Page 304 - Offshore Electrical Engineering Manual

Ships 291

INSTRUMENT/COMMUNICATIONS EARTHS

A separate instrument earth ring will be provided with a lower impedance than the
electrical earth so that interference from the electrical system is minimised.

INTRINSICALLY SAFE EARTH

If intrinsically safe (IS) systems on the platform have the Zener barriers as well as
optically isolated barriers, then an IS earth ring will also be required, in order to
avoid ignition hazard if the Zener barrier fails (Figs 5.2.2 and 5.2.3).

EQUIPOTENTIAL BONDING

This is required to prevent any arcs or sparks occurring between adjacent metal sec-
tions, such as gasketed pipe sections, cable tray and steel tanks. On machinery bolted
to a steel deck, separate bonding connections will be required. It is usual to provide
the diagonal bonding mentioned in following list if resistance is increased by cor-
rosion. Conducting copper grease can be used as a preventative measure. The resis-
tance to earth and to adjacent metal parts should not exceed 10 Ω for any item, and
this should be checked regularly. It is particularly vital that earth bonding is complete
in hazardous areas in order to prevent ignition should a flammable gas leak occur.
A matrix of main earth bonding cables will be provided to bond

1. the casings of electrical and mechanical plant at diagonal corners of the bedplate
and on the casings of electrical motors – cabins and superstructure need to be
provided with earthing points at diagonal corners,
2. sections of cable tray with braided copper links between sections – gasketed
pipe flanges should be provided with braided copper links.

The industry preferred cross-sectional areas are

2
• 70 mm for main earth ring cables,
2
• 35 mm for cables branching to individual earth bosses/lugs.

SHIPS

The major difference with ships is that the low-voltage system is usually an IT system,
i.e., it is unearthed, and must be provided with earth monitoring, so that any earths
can be easily detected and the low earth resistance cleared. Note that it is dangerous
to leave low resistances in the system, as they may cause electrocution if the earth
resistance is low enough. As such, it can contravene the statutory UK Electricity at
Work Regulations on certain installations within British territorial waters.