H. C. (Harry Cooke) Cushing.

Standard wiring for electric light and power; as adopted by the fire underwriters of the United States .. online

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Online LibraryH. C. (Harry Cooke) CushingStandard wiring for electric light and power; as adopted by the fire underwriters of the United States .. → online text (page 1 of 16)
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21.T YEAR. 1915 218T EDITION









Fellow )lm. Inst. Elecl. Engrs.





U. S. A,


THE Author, with the collaboration of Mr. F.
E. Cabot, Chairman of the Electrical Com-
mittee of the National Fire Protection Associa-
tion, and with the co-operation of the National
Electric Light Association Committee on Wiring
Existing Buildings, and the Society for Electrical
Development, has made it his aim in compiling
the following pages to set forth, as clearly as pos-
sible, the essential rules and requirements for
safe and economical exterior and interior wiring
and construction, the object being to standardize,
as much as possible, all work of this nature and to
respectfully suggest to the Electrical Engineer,
Architect, House-owner, Contractor and Wire-
man just what is required by Fire Underwriters
Inspectors throughout the TJaited States.

Copyright, 1915, by A. B. Gushing.


All generators, whether for central station or
isolated lighting or power work, should be located
in a dry place so situated that the surrounding at-
mosphere is cool. If the surrounding air is warm
it reduces the safe carrying capacity of the ma-
chine, and is likely to allow such temperatures to

Proper installation of dynamo or motor on filled wooden base frame.

rise in the machine itself as to burn out either
armature or field, or both. A generator should
not be installed in any place whe/e any hazardous
process is carried on, nor in places where they
would be exposed to inflammable gases or flying
combustible materials, as the liability of occasional
sparks from the commutator or brushes might
cause more or less serious explosions.

3O1 240

Foundations. Wherever it is possible, generat-
ors should be raised or insulated above the sur-
rounding floor on wooden base frames, which should
be kept filled to prevent the absorption of moisture,
and also kept clean and dry. When it is imprac-
ticable to insulate a generator on account of its
great weight or any other reason, the Inspection
Department of the Board of Fire Underwriters
having jurisdiction may, in writing, permit the omis-
sion of the wooden base frame, in which case the
frame should be permanently and effectively
grounded. Generators operating at a potential of
over 550 volts should always have their base frames
permanently grounded. When a frame is, grounded
the insulation of the entire system depends upon
the insulation of the generator conductors from the
frame, and if this breaks down the system is
grounded and should be put in proper condition
at once.

Grounding Generator Frames can be effectually
done by firmly attaching a wire to the frame and
to any main water pipe inside the building, on the
street side of the meter, if there is one. The wire
should be securely fastened to the pipe by screwing
a brass plug into the pipe and soldering the wire
to this plug or by approved ground clamps. When
the generator is direct driven an excellent ground is
attained through the engine coupling and piping.

Wherever high voltage machines have their
frames grounded a small board walk should be
built around them and raised above the floor on
porcelain or glass insulators, in order that the at-


tendant may be protected from shock when adjust-
ing brushes or working about the machine.

Accessibility, Sufficient space should be left on
all sides of the generator, or motor, and especially
at the commutator end, so that there may be ample
room for removing armatures, commutators, or
other parts at any time.

Circuit Breakers and Fuses. Every constant
potential generator should be protected from ex-
cessive current by a fuse, or equivalent device of
approved design, such as a circuit breaker. Such
devices should be placed on or as near the dynamo
as possible.

For two-wire, direct-current generators, single
pole protection will be considered as satisfying the
above rule, provided the safety device is located
and connected that the means for opening same is
actuated by the entire generator current thus com-
pletely opening the generator circuit.

When two-wire, direct-current generators are
used in conjunction with balancer sets to obtain a
neutral for three-wire systems, a protective device
should be installed which will operate and discon-
nect the three-wire system should an excessive un-
balancing of voltage occur. If a generator, not
electrically driven, in a two-wire system has one
terminal grounded, the circuit breaker above men-
tioned should be placed in the g/ounded lead.

