Page 302 - Offshore Electrical Engineering Manual

CHAPTER

Electrical System 2

Earthing

INTRODUCTION
The earthing (grounding) of ‘non-current-carrying parts’ is essential for the safety
of all those on the installation, to avoid both electrocution and voltage gradients
between metal components, which can lead to sparking and hence risk of ignition of
flammable gases and vapours.

PLATFORMS
It is necessary to have several main earth points on the platform, all of which will be
of low resistance and with test facilities which allow the resistance to be measured
without disconnection of the system.

ELECTRICAL EARTHS
This is required for safe operation of the electrical system. It consists of a network of
protective conductors to ensure that fuses and protective relays operate effectively and
that no dangerous voltage gradients exist. The resistance to earth of any part of this
system should not exceed 1 Ω. Some oil companies require this value to be 0.5 Ω to
provide a safety margin against the effects of corrosion and dirt. At main earth bars,
facilities such as duplicate connections and removable links should be provided and
easily accessible to allow for testing and maintenance to be carried out regularly and
safely. Conductor sizes should have been already calculated during the design phase.
A typical platform will have a main electrical earth connected to low-voltage
(secondary transformer star points). The earth system will be in the form of a ring
connecting to all low-voltage switchboards and connected to a main earth on the
platform substructure (Fig. 5.2.1). Concrete leg platforms will be provided with main
earthing points giving low resistance connections to the sea. Medium-voltage distri-
bution transformers will be resistance-earthed at the star point to limit fault currents.
For earth conductor sizes, please refer to

IEC 61892-4: Table 2 Mobile and fixed offshore units: cables
IEC 60092-352: Table 2 Ships: choice and installation of electrical cables

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