May/June 2019
B ULK D ISTRIBUTOR
Road Tanker Earthing
27
Grounding systems Summary
The standards advise that a grounding system, which can measure
and monitor resistance in the grounding circuit, can be utilised. The
system should verify if the ground connection to the road tanker is
complete before loading or unloading is initiated.
The precautionary guidance regarding the grounding of road
tankers in IEC 60079-32 reflects the IEC’s stated goal of providing the
latest state of the art guidance. It states that the ‘earth cable’ earthing
the road tanker should be part of a static grounding system that
continuously monitors the resistance between the road tanker and
the designated grounding point located on the loading gantry. It
states that the grounding system should be interlocked with the
transfer system to shut down the product transfer operation if this
resistance exceeds 10 ohms.
It also states that the grounding system should be capable of
recognising when it is not connected to the chassis/tank of the road
tanker. This ensures that situations where the tank of the road tanker
is not connected to the grounding system, for example, where an
operator could connect the clamp to an isolated metal mud-guard or
wheel-nut, will not result in a permissive condition for the transfer
operation, thereby eliminating the risk of electrostatic charging of the
road tanker.
An additional recommendation in NFPA 77 and API RP 2003 calls for
interlocking the feed system (eg pump) with the grounding system so
that if the grounding system is not connected to the road tanker,
product cannot be transferred. This will ensure that electrostatically
charged product cannot enter or leave the road tanker when the road
tanker has no grounding protection in place. In general, interlocked
grounding systems will complete the grounding circuit when the
driver connects the clamp of the grounding system to the road tanker
and the system sees a circuit resistance of 10 ohms or less.
Although the standards recommend a monitored resistance of 10
ohms, there are many grounding systems on the market today that
monitor well in excess of this level. While it may be claimed that these
systems are capable of dissipating static charges the capacity of a
system to monitor at 10 ohms, not only provides an opportunity to
demonstrate compliance with internationally recognised
recommended practice, it also means that hazardous location
operators know the system’s grounding clamp is making a secure and
reliable connection to the road tanker, every time a product transfer is
carried out. Grounding clamps should be designed to penetrate paint
coatings, rust and dirt build up as they are very effective at impeding
secure electrical contact with the conductive metal of the road tanker.
Additionally, the grounding system must be capable of detecting
minute changes in resistance when the transfer is underway and
should not allow a high degree of change in resistance before
shutting down the pump or alerting personnel. As soon as a
resistance above 10 ohms is present in the grounding circuit, the
grounding system should be capable of detecting this change and
take control of the feed into the road tanker. Systems that permit
resistances higher than 10 ohms have a greater degree of difficulty in
detecting changes in the health and condition of the grounding
circuit. In accordance with the recommendations of industry groups and
fire safety associations, the static grounding of road tankers is a
key safety protocol in the loading or unloading of flammable and
combustible products. Grounding ensures static charges are not
permitted to accumulate on the road tanker eliminating the risk
of the container becoming an ignition source. Additionally,
national and international recommended practice advocates the
adoption of static grounding parameters that will enhance the
safety of the product transfer process including monitoring the
grounding circuit to 10 ohms or less and interlocking the product
feed system with a dedicated grounding system.
When selecting road tanker grounding systems, specifiers
should also consider additional functions that can enhance the
safety of the transfer process. Grounding systems which include
road tanker recognition and static ground connection verification
functions provide additional guarantees that a transfer process
cannot take place unless the road tanker is connected to the
grounding system and the grounding system itself is connected
to a verified ground source.
These features enhance the secure grounding of the road
tanker and enable hazardous area operators demonstrate the
highest levels of compliance with NFPA 77, API RP 2003 and IEC
60079-32.
Road ranker recognition
Because resistance monitoring systems operate when connected to
conductive metal objects, additional features can enhance the
protection of drivers, product and equipment. A ‘road tanker
recognition’ feature can be used to ensure that drivers can only
operate the feed system when the grounding system detects it is
connected to a road tanker.
A system like the Earth-Rite RTR will analyse the capacitance of the
road tanker as part of the grounding circuit. If the capacitance
presented is in the normal range for road tankers, the grounding
system will recognise that it has made a positive connection to a road
tanker.
From the site operator’s perspective, this eliminates the risk of drivers
unknowingly connecting the grounding clamp to parts of the truck
chassis that are electrically isolated from the truck’s container. This
isolation may be due to original design oversight like isolated mud
guards or paint coatings insulating conductive parts like truck light
enclosures from the chassis. In addition drivers have been known to
attach the grounding system’s clamp to the loading rack in order to
obtain a permissive state for the feed system to ‘speed up’ the
transfer. So while a permissive state for the feed system can be
obtained with a standard resistance based monitoring system it does
not necessarily mean the grounding clamp is electrically connected to
the road tanker’s container.
Specifying a grounding system with a road tanker recognition
feature ensures the road tanker is safely grounded before drivers are
in a position to begin filling it with product. Once the system has
verified it is connected to a road tanker it should then monitor the
road tanker’s connection to the grounding point to 10 ohms or less.
The earth source
When a road tanker grounding system is installed it is assumed that
the earth source (eg, buried ground electrode) to which the system
is connected has been independently verified as having a low
resistance connection to earth. This connection is the foundation for
secure and safe transfers and it is incumbent on the site operator to
Specifying a grounding system with a road tanker recognition feature ensures the tanker is
safely grounded before drivers begin filling
conduct seasonal ‘Fall of Potential’ tests to ensure these ground
connections do not deteriorate due to changes in soil composition,
soil resistivity or corrosion of the ground electrode. In winter, ground
temperatures can reduce dramatically and cause an exponential
increase in soil resistance levels. For the ground electrode these
temperatures can have a significant impact on its contact resistance
with the soil potentially impeding the transfer of static charging
currents.
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