For three-wire direct-current generators, either
compound or shunt wound, a safety device should
be placed in each armature lead of sufficient ca-
pacity and so arranged as to take care of the entire
current from the armature.

The safety devices for this service should be a
double-pole, double-coil overload circuit-breaker,
or a four-pole circuit-breaker connected in the main
and equalizer leads and tripped by means of two
overload devices, one in each armature lead. The
safety devices thus required should be so inter-
locked that no one pole can be opened without si-
multaneously disconnecting both sides of the arma-
ture from the system.

Fuses should never be used for this class of

In general, generators should preferably have no
exposed live parts and the leads should be well insu-
lated and thoroughly protected against mechanical
injury. This protection of the bare live parts
against accidental contact would apply also to all
exposed, uninsulated conductors outside of the gen-
erator and not on the switchboard.

Waterproof Covers, though not required, should
be provided for every generator and motor and
placed over each machine as soon as it is shut down.
Negligence in this matter has caused many an arm-
ature or field coil to burn out, as only a few drops
of water are necessary to cause a short circuit, when
the machine is started up again, that might do many
dollars' worth of damage, to say nothing of the
inconvenience of having to shut off light or power
when it is most needed, and for an indefinite length
of time.

Name Plates. Every generator and motor should
be provided with a name plate, giving the maker's
name, the capacity in volts and amperes and nor-
mal speed in revolutions per minute. This will

show exactly what the machine is designed for, and
how it should be run.

Terminal blocks when used on generators should
be made of approved non-combustible non-absorp-
tive, insulating material, such as slate, marble or

Wiring from Generators to switchboards and
thence to outside lines should be in plain sight or
readily accessible, and should be supported entirely
throughout upon non-combustible insulators (such
as glass or porcelain) and in no case should any wire
come in contact with anything except these insula-
tors, and the terminals upon the generators and
switchboard. When it becomes necessary to run
these wires through a wall or floor, the holes should
be protected by some approved non-combustible in-
sulating tube, such as glass , or porcelain, and in
every case the tube should be so fastened that it
shall not slip or pull out. Sections of any conduit,
whether armored or otherwise, that are chopped off
for this purpose, should not be used. All wires for
generator and switchboard work should be kept so
far apart that there is no liability of their coming in
contact with one another, nor of short circuit from
metallic tools used about them. All wire used in
this class of work should be the best quality of
"rubber covered" (see page 66). Bus-bars on
switchboards, may be made of bare metal,
so that additional circuits may be readily
attached. They should have ample carrying
capacity, so as not to heat with the maximum cur-
rent likely to flow through them under natural con-
ditions. (See "Capacity of Wire Table/' page 81.)

So much trouble in past years has arisen from faulty
construction of switchboards, and the apparatus
placed upon them, that strict requirements have
been necessarily adopted by engineers as well as
insurance inspectors, and the following suggestions,
are recommended by the latter ; although it is advis-
able, when possible, that all wires from generators to
switchboards be in plain sight and readily accessi-
ble, wires from generator to switchboard may, how-
ever, be placed in a conduit in the brick or cement
pier on which the generator stands, provided that
proper precautions are taken to protect them against
moisture and to thoroughly insulate them from the
pier or foundation. If lead-covered cable is used,
no further protection will be required, but it should
not be allowed to rest upon sharp edges which in
time might cut into the lead sheath, especially if the
cables were liable to vibration. A smooth runway
is desired. If iron conduit is provided, double
braided rubber-covered wire will be satisfactory.
In wiring switchboards with regard to their ground
detectors, voltmeters, pilot lights, potential trans-
formers or other indicating instruments. Nothing
smaller than No. 14 B. & S. gage "rubber cov-
ered" wire should be used, and no such circuit
should carry over 660 watts. Such circuits should
be protected by approved enclosed fuses. (See p.

