Accidents involving fuel tanker trucks and other vehicles transporting flammable liquids
present major challenges to first responders.
One of the most critical is the danger of electrostatic discharge, which can instantly
turn a potentially hazardous situation into a life-threatening disaster.
To help mitigate this risk, standards such as NFPA 472 mandate that hazmat personnel
set up, and bond the tankers to, a temporary grounding system before offloading flammable
liquids from the damaged tanker to the receiving vehicle.
The purpose of this grounding system is to create an equipotential plane between the
two vehicles and all equipment used to pump the liquid from one to the other.
To help make this process as easy and safe as possible, AEMC has introduced the Static
Ground and Test Kit.
This portable system contains all the necessary equipment for creating and testing a temporary
grounding system for the transfer of hazardous materials, including a test instrument and
accessories to ensure the system is fully functional and in compliance with relevant
standards.
The kit enables responders to ensure the safe transfer of hazardous liquids by minimizing
the risk of spark ignition due to electrostatic discharge.
This video explains how to use the Static Ground and Test Kit.
We begin by describing the kit’s components.
We then demonstrate how to use the kit to set up a temporary grounding system, and then
test this system to ensure it provides the grounding performance required for your location.
Finally, we explain how to connect the grounding system to the damaged and receiving tankers
and other equipment.
The kit is packaged in a hard-shell case designed for rugged environments, and consists of two
parts.
The first includes:
Rods for creating a grounding system.
These are steel rods with copper cladding to prevent corrosion and rust.
The grounding system can consist of a single rod, or multiple connected rods.
Typically, you will create one grounding field for the damaged tanker, and a second for the
receiving tanker.
You can also create a separate grounding field for the transfer pump, in cases where it is
a portable standalone device not mounted to the fire truck.
An insertion/extraction tool for installing and extracting the rods.
Auxiliary electrodes for testing the grounding system.
These electrodes are used for injecting test current and measuring potential
Couplings for connecting rods in situations where the rods must be hammered to a greater
depth to achieve the required resistance reading.
The second part of the kit provides:
The AEMC Ground Resistance Tester Model 3640, a simple-to-use ground tester that can perform
a Fall-of-Potential test with a single push of a button.
You will use this instrument to verify that the grounding system meets local requirements,
as explained later in this video.
Color-coded leads for connecting the auxiliary electrodes to the Model 3640.
Jumpers for connecting the tankers to their grounding systems, and to each other.
These connections create the required equipotential plane.
The jumpers consist of bare stainless steel wire to ensure they have no hidden breaks.
Stainless steel jumpers for connecting rods together, in locations where multiple rods
are necessary to create a satisfactory grounding system.
And an easy-to-follow instruction sheet.
Our first task is to select a location for the damaged tanker’s grounding system.
Ideally, this is outside the “hot zone” that may exist around the tanker.
This is the area in which flammable gasses may be present, for example evaporated fuel
that has leaked onto the ground.
You should first determine the extent of the hot zone with a gas detector designed to detect
these gasses and display the Lower Explosive Limit, or LEL percentage.
If at all practical, install the grounding system well outside any potential hot zone.
Also, if possible choose a location uphill and upwind from the hot zone.
After a suitable site has been selected, you can install a grounding system.
If soil conditions allow, you can insert the rod by hand.
In hard or compacted soil, use the insertion/extraction tool that comes with the kit to install the
rod.
Screw the threaded end of the rod into the tool, and then install the rod with a hammering
motion, to a depth of 24 to 36 inches.
Then unscrew the rod from the tool.
With the first rod installed, we will now measure its effective resistance by performing
a Fall-of-Potential test with the Model 3640.
This involves injecting a low-level current into the ground at a distance from the grounding
rod, and then measuring potential at a point between the grounding electrode and auxiliary
injector electrode.
For more information about performing Fall-of-Potential tests, as well as ground resistance testing
in general, see the workbook “Understanding Ground Resistance Testing,” available free
on the AEMC website.
The Model 3640 runs on AA batteries.
We recommend keeping an unused set of batteries in the responding vehicle and installing them
on-site.
This ensures the instrument always has fresh batteries to provide power.
It is also good practice to remove the batteries after returning to the station, since the
instrument generally will not be used on a frequent basis.
In the Fall-of-Potential test, the injector electrode should be inserted approximately
80 to 100 feet from the grounding rod, and the potential electrode approximately 60 to
62% of the distance from the injector electrode to grounding rod.
Both auxiliary electrodes should be placed outside the hot zone.
Connect the red lead to the injector electrode.
Insert the spindle tool that comes with the kit into the spool to allow it to spin freely,
and bring the spool back to the instrument.
When a sufficient length of lead has been played out, push the lever on the spool to
release the tool.
