Page 366 - Handbook of Electrical Engineering

354 HANDBOOK OF ELECTRICAL ENGINEERING

13.2.1 Steel Structures

Some processing plants are constructed predominantly from steel, e.g. ships, offshore platforms,
drilling vessels, compact refineries and chemical plants. In these plants the superstructures and pro-
cessing equipment are generally made of steel beams, steel plating, steel flooring, steel vessels and
pipe-work. These items are either welded or bolted together, and by so doing they tend to form a
continuous electrical circuit as far as the passage of ‘earth’ currents are concerned. In some situations
where bolting is used it is necessary to provide additional copper bonding conductors across the
bolted surfaces, e.g. piping flanges, cable racking, machinery footings. It is essential to maintain a
low impedance continuous circuit, in order to minimise the risk of electric shock when fault currents
pass in the steelwork.
In certain parts of a power system it can be seen that very large earth currents can flow in
adjacent steel-work, e.g. generator frames, high power switchboards. These locations are often pro-
vided with a specially designed sub-system of interconnected copper busbars and common reference
earth points. The principle behind this sub-system is to provide what is in effect a set of very low
impedance conductors in parallel with the steelwork. The sub-system has the effect of forcing the
earth currents to pass in well-defined routes, in which the interconnecting conductors are situated.
This occurs because the impedance of each ‘copper route’ is designed to be much lower than the
‘steel route’. In general it is extremely difficult to calculate the impedance between any two points
in a typical steelwork electric circuit because of its three-dimensional nature. Even calculating the
low frequency impedance of a simple steel plate or ‘H’ section beam to the passage of alternating
current is difficult due to the creation of eddy currents, skin effect and local magnetic saturation of
the steel. The impedance would be a complicated function of the current magnitude. Consequently
the calculation of the sizes of earthing busbars and their interconnectors is based on assuming that
all the current flows in the copper and none in the parallel steel. This leads to a conservative and
safe result.
The method of calculating the cross-sectional area of busbars, interconnectors and bonding
conductors is given in 9.4.3.5.

13.2.2 Land-Based Plants

Processing plants located on land frequently have the benefit of space, wherein the plant is sub-
divided into discrete units. Each unit occupies a separate plot of land. Hence the plant is horizontally
distributed as opposed to an offshore platform in which the plant is both vertically and horizontally
distributed.
Each discrete unit is usually supplied with power from one or two main circuits, called feeders,
from a central high voltage source, e.g. local captive generators, supply authority overhead line intake.
The high voltage supply is used in two forms. Firstly to supply a few large consumers such as large
gas compressors, oil transporting pumps or large cooling water pumps. Secondly it is transformed
down to a lower voltage for all the small process motors, heaters, utilities, lighting and small power.
This two-fold situation requires the earthing to be dealt with in two distinct ways, one for
the high voltage feeders and one for the low voltage distributors. With a land-based plant the high
voltage feeders may be routed over reasonably long distances, i.e. 0.5 km to 2.0 km, at voltages
between 3000 V and 13,800 V (longer distances may require voltages up to 66,000 V).