The Switchboard should be so placed as to re-
duce to a minimum the danger of communicating
fire to adjacent combustible material, and, like the
generator, should be erected in a dry place and kept
free from moisture. It is necessary that it should

be accessible from all sides when the wiring is done
on the back of the board, but may be placed against
a brick, stone or cement wall when all wiring is on
the face of the switchboard.

The board should be constructed wholly of non-
combustible material and never built up to the ceil-
ing; a space of three feet, at least, should separate
the top of the board from the ceiling and at least
eighteen inches should separate the wall from the
instruments or connections, when the wiring is done
on the back of the board. Every instrument,
switch or apparatus of any kind placed upon the
switchboard should have its own non-combustible
insulating base. This is required of every piece of
apparatus connected in any way with any circuit.
If it is found impossible to place the resistance box,
rheostat, or regulator, which should, in every case,
be made entirely of non-combustible material, upon
the switchboard, it should be placed at least one
foot from combustible material or separated there-
from by a non-inflammable, non-absorptive insulat-
ing material. This will require the use of a slab or
panel of non-combustible, non-absorptive insulating
material such as slate, soapstone or marble, some-
what larger than the rheostat, which should be
secured in position independently of the rheostat
supports. Bolts for supporting the rheostat should
be countersunk at least J /6 inch below the surface
at the back of the slab and the holes over the heads
of the bolts filled with insulating material. For
proper mechanical strength, the slab should be of a
thickness consistent with the size and weight of the
rheostat, and in no case to be less than y*> inch.


If resistance devices are installed in rooms where
dust or combustible flyings would be libale to accu-
mulate on them, they should be equipped with dust-
proof face plates. Where protective resistances are
necessary in connection with automatic rheostats,
incandescent lamps may be used, provided that they
do not carry or control the main current nor con-
stitute the regulating resistance of the device.

When so used, lamps should be mounted in por-
celain receptacles upon non-combustible supports,
and should be so arranged that they cannot have
impressed upon them a voltage greater than that for
which they are rated. They should in all cases be
provided with a name-plate, which should be per-
manently attached beside the porcelain receptacle
or receptacles and stamped with the candle-power
and voltage of the lamp or lamps to be used in each

Wherever insulated wire is used for connection
between resistances and the contact device of a
rheostat, the insulation should be "slow burning."
(See page 67.) For large rheostats and similar
resistances, where the contact devices are not
mounted upon them, the connecting wires may be
run together in groups so arranged that the maxi-
mum difference of potential between any two wires
in any group shall not exceed 75 volts. Each group
of wires should either be mounted on no-combust-
ible, non-absorptive insulators giving at. least */
inch separation from surface wired over, or, where
it is necessary to protect the wires from mechanical
injury or moisture, be run in approved conduit or

equivalent. Special attention is again called to the


fact that switchboards should not be built down to
the floor, nor up to the ceiling, but a space of at
least ten or twelve inches should be left between
the floor and the board, and thirty-six inches be-
tween the ceiling and the board, when possible, in
order to prevent possible fire from communicating
from the switchboard to the ceiling, and also to
prevent the forming of a partially concealed space
very liable to be used for storage of rubbish and
oily waste. Where floor is of brick, stone or con-
crete, the switchboard may go to the floor, but for
cleanliness and safety space should always be pro-
vided when possible.

Lightning Arresters should be attached to each
wire of every overhead circuit connected with the

It is recommended to all electric light and power
companies that arresters be connected at intervals
over systems in such numbers and so located as to
prevent ordinary discharges entering (over the
wires) buildings connected to the lines (see p. 48).

Arresters for Stations and Sub-stations should
be located in readily accessible places away from
combustible materials, and as near as practicable to
the point where the wires enter the building.

Station arresters are often placed in plain sight
on the switchboard. The switchboard, however,
does not necessarily afford the only location meet-
ing these requirements. In fact, if the arresters
can be located in a safe and accessible place away
from the board, this should be done, for, in case the
arrester should fail or be seriously damaged there


would then be no chance of starting arcs on the

Fire Extinguishers. At least one, or more if
the size of the installation demands it, good ap-
proved extinguisher should be in plain sight and
readily accessible, one which is capable of extin-
guishing electrical fires or arcs without danger of
transmitting a shock to the operator (see page 194).

