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ART (ATTACHED
ROD TECHNIQUE)
TESTING CAPABILITY
The model DET3TC includes the
additional testing capability that we have termed ART, for Attached
Rod Technique. A nagging problem with traditional ground testing has
been the requirement to “lift” (i.e., disconnect) the utility
connection. Once the grounding conductor (the main conductor that
connects the facility to the ground rod or grid) has been attached
to the grounding electrode, the utility ground becomes a parallel
resistance.
The utility neutral is
typically bonded to the ground bus at the service entrance and this
connection, during a ground test, causes test current to flow back
through the utility ground as well as through the test electrode.
Test current divides according to Law of Parallel Resistance, but
the tester makes its measurement based on total current flow. The
reading is the combined parallel resistance of the on-site ground
and the utility protection. This is a valid measurement, but not of
the test electrode exclusively. This poses a considerable problem in
many common testing situations.
If a commissioning test
were required to determine if design specifications had been met for
a new facility, such a reading would be insufficient. Lightning
protection requiring a short, straight path into the earth, could
also not be properly validated. But lifting the utility connection
poses several problems, not the least of which is the breaking of
what is often a welded bond, in addition to the temporary loss of
protection. Clamp-on ground testers, which measure ground resistance
by clamping around the rod and inducing a test current onto it, are
only a limited solution. They can accurately measure resistance of a
single rod in a parallel system by inducing the test current onto
the clamped rod and utilizing all the parallel grounds as the
return.
Collectively, these
returns, typically the multiple grounds of the utility, contribute
little to the loop measurement. This is essentially the reverse of
the operation of a traditional tester, which uses the current probe
as the return while current “goes to ground” through all parallels
collectively. This technique solves the problem of separately
measuring an attached rod, but leaves the problem that it cannot be
proven. A clamp-on measurement has to be accepted on faith and its
reliability is based squarely on the knowledge and experience of the
operator, leaving a large margin for “human error.” In complex,
multiply connected grids and other grounding schemes, return paths
may exist that are entirely metallic, not including earth at all.
The clamp-on test current
will circulate through such paths and give a reading, essentially a
continuity reading of the grid structure having nothing to do with
soil resistance. Such readings will be low, and appear to the
uninformed as acceptable grounds. The responsibility for making
these determinations falls squarely on the operator. But even when
properly addressed, there is no way of demonstrating the competence
of the readings to a third party, such as a client. They must simply
be accepted. The ART testing capability combines the advantages of
both of these technologies to produce a method that can reliably
measure an attached ground, and prove it! A built-in clamp input,
used in conjunction with the optional ICLAMP accessory, connected
below the point of separation of the parallel test currents,
measures only the current flowing through the test ground, not that
going back through the utility. |
