Page 303 - International safety guide for oil tankers and terminals

TERMINAL SYSTEMS AND EQUIPMENT
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17.5.5.3
17.6 Earthing and Bonding Practice in the Terminal
Earthing and bonding minimises the dangers arising from:
Faults between electrically live conductors and non-current carrying metalwork.
Atmospheric discharges (lightning).
Accumulations of electrostatic charge.
Therefore, even if the resistance of the flange drops below the
1,000 ohms quoted above due, for example, to ice, salt spray or
product residue, any current flow will still be limited to a few
milliamps as the potential difference across the flange will be far
less than is required to initiate an arc during connection or
disconnection of loading arms or hoses. Conversely, trying to
earth (ground) a low voltage/high current circuit with a bonding
cable is difficult, even if a very low resistance cable is used. The
total resistances of the cable circuit connections and any
switching device, combined with the availability of a very large
current, will effectively prevent the potential difference between
the ship and shore becoming zero and will render this circuit
ineffective as a means of eliminating ship/shore currents in
loading arms.
Typical DC insulation testers are often arranged with a user
selectable test voltage (500/250/50 V etc) but are not normally
accurately ranged or capable of adequately applying voltages to
resistances as low as 1,000 ohms. These instruments are
therefore not best suited for routine testing, but could be used for
new installations where there will be no contamination of the
flange and insulation readings will be many times higher. Routine
testing should therefore be undertaken with an insulation tester
specifically designed to have a typical driving voltage of 5 V or
ึ
more when applied to a resistance of 1,000 ohms or greater.
It is recommended that handheld multimeters are not used for
resistance testing of insulating flanges. Although it is understood
that there may be multimeters with a capability to undertake this
testing, they do not typically apply sufficient test energy to be
effective in determining flange resistance, and may therefore
falsely show a flange as having adequate resistance. However,
should a potentially suitable multimeter be identified, it is
recommended that users take care to verify that the equipment
meets the strict interpretation of the recommendations contained
in this Section before carrying out the tests.
Safety
Testing should be undertaken with instruments and methods
selected to be compatible with any hazardous area associated
with the location of the flange. Where testing of an insulating
flange is carried out in a hazardous area with testing equipment
not certified for use in such an area, the testing should be
performed under the control of a Permit to Work (see
Section 19.1.3).
© ICS/OCIMF/IAPH 2006 269