In all cases, kinks, coils and sharp bends in the
wires between the arresters and the outdoor lines
should be avoided as far as possible.

They should be connected with a thoroughly good
and permanent ground connection by metallic strips
or wires having a conductivity not less than, that of
a No. 6 B. & S. copper wire, and these should be
run as nearly in a straight line as possible from the
arresters to the earth connection.

Ground wires from lightning arresters should not
be attached to gas-pipes within the buildings.

It is often desirable to introduce a choke coil in
circuit between the arresters and the dynamo. In
no case should the ground wire from a lightning
arrester be put into iron pipes, as these would tend
to impede the discharge.

Unless a good damp ground is used in connection
with all lightning arresters, they are little better
than useless. Ground connections should be of the
most approved construction, and should be made
where permanently damp earth can be conveniently
reached. For a bank of arresters such as is com-
monly found in a power house, the following in-
structions will be found valuable: First, dig a hole
six feet square directly under the arresters until


permanently damp earth has been reached; second,
cover the bottom of this hole with two feet of
crushed coke or charcoal (about pea-size) ; third,
over this lay 25 square feet of No. 16 copper plate;
fourth, solder at least two ground wires, which
should not be smaller than No. 4, securely across
the entire surface of the ground plate ; fifth, now
cover the ground plate with two feet of crushed
coke or charcoal; sixth, fill in the hole with earth,
using running water to settle.

A practical and effective method of installing an
outside line arrester is shown on page 48.

All lightning arresters should be mounted on
non-combustible bases, and be so constructed as not
to maintain an arc after the discharge has passed.

Testing of Insulation Resistance. All circuits
except such as are permanently grounded, as de-
scribed on pages 45 and 46, should be provided with
reliable ground detectors. Detectors which indicate
continuously and give an instant and permanent
indication of a ground are preferable. Ground
wires from detectors should not be attached to gas
pipes within the building.

Where continuously indicating detectors are not
used, the circuits should be tested at least or^e
per day (see page 65), and preferably oftener.

Data obtained from all tests should be recorded
and preserved for examination.

Storage or Secondary Batteries should be in-
stalled with as much care as generators, and in
wiring to and from them the same precautions and
rules should be adopted for safety and the preven-
tion of leaks. The room in which they are placed


should not only be kept dry, but exceptionally well
ventilated, to carry off all fumes which are bound
to arise. The insulators for the support of the
secondary batteries should be glass or porcelain,
as filled wood alone would not be approved. The
use of any metal liable to corrosion should be
avoided in cell connections of secondary batteries
of the lead or sulphuric acid type.

Care of Generators. A few suggestions as to
the care of the generator, as well as its installation,
may be of value, and one of the important points
under this head is that the driving power should
have characteristics of steadiness and regularity of
speed, and should always be sufficient to drive the
generator with its full load, in additions to the
other work which it may be called upon to sustain.
Unsatisfactory results are always obtained if at-
tempting to run a generator on an engine operating
anything other than its own generator or gen-

Wooden bed plates are supplied, when ordered,
for all generators, except in the larger or direct-
connected machines.

Most belt-driven generators and motors are
fitted with a ratchet or screw bolt, so that they may
be moved backward or forward on the bed plate
in a direction at right angles to the armature shaft.
By this means the driving belt may be tightened or
loosened at will while the machine is in operation.
Care should be taken in tightening the belt not to
bind the bearings of the armature and force the oil
from between the surface of the shaft and boxes.


Such practice will inevitably cause heating of the
bearings and consequent injury.

Generators are usually assembled, unless ordered
otherwise, so that the armature revolves from left
to right when the observer faces the pulley end of
the shaft. All generators, however, may be driven
in either direction by reversing the brush leads and
changing field connections.

The generator, if belt driven, is provided with a
pulley of the proper size to take care of the power
necessary to drive it, and one of different size
should not be substituted unless approval be ob-
tained from the generator makers.

When driving from a countershaft which, at best,
is bad practice, or when belted directly to the main
shaft, a louse pulley or belt holder should be used,
to admit of starting and stopping the generator
while the shafting is running.

Belts. A thin double or heavy single belt
should be used, about a half -inch narrower than the
face of the pulley on the generator. An endless
belt, one without lacing, gives the greatest steadi-
ness to the lights. For proper length of belts see
formula on page 213.

All Bolts and nuts should be firmly screwed
down. All nuts which form part of electrical con-
nections should receive special attention.

When copper commutator brrshes are used, al-
though now almost obsolete, they are carefully
ground to fit the commutator, and they should be
set in the holders so as to bear evenly upon its

On machines where two or more copper brushes


are supported on one spindle, the brushes on the
same side of the commutator must be set so that
they touch the same segments in the same manner.
The brushes on the other side of the commutator
must be. set so as to bear on the segments diamet-
rically opposite. When the brushes are not so set
it is impossible to run the machine without spark-
ing. A convenient method of determining the
proper bearing point for the brushes. is to set the
toe of one brush at the line of insulation, dividing
two segments to the commutator; then count the
dividing lines for one-half the way around the sur-
face, and set the other brush or brushes at the line
diametrically opposite the first. Thus, on a forty-
four segment commutator, after setting the tip of
one brush at a line of insulation, count around
twenty-three lines, setting the other brush at the
twenty-third line, thus bringing the tips directly
opposite each other. The angle which the brushes
form with the surface of the commutator should be
carefully noted, and the brushes should not be al-
lowed to wear, so as to increase or decrease this
angle. Careless handling of the machine is at once
indicated by the brushes being worn either to a
nearly square end or to a long taper, in which the
forward wires of the brush far outrun the back or
inside wires. Either condition cannot fail to be at-
tended with excessive wear of both commutator
and brushes.

After copper brushes are set in contact with the
commutator, the armature should never be rotated
backward. If it is required to turn the armature
back, raise the brushes from the commutator by the


thumb screw on the holder provided for that pur-
pose, before allowing such rotation.

Carbon Brushes are now almost universally
used and require little or no adjustment or care
other than keeping them clean.

Bearings. See that the bearings of the ma-
chine are clean and free from grit, and that the oil
reservoirs are rilled with a good quality of lubricat-
ing oil.

The Oil Reservoirs should always be axamined
before starting, and all loose grit removed. The oil
should all be drawn off at the end of each day's
run for the first three or four days and filtered,
after which it can be assumed that any grit has
been carried off with the filtration, and it will only
be necessary to add a little fresh oil once in seven
to ten days. These instructions apply only to ma-
chines using loose ring oilers attached to each end
of the armature shaft.

In starting up a Generator or motor fill the oil
reservoirs and see that the oiling rings are free to
move. In the case of generators fitted with oil
cups, start the oil running at a moderate rate. Too
little oil will result in heating and injury of the
bearing; but, on the other hand, excessive lubrica-
tion is unnecessary, wasteful, and sometimes pro-
ductive of harm.

When the generator is ready to be started, place
the driving belt on the pulley on the armature shaft,
and then slip it from the loose pulley or belt holder
on to the driving pulley on the counter-shaft.
Tighten the belt, by means of the ratchet on the
bed plate, just sufficiently to keep it from slipping.

Care should be taken not to put more pressure than
is necessary on bearings ; carelessness in this respect
is often followed by heating of the boxes, and pos-
sible permanent injury.

The brushes may now be let down upon the com-
mutator, if copper brushes are used.

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Online LibraryH. C. (Harry Cooke) CushingStandard wiring for electric light and power; as adopted by the fire underwriters of the United States .. → online text (page 1 of 16)