Insert the red jumper into the spool’s banana jack.
In the other end of the jumper insert a provided spade clip, and attach the clip to the red
terminal on the instrument, labeled Z.
When you do this, ensure the metal jumper strap attached to the red terminal is not
connected to the blue terminal.
This jumper is not used when performing a Fall-of-Potential test.
Similarly, use the blue lead to attach the potential electrode to the instrument’s
blue Y terminal.
Finally, use the green lead to attach the instrument’s green X terminal to the grounding
rod.
To take the measurement, simply press the button and wait a few seconds for the reading
on the display to stabilize.
In some jurisdictions, a resistance under 1000 ohms is acceptable for a temporary grounding
system in this application.
Other jurisdictions require the resistance to be under 25 ohms.
Be sure you know the requirements of your location before connecting any cables and
pumping equipment to the damaged tanker.
When performing a test, note the three fault indicators on the Model 3640’s front panel.
If any of these lights are lit, check all connections between the instrument, auxiliary
electrodes, and grounding rod.
If the measurement does not meet the standard mandated by your area, there are a number
of measures you can take.
For example, to improve conductivity you can moisten the soil around the temporary grounding
system by pouring water on it.
Another option is to add one or more additional ground rods.
Insert the second ground rod a minimum of one rod length away from the first, and bond
it to the original via the green jumper cable provided with the kit.
Similarly you can add a third rod, placed a rod’s length away from both the first
and second rods.
You can also use the provided connectors to attach a second rod to the original, and then
use the insertion/extraction tool to hammer them deeper into the ground.
And if a guard rail is available, use the jumpers to attach it to the grounding system.
In some locations, the guard rail by itself can provide a satisfactory grounding system.
In every case, perform a Fall-of-Potential test afterward to ensure the grounding system
meets your local requirements.
When the readings are acceptable – and if time permits – we recommend taking a few
minutes to perform two additional measurements.
These involve moving the potential auxiliary electrode to 52% and 72% of the distance between
the injector electrode and grounding system, and taking a measurement at each location.
If the three measurements are within 3% of each other, the injector electrode is sufficiently
distant from the grounding system to provide a reliable and accurate measurement.
After you have set up a grounding field for the damaged tanker, use the same procedure
to create a separate grounding field for the receiving tanker.
Although it’s possible to use a single grounding field for both the damaged and receiving tankers,
we recommend separate fields for each to ensure redundancy – for example, in case a cable
is accidently disconnected during liquid transfer.
And as an extra safety measure, you can also create a separate grounding field for the
transfer pump equipment.
The final step is to connect the grounding fields to the tankers.
Start with the damaged tanker.
Connect the grounding jumper to a point on the tanker directly welded to the vehicle
frame.
The reason we connect the damaged tanker first is to ensure that any electrostatic spark
created as a result of the connection occurs outside the hot zone.
Connect the other end of the grounding jumper to the grounding field.
Next, connect the receiving tanker to its grounding field.
Then connect the damaged tanker to the receiving tanker.
Again connect to the damaged tanker first, to avoid electrostatic spark within the hot
zone.
Finally, connect the transfer pump to its grounding field if you have created one for
it, and then connect the damaged tanker to the pump.
You have now created an equipotential plane that will minimize the risk of static spark
during flammable liquid transfer.
Note that you can use this plane to ground any buckets used to capture leaks from the
damaged tanker.
To do this, place the bucket outside the hot zone, connect a grounding jumper to the tanker,
and connect the other end to the bucket.
You can now bring the bucket into the hot zone to catch any leaking fluid.
When all fluid has been offloaded, remove all connecting wires from the ground rods,
tankers, and other equipment.
As you do so, be aware that volatile vapors may still be present on or around the empty
damaged tanker.
Then extract the rods using the insertion/extraction tool provided with the kit.
Let’s take a moment to review the main tasks required to create a temporary grounding system
that provides an equipotential plane for the offloading of hazardous liquids:
Determine an appropriate location for the grounding systems, outside the hot zone that
may surround the damaged tanker.
Install the ground fields, using the grounding rods supplied with the kit.
Create separate fields for the two tankers, and ideally the pumping equipment.
Perform a Fall-of-Potential test to ensure each field provides satisfactory grounding.
If not, install additional rods to lower the resistance of the field.
Connect each grounding field to its tanker, and the pumping equipment.
After offloading, disconnect the grounding fields, dismantle them, and return all components
to the kit.
This concludes our demonstration of the Static Ground and Test Kit.
For more information about the kit or the Model 3640, visit the AEMC web site.
And be sure to check our YouTube channel for instructional videos about other topics in
electronics, including the many products offered by AEMC.