The Diseases Of Electrical Machinery 1904
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24
THE DISEASES OF
where the machines are not insulated from earth, the
occurrence of a break in the insulation in one of the
loads, if ti ere is at the same time a short-circuit
between armature winding
and armature iron, may
"y result in the destruction of
\ the winding. Fig. 3 explains
this diagrammatically.
fy /C/ A casual connection
[ I between the brush-holder
1 T
„ spindle and the rocker is
Fig. 3 . .
in itself of no account until
another short-circuit takes place at some second spot.
To follow up the direct bearing of these remarks, let
it be remembered that a breakdown cannot occur
unless or until a wrong path is opened to the current
along which it can pass. There must be an entrance
and an exit for the current. If there is only one place
where the winding and the iron are in contact, the
current has an entrance but no exit. So nothing
occurs until there is a second contact somewhere else
between some live part of the circuit of the machine,
or of the leads and the iron of the machine, or some-
thing that is connected to the iron.
It is not difficult to locate the short-circuit in a
machine by means of a galvanoscope or simple
galvanometer, such as a common linesman’s detector
and a single Leclanché cell, or a “dry” cell. If no
galvanometer is available, an ordinary electric beil