Thorn Alpha 4

Lantern acquired in November 2004.

The second Alpha 4 to enter the Collection is designed to run 135 Watt SOX lamps (the first being designed for 90 Watt lamps); therefore, it is intended for attachment to columns of a mounting height of around 10 m, whereas, the 90 Watt version is more suited to 8 m columns. Both the 90 Watt and 135 Watt versions (the latter type being available with the lamp control gear housed remotely, or, as with this lantern, within the lantern itself) were popular lanterns in Derbyshire, with the 135 Watt version replacing failed lanterns of the same wattage from other manufacturers. Following the Street Lighting PFI in Derby itself, numbers declined considerably, but odd examples remain. Within the wider County of Derbyshire, examples were still relatively common, until LED replacements saw their numbers reduce significantly within only a few years.

The familiar white GRP [Glass Reinforced Plastic] canopy, black plastic clips and aluminium rear shoe of the 90 Watt version are retained with this version; however, the shoe is elongated here, in order to accommodate the bulky lamp control gear. The lantern's appearance is somewhat reminiscent of a steam locomotive, with the shoe serving as the tender. The idea of having the gear located in a completely separate part of the lantern was a design characteristic that Thorn also applied to their Alpha 6 SLI/H lantern.


The lantern is in relatively clean condition for its age (which we will discover later), and the GRP canopy isn't too flaky, though it did spend some time out of service in a room used for the safe destruction of failed lamps at the Derby City Council Highways depot on Stores Road, which will have helped to preserve its appearance somewhat.


Unusually, the lantern is not equipped with a NEMA socket, although the position of where one would be fitted is visible on the casting.


Unlike the 90 Watt version, the gear area cover on this lantern is aluminium, instead of being GRP as well. The curve of the cover matches that of the bowl. A small hole exists in the rear of the cover as a way of allowing any water that has accumulated to drain out; the shape of the cover allowing liquid to fill it without this feature.


A useful feature of the lantern is that the bowl is held by the clips on one side when all four are released. This allows the lamp to be changed without having to remove the bowl completely; however, should the need arise, the bowl can be removed by releasing the two retaining clips from the lantern using a flat-bladed screwdriver.


The identification label on the inside front of the lantern confirms the lamp type to be used.


The control gear comprises a Thorn G53260.T 135 Watt / 180 Watt SOX leak transformer, and BICC GC 2281 440 V 13 F capacitor, which was made in February 1980. This provides an indicator as to the age of the lantern, and proves that it was an early example, with the design being launched in 1979 as a replacement for the Alpha 5; the production of which was discontinued in the same year. Oddly, the two screws used in securing the gear cover are missing, as are the screws used for the cable clamp and earth terminal.


The terminals for the ballast are as follows: 1 and 6 connect to the lamp, 2 and 5 connect to the capacitor (terminal 2 also providing the neutral path) and 3 or 4 connect the live incoming supply (depending on whether the local voltage is closer to 220 V (3) or 240 V (4)). The wires to the lamp area pass through a small recess in the casting that is beneath (or above, in normal operation) the ballast.


Heavy corrosion is present on the inside of the gear cover.


These pictures demonstrate the size difference between the 90 Watt and 135 Watt versions.

The 90 Watt version is 1067 mm [3.5 ft] in length, whilst the 135 Watt version is 1258 mm [4 ft 112 inches] along the same dimension. Both lanterns are 146 mm [534 inches] in depth when the semi-cut-off bowl is employed, and the same figure in width.


The apparent colour difference between the two bowls is the result of the 90 Watt version being somewhat cleaner than the 135 Watt version is!


This view shows the difference within the lamp area - the shorter 90 Watt lamp meaning that the lampholder is situated further forward in this version.


The lantern was deconstructed on Thursday, 2nd November 2017 in preparation for its restoration. As can be seen by the state of the sheet, this was a rather messy task, all told!


The canopy was cleaned using a pressure washer, and then coated with Rust-Oleum Mode cream spray paint, both externally...


...and internally.


Meanwhile, the rear shoe and gear cover were sent away to be bead blasted and repainted in black and light grey, respectively. Thanks to a local refurbishment company for their assistance here.


A slight dent in the gear cover was also hammered out at this time.


The individual components began to be re-assembled into a lantern on Friday, 4th May 2018. The bowl, clips and gear cover were yet to be reinstated at this stage.


The two sections looked as though they had never been apart!


The refurbished interior was all set to receive its first lamp in many years.


The gear components were given a surface cleaning, but other than that, were left alone.


The lantern is probably in better condition now than it was upon leaving Thorn's factory!


With the gear cover back in place, the lantern was fitted to a wall bracket on Friday, 11th May 2018. A new Radium lamp was installed at this time too.


The final component to be re-attached was the (now clean) bowl.


All that was left to do was to switch the lantern on and hope for the best!


With the lamp being new, the electrodes emitted a vibrant mauve glow for a brief period while the excess emitter coating was being burned off.


The lamp was running at full power within about fifteen minutes.


Lamp warm-up video:

Testing the lantern with my energy monitoring device revealed the following results; the 37-year-old capacitor was revealed to be no longer providing much Power Factor correction:

Test Voltage (V) Current being drawn at full power (A) Measured wattage (W) Apparent Power (VA) Frequency (Hz) Power Factor True Power (W) Difference to rated wattage Percentage Difference
240.7 1.5 194 361 49.8 0.53 191.36 56.36 42%

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