Short-circuit problem: Mathmos replaces "Rokit" lava lamps

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Lava lamps are cult. They were invented in the 1960s by the British manufacturer Mathmos, which still sells them in various versions today. Mathmos prides itself on its particularly high quality and charges significantly higher prices than imitators. The recipe for the liquid in the bottle, in which the heated wax rises, cools and sinks back to the bottom, is a secret. Density, temperature and choice of color make the difference here.

Since 2020, Mathmos has been selling a larger rocket-shaped model called "Rokit". According to the manufacturer, it can be ordered primarily from the Mathmos online store at the hefty price of 375 euros. To heat up the large bottle, the base of the lamp contains a class C halogen lamp with 80 watts. Mathmos can continue to sell these in the EU with a special permit. The bottles are manufactured in the UK and filled in the Mathmos factory there. The author ordered a model as an ordinary customer, which arrived in the post a few days later.

However, when the lamp was switched on, the fuse blew with a big bang. On a second attempt with a new bulb (three are included), this initially burned for a few minutes before the fuse tripped again. The switch integrated in the cable was damaged. To get to the bottom of the cause, he took a closer look at the base with the lamp. The holder of the halogen lamp is screwed to a reflector screen using two small nuts.

Inside you can see the supply cable, which is connected to the lamp holder via two luster terminals. The luster terminals are enclosed in an insulating shrink sleeve, which can be easily pierced at the points where the fixing screws are located in order to unscrew the cables. Everything is very simple: All parts are freely accessible and can be loosened with a small screwdriver and a screwdriver socket.

Small screw, big problem

The power cable is fed through an opening with a metal housing. In this metal passage is another small screw that secures the power cable. However, this screw is made of metal and presses directly on the insulation in the cable. The assembly force at the Mathmos factory had apparently overtightened the metal screw, crushing the insulation and short-circuiting the conductors.

The Mathmos customer support team contacted by email got back to me by phone the next day and offered a new lamp base with five replacement lamps. The replacement base arrived about a week and a half later. Unfortunately, the fault had not been rectified, but revealed a general problem in the production process: Although the package arrived intact on the outside, the replacement base was badly dented – and revealed the same problem as with the first lamp: the metal screw had been tightened too tightly and had drilled deep into the insulating layer of the cable. Here, too, there was an inevitable risk of a short circuit. If not immediately, then later in daily use. Meticulous quality control should have noticed the fault and the danger.

Now the previously private lamp purchase became a story for c't. The author contacted support again and this time also the Mathmos press department because the problem could affect the entire production of the Rokit model. Other smaller Mathmos models of the types "Astro" and "Astro Baby" in the author's possession are not affected as they have a different cable routing.