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For example, when inspecting a good distance of welded pipeline, it would
be advisable to perform Field Calibrations at certain intervals along the
pipeline to verify a known holiday can still be easily detected at the specified
inspection voltage. This will assure the operator that he is inspecting the pipe
under optimum conditions and is not missing any holidays. A Field Calibration
would also allow the operator to make any voltage adjustments to the Holiday
Detector to compensate for any voltage loss in the soil between ground rods,
if conditions do change.
CONSIDERATIONS – ‘’INDIRECT GROUNDING USING THE TRAILING
GROUND WIRE’’ METHOD
This grounding alternative, Illustration #3, allows holiday testing without an
obstructing cable, if direct grounding is not possible.
Important: When using this method “Soil Moisture Content” is very
important to establish a good ground. The moisture in the soil acts as a return
path for a low current that flows back to the holiday detector allowing the unit
to “Jeep” when a holiday is located. Without this low resistance return path,
the unit would not Jeep and the operator would miss valid holidays.
Caution: This method should not be used when a surface becomes an
“insulator” and can’t provide a low current return path.
For example, if the soil is very dry with low moisture content such as sand, dry
clay or a rocky surface, when the soil is frozen or there is ice on the surface.
these surfaces have become “insulators”. DO NOT use this method when the
trailing ground must be dragged across concrete, cement or asphalt surfaces.
These too are considered “insulating” surfaces.
CONSIDERATIONS – ‘’CAPACITIVE GROUNDING USING A GROUNDING
COLLAR’’ METHOD
This grounding method, Illustration #4, can be used only with Pulsed DC
holiday detectors such as the SPY 785 or 790. It cannot be used with the SPY
780 detector since it has a continuous DC voltage output. The DC low current
that would normally be produced when a holiday is located is blocked by the
capacitance of the grounding collar. Therefore, no holidays can be detected.
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