The Washington times., 1904-05-08
WHAT HAPPENS WHEN
FUSES BLOW OUT

One often sees a bright flash and n
sudden stop of an electric car. with ali
lights out This is frequently described
as a more or less serious accident. and
II often causes a panic among the ms
| sengers, especially on elevated trains.
The greatest danger to passengers
However. is a possible panic. which
might be avoided If It was generally
Known how little chance there is of IRE
or other danger. To help people to uh-
derstand what happens we make the
following explanation, which should be
plain to lay minds: If a steam engine
is overloaded it Will stop and refuse to
work, although the fun pressure of
steam may remain upon the piston, and
not cause any damage. On the other
hand. a motor when overloaded, tries
Its best 10 do the work thrown upon IL
II IL cannot run at full speed it Will
run at whatever speed it can. AS the
sDeeG decreases the current through
the motor increases, and the motor ad-
Justs itself to that speed at which the
turning effort is sufficient to cause rota-
ton and do the work If the effort de-
manded of the motor is so great th2t
the current which passes through IL
when standing still is not sufficient 10
cause rotation, the motor will. Of
course. not turn. but the current will
continue to flow unless interrupted by
a suitable mechanism.
The current which will flow through
motor when 1t is standing still is in
almost all cases far in excess of that
which the motor is designed to carry:
and indeed. in q well-designed motor a
current dangerous for the motor will
be reached before the motor has been
M
rent on the motor. if allowed to con-
TIME. is 10 heat the windings 70 a dan
gerOus degree and destroy the insula-
ton, poSS' bly, setting il on fire, and
IL is 10 prevent this occurrence, whether
due TO careless handling of the car or
to unexpected causes, that the fuses
arc used
A fuse Is simply a short piece OF wire
of such size that It will be melted by a
current which, If allowed to flow through
the motor for any time, will damage Ii.
When fuse blows, then. it simply
means that one of the safety devices
on the car has operated to prevent dam
age 10 the motor The melting of the
use opens the circuit and cuts off the
current from the motor. To protect the
car the fuse is inclosed in a fireproOf
box.
There is another device for accomplish
ing this purpose, which is known as the
circuit breaker. This is a switch con-
trOlled by an electromagnetic, which opens
whenever the current reaches a certain
dangerous value. This mechanism is now
generally installed upon electric cars in
addition TO the fuse. 1t Is often placed
on the roof Of the platform over the
mOtorman's head, where IL is easily
reached, and it is set to operate at
higher current value than the fuse, be-
cause the circuit breaker acts almost
instantaneously. while 1t takes q little
time for the fuse to be melted. Now a
motor car can stand for a second or
two current which would destroy IL II
applied for a longer period. The circuit
breaker, then, takes care OF heavy over
loads and the fuse protects the motor
against these smaller currents which
are dangerous if applied for a consid-
cradle time.
When a fuse blows there rs generally q
rehabilitation OF the metal of the fuse
and a slight explosion. These eXplo-
sions usually cause a report and some
smoke When the circuit breaker is
opened IL draws an electric arc in break
ing the circuit, and as in this arc a con-
siderable amount of energy Is dissipated
in heating the air. there may be here
also something of an explosion; but in
neither case is there any danger 70 the
passengers when the apparatus is prop
eriy installed. The fuse and the circuit
breaker are safety devices, the opera
ton OF which indicates, not that there Is
danger TO those on the car. but that
danger TO the motors has been averueu--
Electrical Review.