7
principles
of
h o l i day
detector
operation
Holiday detectors are devices that are used to detect the presence of
defects in the non-conductive coatings applied to surfaces in order to
minimize ion flow from a conductive substrate. This is accomplished
by attempting to create an electrical circuit by passing an electrode
over the non-conductive coating. A sufficient voltage is generated in
the electrode to cause a spark that will jump from the electrode to the
substrate if a defect is found. When a defect is found a horn will sound
and a LED light will turn off.
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.
Environmental Conditions such as humidity, temperature, and other
environmental factors will also affect voltage required to jump a given
distance.
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 allow a spark to jump
or “arc” from the electrode to the pipe to complete the circuit. When a
complete circuit is formed, an audible and visual signal is activated on
the holiday detector.
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