Thorn Beta 5

This Beta 5 was entered the Collection on the same date as the Z5698U - Wednesday, 14th May 2003. It was used somewhere within Derby, although its original location is unknown, as this style of Beta 5 was used all over the City - they started appearing in the early 1980s to replace Group 'B' mercury lanterns; indeed, the capacitor in this example carries August 1983 as a manufacturing date.

The original aluminium canopy seen with the AEI Amber Minor lantern of the 1960s is retained for this example, except that it has been modified to accommodate a NEMA photocell socket in the centre, and the stainless steel bowl toggle seen in the 1982 version has become a black plastic equivalent instead.

After acquisition, I rubbed the canopy down with a 'Brillo' soap pad, in order to remove the 20 years of roadside dirt that had gathered on it. Although a photocell would have been fitted when the lantern was in use, none was included at acquisition.

This version of the Beta 5 can only accommodate the original deeper bowl, owing to the positioning of the lamp within the lantern. The shallow bowl would not become an option until the slimmed-down Beta 5 appeared later in the 1980s. Whilst the earliest Amber Minor bowls were seated within an aluminium frame, by the time that this lantern was produced, the design had been revised to remove the need for the separate frame, with one end featuring an indentation for the toggle to fasten into; the other including a thin metal strip that served as a hinge.

No lamp was supplied with the lantern, and so I fitted a period Thorn 35 Watt lamp, dating from 1985.

The gear tray / reflector does not extend the full length of the inside of the canopy, allowing easy access to the two grub screws that would be used in securing the lantern to a bracket. The identification label states that this lantern carries a sealing rating of IP23 (this was improved in later incarnations), and is a Mark 16 version.

Unscrewing the single bolt that holds the gear tray in place allows it to swing downwards, revealing the full extent of the canopy internals. As the NEMA socket is no longer required, the former incoming live and switched load wires are now linked together within a single connector block.

The ballast, ignitor and capacitor are all positioned on the rear of the reflector, though the August 1983 capacitor is the only original component. As can be seen, the ignitor is a replacement that I fitted on the 25th December 2007 (the old one picked a great day to fail!), and in the process of replacing this, I noticed that the ballast's terminals had become yellowed and brittle, and so I swapped this out for a slightly newer Thorn EMI ballast instead. This ballast looks rather worn as it had been used in a column base, supplying a separate gear Beta 5, previously.

A comparison of the original and replacement ballasts - the original is a G53332.4; the replacement is a G53331.4, with the only difference being that the replacement is suitable for running a 35 or 55 Watt lamp, whereas the original is for 35 Watt lamps only. The original also doesn't have an earth bonding bolt attached, although this may be because it was destined for use in a lantern (with an earthed metallic gear tray anyway) from the outset.

The outer face of the spigot entry is relatively angular, but forms into the correct circular shape inside the lantern. Barely visible within this is '34', which is likely to represent a canopy designed for a 34 mm diameter bracket - at the time, Thorn offered a 27 mm option too, for use with older 3/4" BSP brackets.

The lantern was mounted to an AC Ford AC872 wall bracket on Monday, 15th August 2005. A 1980s' Royce Thompson P5 photocell was used to plug the NEMA socket.

Operational view:

Lamp warm-up video:

Testing with my energy monitoring device revealed the following results:

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
242.1 0.57 56 138 49.9 0.4 55.20 20.20 58%

Thorn Beta 5s in the Collection











GEC Z5698U | Atlas Alpha 3




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