Electrolytic hazards depend not on the heating
Electrolytic hazards depend not on the heating effects of currents but
on their chemical effects. The same natural law which enables primary
and secondary batteries to be useful provides a hazard which menaces
telephone-cable sheaths and other conductors. When a current leaves a
metal in contact with an electrolyte, the metal tends to dissolve into
the electrolyte. In the processes of electroplating and electrotyping,
current enters the bath at the anode, passes from the anode through
the solution to the cathode, removing metal from the former and
depositing it upon the latter. In a primary battery using zinc as the
positive element and the negative terminal, current is caused to pass,
within the cell, from the zinc to the negative element and zinc is
dissolved. Following the same law, any pipe buried in the earth may
serve to carry current from one region to another. As single-trolley
tractiosystems with positive trolley wires constantly are sending
large currents through the earth toward their power stations, such a
pipe may be of positive potential with relation to moist earth at some
point in its length. Current leaving it at such a point may cause its
metal to dissolve enough to destroy the usefulness of the pipe for its
intended purpose.
Lead-sheathed telephone cables in the earth are particularly exposed
to such damage by electrolysis. The reasons are that such cables often
are long, have a good conductor as the sheath-metal, and that metal
dissolves readily in the presence of most aqueous solutions when
electrolytic differences of potential exist. The length of the cables
enables them to connect between points of considerable difference of
potential. It is lack of this length which prevents electrolytic
damage to masses of structural metal in the earth.
Electrical power is supplied to single-trolley railroads principally
in the form of direct current. Usually all the trolley wires of a city
are so connected to the generating units as to be positive to the
rails. This causes current to flow from the cars toward the power
stations, the return path being made up jointly of the rails, the
earth itself, actual return wires which may supplement the rails, and
also all other conducting things in the earth, these being principally
lead-covered cables and other pipes. These conditions establish
definite areas in which the currents tend to leave the cables and
pipes, _i.e._, in which the latter are positive to other things. These
positive areas usually are much smaller than the negative areas, that
is, the regions in which currents tend _to enter_ the cables form a
larger total than the regions in which the currents tend _to leave_
the cables. These facts simplify the ways in which the cables may be
protected against damage by direct currents leaving them and also they
reduce the amount, complication, and cost of applying the corrective
and preventive measures.
All electric roads do not use direct current. Certain simplifications
in the use of single-phase alternating currents in traction motors
have increased the number of roads using a system of
alternating-current power supply. Where alternating current is used,
the electrolytic conditions are different and a new problem is set,
for, as the current flows in recurrently different directions, an area
which at one instant is positive to others, is changed the next
instant into a negative area. The protective means, therefore, must be
adapted to the changed requirements.
CHAPTER XIX
PROTECTIVE MEANS
Any of the heating hazards described in the foregoing chapter may
cause currents which will damage apparatus. All devices for the
protection of apparatus from such damage, operate either to stop the
flow of the dangerous current, or to send that flow over some other
path.