Principles of Operation
Metal objects such as pipelines, reinforcing bar (rebar), storage
tanks or structural steel are normally covered with a protective
coating to prevent corrosion. Holiday detectors are used to inspect
these coatings for pin holes, scratches or other coating faults. They
work by generating a voltage high enough to jump a gap that is
longer than the thickness of the coating.
The laws of physics determine the required voltage level to
jump a given distance, or gap. Coating type also affects voltage
level requirements.
A holiday detector simply applies a voltage to the outside of the
coating. With the pipe connected to ground and with the holiday
detector connected to ground, a hole in the coating will cause a
spark to jump or “arc” from the electrode to the pipe to complete the
circuit. When a complete circuit is formed, a signal is activated on the
Holiday Detector.
A brief description of the steps necessary to operate a SPY
®
‚ Model
780, 785 or 790 holiday detector is presented here with detailed
steps following later in the manual.
To operate the detector, make sure the control switch is in the OFF
position (fully ccw), then install the battery. The battery is keyed so it
can only be installed in one way. Then install the electrode wand on
the front of detector and insert the ground cable into the rear
of detector (inside handle). Ground the pipeline and connect the
electrode, spring or brush. Turn the control knob to the middle posi-
tion (set voltage). This activates the internal Jeepmeter. A pulsing tone
and light warn that this is the “set voltage” mode. Set the voltage to
the required value. Turn the unit on (fully cw). A running tone and
light indicates the detector is operating. Roll the spring toward the
bare metal end of the pipe. When the electrode is sufficiently close,
a spark will jump from the electrode to the pipe, the running tone and
light will go out, and a loud signal horn will sound to designate a
holiday. Of course it is not a real holiday but it demonstrates what
happens when the electrode encounters a holiday and verifies correct
detector operation.
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