.

  2. Hay nice article mate… lucky for me my work is replacing its old phone system and these plugs were used for incoming call lamp for the phones, so i will probably end up with 30-40 of them…

  3. If you ever shifted back to NZ?
    Why would that be the only reason to use an interrupted phase control tapon plug?
    It’s a BRILLIANT piece of equipment especially when using a push-on or push-off “foot switch” to control the likes of a circular saw bench – or a drill press, where BOTH hands are required to hold the item being drilled – or saw-cut, whereby the use of a FOOT SWITCH allows both hands to hold the item being worked..

    I myself picked up several (not sure how many – but well over FOUR of these), when working at a site, as an industrial electrician in the 1980’s.
    They were on remote switched devices used to control electric fence energisers, by way of a simple plug/play method, where the controller was fitted with the white/orange tapon and the energiser simply plugged directly into the tapon at the outlet.

    I have one on a long lead – that controls the supply a small wall lamp (on a hook) with the cable from the 40A being run to the headboard of the bed, where a round surface mount switch is located (thus is itself run from an extension cord from an outlet further away) – why?
    Well why bother getting out of bed to turn the light on/off when it can be done from the bedhead itself?
    The lamp extension cord simply taps onto the back of the tapon interruptor plug, at the wall switched outlet.
    When one is OUT of bed, simply turning off the wall switch kills the lamp, and at night switching the socket switch ON at the wall near the door, turns the lamp ON which is then turned off from the bed.
    Bingo – no torch required.

    Another use, is on a firewood / saw-bench – where it allows me to use a momentary ON foot control – to operate the saw whilst having FULL control of “both sides” of a cut log.
    It’s a LOT safer than trying to hold a saw-switch (on the saw) – with just ONE hand to hold both cut pieces of firewood.

    Another use I have for another one, is for the drill press, meaning I have a foot-switch with emergency OFF – that allows the drill press to remain running as I hold parts that I am drilling yet allows me to have an EMERGENCY OFF function, without actually letting go – of whatever MY TWO HANDS are holding.

    That – is why – you won’t need to come back to NZ – just to find somewhere to use one.

  4. So – how many (originally 40A PDL) Clipsal Interrupted Phase Tapon’s do you require.. ?
    Clipsal still makes thes – as they are STILL a brilliant remote control (piggyback) plug/socket which allows direct plugging of appliances at the wall socket outlet, whilst having a user friendly remote control switching device – located close to the operator, without having to extensively rewire the premises.
    Especially useful when BOTH hands are required to hold something allowing a foot-switch to be used for the active control
    A suitable (Push/on or push/off) control unit is the Jaycar Foot Switch

    Directly wired via a flexible cord – to an Interrupted_Phase TAPON plug (Clipsal 461 SUA-WE Control Adaptor).
    One can directly PLUG IN any usable appliance to the switched wall outlet and have an INSTANT remote control of that device.
    I have absolutely no idea why the USA website states these as being in excess of $2k – but here in NZ at any of the Electrical Trade Outlets, they are around $NZ26 each.

    Perfect for intermittent (or permanent) control of plugable appliances and other devices, where the use of FIXED wiring – would be an impossible if not extremely expensive option.
    Plus – these are relocatable at a moments notice, simply by unplugging them and moving to a different outlet, to plug another or the same portable device/appliance into – such as when using skill-saws (upside down) as firewood bench saws , mounted under a table, where the operator HAS to have both hands free to hold firewood being cut – hence the use of a foot switch to control the device.

  5. I’ve fitted one to a ripple control receiver to power a clamp lamp to monitor dropouts on telegrams the network sends out. They fail a couple of times a year and I end up with either a cold shower or a high water heating bill.

    And then there’s the unheralded interrupted earth tapon, very useful for removing earth loops on audio equipment. Can’t buy those for some reason.

  6. I heard a *rumour* that they were discontinued because someone was walking across a room carrying a mold, and dropped it. The volumes were so low (and apparently clipsal makes an equivalent – I will have to see if I can get some) that it wasn’t worth getting new molds made.

    Might be wrong, who knows.

    1. It actually disappeared about the same time as the 38/40 was introduced. If it’s anything to do with molds, I would have thought it was probably that it wasn’t worth making a mold for a “38/40A”.

      Even that theory is doubtful as I’d have thought these were both made in the same mold, only changing an insert at the top for the model number?

  7. I used these for many years, for water pumping and filtration. so A water pump up to 2hp could be plugged into this plug and a thermo-cutout put on that extension with a thermostat probe sensing max water temperature of say 40c for a run dry protection.
    Also used for a waterpump plugged into plug and a Ultraviolet sterilizer filter connected on that phase extension. So the UV filter only worked when the waterpump was operating so the bulb did not cook inside, and so no need for a waterflow switch.
    You can use any protection device on that lead out. I have one left in stock which has a red loop wire out of so I use to clamp test run amps. I bought two Clipsal plugs and they are sold to you unassembled with no instruction how to assemble the small brass baynet connectors etc. The wholesaler had no idea, so sadly I took them back for refund. Really stupid not assembling them. I would like some one to find instructions on how to assemble the Clipsal plug

  8. They were originally made for hot water cylinders that didn’t have themostats. There was a range of elements with an additional thermostat pocket, so you could retro-fit a themostat and control the element. I think the element was on a lead with a plug.

    That would have been in the 60s and 70s.

    I worked at a power board in the 70s and they held stocks of both the element ant the plugs. Back then they controlled the hot water via DC injection on the neutral (Wellington). We added the first ripple control (Zellweger Decabit) in 1978, injecting at 33kV at the Karori 33kV zone substation.

  9. I too am an ex Kiwi, living in Oz, and brought one with a loot of red wire for a clamp meter when I moved here in 1979. I also bought for a new ones at the time before coming to Oz, and have used one for a thermostat so that I could run a chest freezer as a fridge, but unfortunately cannot find the other two or three that are now I have somewhere. Oh well, the kids will probably find them when they clean the shed out after I pass, and throw them out, not knowing what they are. I too will now try and obtain the Clipsal version.

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