Handbook of Electrical Installation Practice - BeKnowledge

192 Handbook of Electrical Installation Practice
Carbonaceous backfill
Carbonaceous backfill is manufactured as coke breeze or calcined fluid petroleum
coke. The material comprises chips of coke which must conform to sizing as specified.
A maximum amount of ‘fines’ is permitted by specification and it may sometimes
be mixed with slaked lime (usually 10% by weight). Carbonaceous backfill
must be carefully installed around the anodes and tamped lightly so that satisfactory
electrical contact is achieved to the electrolyte. The calcined fluid petroleum
coke is a round grained material. In all cases the electrical resistivity of the material
should be 50 W cm or less.
Distribution boxes
Distribution boxes are manufactured of steel or glass fibre, and where located in
flameproof zones they must meet relevant classification requirements. Each box
is equipped with terminals, resistance control and other facilities as required by
specification.
Cable connections to structures
Several methods are used to attach the d.c. negative or test cables to the structure.
A summary of the common types of cable attachment methods is indicated.
Direct connection
Cables are attached directly to the structure using pin brazing or thermit welding
techniques. Thermit welding systems are not favoured by all users as damage to
high-strength materials may occur. Brazing systems are now widely accepted as a
direct connection method.
Cable attachment plates
A steel plate approximately 50mm square is attached to the structure. The plate is
welded direct to the metal surface. A bolt is welded to the plate to accept a crimped
or sweated lug cable connection.
Collar/bracelet connections
In the case of pipelines or pipework a collar or bracelet may be attached to the pipe
to facilitate a cable connection.
Test stations
Test stations may be of concrete (in the form of a location designation post), glass
fibre, steel, cast iron or plastics materials. They contain the necessary terminals,
resistors and other items of equipment.
Safety and security
In certain applications the use of security or safety equipment may be required.
These would include: