Electrokinetica The Electro-mechanical Museum

The fuse box

Domestic electrical distribution boards

Every electrical installation needs protection from overloading and short circuits which could otherwise overheat the cables and fittings and set them on fire. Apart from early wiring schemes using the 'tree system' there is usually at least one distribution board in every installation where the several circuits are fed from a source of supply and protected by fuses or circuit breakers of appropriate current rating. Some of the early boards lacked safety features now taken for granted; it was easy to touch live connections and have molten metal droplets spray out if one forgot to turn off the main switch before changing a fuse. The operation also involved extracting bits of failed fusewire from the fuse bridges, poking in a new wire and screwing it to the terminals, a rather involved procedure often carried out in the dark after a fault left the lighting circuit dead. A further risk arose from the possibility of fitting the wrong rating of fuse wire to a circuit, or even a piece of wire wire heavier than the circuit cable itself, leaving the circuit unprotected and at risk of fire in the event of overload. One development widely adopted during the 1930s was the distribution board with built-in main switch, interlocked to prevent the cover being opened with the switch on. Later boards used cartridge fuses, which can simply be replaced by snapping the old one out of the clips and inserting a new one. Cartridges of different rating are usually of different sizes, a safety feature which prevents oversized fuses being fitted. They are also more resistant to disintegration if a very heavy fault current occurs, and cut the power off faster and more predictably than rewireable fuses. Nowadays the miniature circuit breaker is standard, which simply trips to 'off' in the event of a fault and can be reset to 'on' once the fault is cleared. It has taken a long time to become the accepted mode of circuit protection; the circuit breaker was actually introduced in the 1890s but was expensive on account of its complexity.

Tucker Telac 3-way double-pole distribution board

Telac 3-way dp board

Telac 3-way dp board

This handsome glass-fronted board was made to a very high specification. Enamelled slate is used for the insulating bases, separate slabs being provided for the positive and negative poles, to guard against surface leakage. Substantial brass busbars are fitted and the well-jointed teak case is lined with fire-resisting material. A recessed pushbutton catch holds the front closed. These latter features were supplied at additional cost and needed to be specified when the board was ordered. The maker’s trademark is printed on the fuse bridges, which would have been suitable for up to 15 amp circuits although no rating is actually given. A board of this type might have been chosen for the best quality domestic or light commercial work of the 1920s / 30's.

Fuse wire

Fusewire dispenser

Fusewire dispenser

Where rewireable fuses were used, there was always the possibility of needing some fuse wire at short notice. In the UK, one is accustomed to the use of fused plugs, but before these were standard any fault such as a damaged flex or malfunctioning appliance could cause a fuse in the distribution board to blow. Early light bulbs were also more prone to blowing the lighting fuse when they failed, leaving the householder to rewire the fuse by torchlight. A dispenser loaded with fusewire, hung on the distribution board, would be a boon on such occasions; tinplate ones carrying many yards of wire usually had a screw-hole to allow them to be fixed or hung up in a convenient place. Cards of fusewire were also equipped with the necessary hole. Early dispensers normally offered 5-amp fusewire for lighting and small socket-outlet circuits and 10-amp or 15-amp for heating etc. Later versions added 30-amp fusewire for ring-main circuits and occasionally 20-amp for radial circuits.

-----

All content © copyright Electrokinetica 2007-2017 except where otherwise stated • Valid XHTML 1.0Valid CSS