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EARTHING Of EQUIPMENT
Reasons for earthing equipment.
Earthing is required to provide a low electrical resistance path from the fault to the supply. This
is to ensure the operation of the fuses or circuit breakers, which are defined as safety critical
items. The damage done to electrical equipment is proportional to the length of time the fault
current is allowed to flow. The quicker a fuse or circuit breaker operates the less damage to the
equipment occurs. The lower the electrical resistance, from the fault to the supply, the higher the
current and the quicker the protection device will operate, limiting the damage to equipment
This low resistance earth path also reduces the voltage at the fault point to as close as possible to
earth potential. This reduces the voltage, which is present at the fault point to a level which
minimises electrical shock, injury or death to personnel who may be in contact with the
equipment at the time. The quicker the protection devices operate the less time there is for
personnel to be in danger.
API Recommendations
API 14F sect 4.10.b
API 14F sect 6.10.3.1
(1991)
(1999)
Equipment earth bonding / Grounding
All exposed non-current carrying metal parts which may become live under fault conditions are
too be earthed.
Any equipment welded to the structure is classed as adequately connected. Skids and structures
are to be connected to the main earthing system by cables of suitable size. The use of mounting
bolts is not an acceptable method, and cannot adequately provide the low resistance path.
Earth cables to the size given in the following tables are recommended
Cables are to be bare or fully insulated and be coloured Green or Green with one or more yellow
stripes. (Cable of other colours can be used as long as each end and the point of entry into
enclosures, is clearly identified with permanent markings (Green & Yellow PVC tape))
These earth cables, green or green & yellow, are not to be used for any other purpose.
EARTH CABLE SIZING AP1
Supply Cable Cross Sectional
Size of Earth Cable Copper mm2
Area S mm2
S < 16
16 < S < 35
S > 35
S ( minimum 4mm2)
16
S/2
SUPPLY BONDING CABLES TAKING NEUTRAL CABLE SIZE.
Cross section of neutral or main phase
Minimum Cross sectional area of main
conductor S mm2
equipotential bonding conductor mm2
S < 35
10
35 < S >50
16
50 < S > 95
25
95 < S > 150
35
150 < S
50
IEE offshore rules
Table 2.2
Up to 3mm
3-125mm
Over 125mm
1.5 stranded 3mm unstranded
50% but min3mm
64mm = 70mm ( standard size of cable)
The neutral size is common to be smaller than the phase conductor on onshore distribution
systems. It is common for rigs not to have neutral supplies.
Caterpillar alternators have specific earthing requirements as indicated on the data plate, it gives
the minimum size of earth conductors and is determined by the output of the alternator.
Several rigs have DC generators for the DC motors. The earthing arrangements are to be the
same with all exposed metal earthed to the skids and these skids connected by a cable with a size
equal to the tables above for the largest cable supplying the skid.
It would be envisaged 35mm2 would be the minimum.
Flexible cables & Portable equipment
Flexible cables with an internal earth core, of the same size as the conductor, used to supply
equipment are classified as adequately earthed as the cable forms a direct path back to the supply
point. This cable must be connected to earth at each end in addition to any junction boxes on the
cable run.
Land Rigs
The use of flexible cables with internal earth cores needs to be carefully considered.
On installations, which are disconnected for moves regularly, i.e. Land rigs, these must have a
policy in place to test and record the earth connections on completion of any move.
As this will add time and effort to the work the addition of external earth cables is recommended,
as these will not be disturbed, and therefore will not be forgotten, during the moving of skid
mounted equipment.
ABS CABLE DATA
ABS
To 2.5 earth 2.5
2.5 to 8mm earth 4
8 and above earth A/2
120+ 70mm
Earthing general requirements API 14F, IEE, AP1,Page 3 of 3
REV 1.0. 10-11-2001
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