The San Francisco call., 1904-03-13
B 0 wing
a Flu s c

n Ne sees occasionally in. the
daily press an account Of the
~ blowing out Of the fuse on an
electric car. Now the greatest
danger TO the passengers in such a case
lies in the possibility of some uneX-
pected happening causing panic. II Is
therefore most desirable that everyone
should know what may be expected &c
happen on a car. and that the happen
ing is not necessarily an indication Of
danger.
II d steam engine is overloaded, ii
viii stop and refuse to work, although
the full pressure Of steam may i'm
pinge upon the piston, and not cause
any danger. On the other hand, a ma
tor, when overloaded, tries its best 10
do the work thrown upon il. If l. can
not run at fun speed it VIII run at
whatever speed it can AS the speed
decreases. the current through the
motor increases, and the motor adjusts
itself 70 that speed at which the turn
ing effort is sufficient TO cause rota-
tion and do the work.
The current which viii flow through
e motor when it is standing still is in
almost all cases far in excess Of that
which the motor is designed 10 carry;
snc. indeed, in a well-designed motor
q current dangerous for the m otor viii
be reached before the motor has been
stalled. The effect Of this heavy cur
rert on the motor, If allowed 70 con-
tinue. is TO heat the windings 10 a dan
gerous degree and destroy the insula-
tion, possibly setting it on fire; and il
is 10 prevent this occurrence, whether
due to careless handling Of the car or
10 unexpected causes, that the fuses
are used. A fuse is simply a short
piece Of wire of such size that IT will be
melted by a current which. if allowed
10 flow through the motor for any time,
viii damage il. When a fuse blows.
then. it simply means that one Of the
safety devices on the car has operated
70 prevent damage 10 the motor. The
melting Of the fuse opens the circuit
and cuts of the current from the mo-
tor. To protect the car the fuse is in
closed in a fireproof box.
There is another device for accom-
plishing this purpose which is known
as the circuit breaker. This is a
switch controlled by an electromagnet,
which opens whenever the current
reaches a certain dangerous value. This
mechanism is now generally installed
upon electric cars in addition TO the
fuse. II is often placed on the roof of
the platform over the motorman's
head, where II is easily reached, and 1t
is set 10 operate at a higher current
value than the fuse, because the cir
cult breaker acts almost instantane-
ously, while il takes a little time for the
fuse 10 be melted. Now, a motor can
stand for a second or two a current
which would destroy it If applied for a
longer period. The circuit breaker
then, takes care of heavy overloads
and the fuse protects the motor against
those smaller currents which are dan
gerous if applied for a considerable
time
When a fuse blows there is generally
q volatilization 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 it draws an electric arc in
breaking the circuit, and as in this arc
a considerable amount Of energy is dis
sipated in heating the air, there may be
here also something Of an explosion;
but in neither case is there any danger
TO the passengers when the apparatus
is properly installed. The fuse and the
circuit breaker are safety devices, the
operation Of which indicates, not that
there is danger to those on the car, but
that danger 10 the motors has been
averted.-Electrical Review.