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350 Cable Survey System
DPN 402197 © TSS (International) Ltd Page 2 of 10
A.1 ELECTROMAGNETIC FIELDS
The 350 System uses an array of sensing coils to detect the presence of alternating
magnetic fields and applies complex and powerful signal-processing techniques to
locate the origin of these magnetic fields.
Alternating magnetic fields exist around any conductor that carries an alternating cur-
rent and are of a strength that varies directly with the instantaneous magnitude of the
current.
Figure A–1: Lines of magnetic flux
Figure A–1 shows the situation when a conductor carries a current. In this example,
the conductor appears in cross-section with conventional current flow rising upwards
out of the page. As the current begins to flow, lines of magnetic flux expand concentri-
cally from the centre of the conductor with their polarity in an anti-clockwise direction
as shown. The strength of the flux varies inversely with distance from the conductor.
When the amplitude of current varies, the flux lines will expand or collapse simultane-
ously. The flux lines will reverse polarity when the current changes direction.
A.2 FIELD DETECTION
Each sensing coil of the 350 System consists of a very large number of turns of fine
copper wire wound around a ferrous metal core. The continuously changing magnetic
fields that exist around a tone-carrying conductor act upon the sensing coils and
induce in them an alternating voltage v. This voltage varies in magnitude according to
the following relationships:
Where d = distance from the conductor to the sensing coil
f = frequency of current in the conductor
i = magnitude of current in the conductor
v
1
d
--- vf vi
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