Cable-operated excavators

Mechanization of loading in quarries started with the introduction of the cable-operated excavator, more particularly the face shovel, which is still on the market and available from many manufacturers and in many sizes. With its bucket fixed immovably to the arm, the diesel or electrically powered face shovel is purely a loading machine. Its relatively high capital cost can be justified by long service life, often twenty years or more. Larger machines generally last longer than smaller ones. 

On the other hand, hydraulic excavators are usually at a disadvantage in having a shorter service life and a lower degree of operational availability than the cable-operated excavator. Although the drive and hydraulic units are generally so designed as to be readily exchangeable and renewable, repairs nevertheless require more skill and care. 

The declining use of cable-operated excavators as loading machines in quarrying is attributable to several drawbacks ... 

The trend in loading machines in the last ten years has been steadily away from cable-operated face shovels and towards the increasing use of wheel loaders and hydraulic excavators. The diagram in Fig. 4 illustrates this development, which is representative of about 65% of German cement production. 

Shovel An excavating or coal-loading machine that uses a bucket mounted on and operated by means of a handle or dipper stick that moves longitudinally on gears and is lifted or lowered by cable. The entire machine is mounted on crawlers for mobility, and the upper structure is mounted on rollers and rail for swing or turn. 

Supervisors must have received training in COSHH appreciation, general site safety, theory and practice of excavation work. Operatives must have received training in excavation support procedures and use of cable location devices. This applies to sub-contractors as well as direct employees. 

There are three operations that carry the highest risk of electrocution use of portable tools, striking a buried cable, and cranes and excavators making contact with overhead power lines. 

The burn accidents invariably result from manual excavation when, for example, an operator using a pneumatic drill penetrates a concealed, buried cable with the tool. A short circuit will occur when the bit comes into contact with one or more of the live conductors and the metallic armouring and/or sheath of SNE cables or the CNE conductor where a PME distribution system is in use. The initial phase/earth or phase/neutral fault usually develops and involves the other phases, causing arcing which may emerge as a flame arc and blast from the hole made by the tool, injuring the operator. 

Cable locators should be used only by personnel trained in their use. It is good practice to use them from time to time as the excavation work proceeds until the cable is exposed. Quite often, and particularly in urban areas, there may be more than one cable laid at the same or different depths. Care in the use of the cable plans and locators is needed to ensure that the location of all the cables is determined and that cables located in close proximity to other cables, including above other cables, are properly resolved. Cables are not always distinguishable by sight from other services, but the use of a locator should enable the operator to be sure. 

Another type of accident common to all types of excavation work is that caused by machinery or operatives striking buried electric power cables. Fortunately injuries from such accidents are usually limited to burns, fatalities being relatively few compared with the high proportion of fatal accidents when overhead power cables are struck. Nevertheless the buried cable (or gas and water main) hazard remains a serious one and is a frequent cause of delays and high costs to the contractor. It is the subject of a special study by the Construction Industry Research and Information Association. 

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