OMG. It’s a UK (BS1363) Piggyback Plug

So what I hear you say.

If like me you’ve spent any amount of time searching for such a thing, you may have also noticed there is virtually bupkis in the way of such products to choose from.

I come from the land of piggy back plugs: New Zealand. I’ve very much missed their convenience since moving to the UK.

Okay, so they’re not as common in New Zealand as they used to be. Thanks to regulatory crackdowns and changes in consumption habits,  we can say in retrospect the 1990s was zenith of piggy back plugs (or tap-ons as we apparently call them).

Piggy back plugs galore. Without them you’d need three of these socket strips to plug all that in

While the days of popping down the supermarket to buy one are unlikely to return, at least you can still get them on pre-made appliance and extension cords, and re-wirable ones can be purchased from electrical wholesalers.

A selection of PDL “40” piggy back plugs from the 1980s and 1990s – including a couple of examples of the “40A” interrupted phase version

Changes to Australian electrical regulations have crimped Clipsal’s ability to manufacture these items but PDL still makes them (cat# 940).

There are 5 appliances in this cramped domestic data cabinet, but only 4 socket outlets. Two PDL 940 piggyback plugs to the rescue!

But anyway, back to the UK…

A very long time there was a company called Clix who manufactured the first BS1363 piggyback plug (more information here). As those are now collectors items, a modern replacement is desperately desired.

Clix 13A piggy back plug. Credit: Image was taken from forums (stevehertz)

Let’s take a look at the UK’s only purchasable piggy back plug. The seller describes it as a “Surged pass through”, somewhat diminishing the piggybackness of it. Let’s open it up and take look…

Fortunately there are a pair of screws on the underside which let us look at the guts of it. These don’t need to be undone to wire the plug.

We are first presented with a plastic spacer which surrounds the socket contacts, and we can see the surge protection gubbins waving at us down by the neutral pin. This spacer also holds the (pointless) neon light.

Quickly we can see my biggest concern with these plugs. That contact is only just barely on the fuse. I’ve purchased a number of these, and can say they vary from unit to unit. This one isn’t so great. If concerned I’ve found they can easily be bent back into a sensible position with pliers.

But regardless I’d pull that 13 amp fuse out and put either a 3 or 5 amp fuse in its place. I’d not be in the habit of using these plugs with 13 amps.

Lifting up the spacer we can see the socket contacts and surge gubbins clearly. Once again, quality is less than spectacular. I cannot comment on the efficacy of the surge protection. In my opinion surge protection is of little value, and in my case I have de-soldered all of these components, as well as the neon, because all I wanted was a plug.

The one last gripe I have is with two protruding corners on the cradle which catch your screwdriver when you are tightening the line and neutral screws. I’ve clipped them off with side cutters (circled).


Surge protection device? Even if the surge protection is effective, it’s not anything to get excited about. There are plenty of other better made surge protection devices to choose from.

Piggy back plug? Definitely. Why the hell the seller isn’t advertising it as this, is beyond me.

Apparently the idea of such a thing is so alien to the British that it has to have some useless surge protection jazz stuffed in it to make the sale?

As I’ve said the quality of the contacts isn’t amazing, but it is acceptable, as this is the UK’s only piggy back plug, you’ve not got another choice.

If you need something like this – buy a box of them now. Who knows how long these will be available for.

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.