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Porcelain Insulators - The
making of GPO insulators
History and Development of Insulators
P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
LINES
OVERHEAD
F 3035
Issue 3, 15.2.39
WIRING
Insulators
1. General
This Instruction describes the standard types of line insulator and their
conditions of use.
2. Design
The essential feature of the design of telegraph and telephone insulators is the
provision of a long leakage path to earth, i.e. to the spindle. This is ensured
by the "double-shed" type of insulator, used at all positions except leading-in
points. Since questions of electrical stress are not involved, telegraph and
telephone insulators can be made to a compact design, with relatively-close
clearances between the spindle and the inner shed and between the inner and
outer sheds. The under surfaces of the insulators are therefore well protected
against wetting by heavy rain.
3. Material and colour
Insulators are made of white or brown porcelain, brown stoneware or black
composition. White and brown insulators are, for all practical purposes,
identical as regards electrical and physical properties, and both are superior
to black composition as regards electrical characteristics.
Black composition weathers more rapidly than porcelain but is
much more resistant to impact, and therefore has greater immunity from damage by
stone-throwing.
Table 1
Details of Insulators
4. Identification Markings
To facilitate assessment of their residual life (see para. 12) and for
identification, black composition insulators have - since 1929 - been marked
upon the top with the last two figures of the year of manufacture and the code
letter or trade mark of the manufacturer.
5. Appearance
For the sake of appearance, a pole should preferably be fitted with insulators
entirely of one colour.
6. Sizes
The insulators used by the Department are mainly of two sizes - "large" and
"small". The large size is used chiefly on main lines, for wires of 150lb. (or
more) per mile; the small size on minor lines, for wires not exceeding 100lb.
per mile. The various types are illustrated in Fig. 1.
7. Conditions of use.
As a long leakage path is needed to provide satisfactory insulation between
conductors, a separate insulator should be used for every conductor. The use of
a two-groove insulator to carry two separate conductors results in an inadequate
insulation resistance between them, and is therefore prohibited.
-
White porcelain insulators should normally be used on main
lines, and for short trunk and junction circuits on minor lines.
-
Brown insulators may be used in lieu of white where it is
desired to render a line inconspicuous.
-
Black insulators should be used in sections of line where
heavy damage from stone-throwing is experienced or anticipated. In other
sections, the use of black insulators for trunk and junction circuits should
be avoided as far as possible.
8. Conditions of use
These are given in Table 1. It will be noticed that, with the exception of
"Insulators No. 5", insulators identical in type, but differing in colour, are
grouped together under one designation. Where a particular colour is required,
therefore, it should be specified on the requisition; otherwise, the colour
supplied will be dependent on the stock position.
9. Fitting
Before insulators are fixed in position, all traces of dirt, dust or packing
material adhering to them should be removed, otherwise insulation will be
impaired.
10. "Rings, Insulator"
A rubber washer should be placed on the upper shoulder of every spindle before
the insulator is screwed on. The washer minimizes the effects of unequal
expansion of the spindle and insulator material, enables the insulator to be
screwed down tightly without risk of damage to the thread, awl tends to prevent
unscrewing through vibration. The washers, of 5/32in. square section, are
available in two sizes under the following designations:-
"Rings, Insulator, No. 1" for spindles with 5/8in. insulator threads
"Rings, Insulator, No. 2" for spindles with 3/4in. insulator threads.
11. Cleaning
The cleaning of insulators in situ should rarely be necessary. It may
occasionally be found desirable to remove the accumulation of cobwebs, etc. from
the spindle cavity and from between the sheds; "Brushes, Insulator" are
available for this purpose. In the event of trouble arising through severe
fouling of insulators - for example, by tarry or sooty deposits in city areas -
new insulators should be substituted and the dirty ones cleaned in bulk or
scrapped (see E 5035 for detailed instructions).
12. Recoveries
Undue labour charges should not be incurred in the recovery of the following
types of insulators, which should be scrapped upon recovery whatever their
condition.
-
Black composition insulators, all types, having no date mark
or having been in position for more than five years under average
atmospheric conditions.
-
White or brown glazed-porcelain insulators of non-standard
types which cannot be used in place of existing types. (The medium No. 8
insulators, now obsolete, are suitable for use on junction or subscribers'
circuits).
-
Any insulators which are cracked or otherwise defective.
13. Recoveries
Recovered porcelain and stoneware insulators not included in the above
categories may, if necessary, be cleaned as described in E 5035.
14. Other instructions concerning the method of disposal of
insulators are given in STORES, Workmen's Procedure, Section J.
15. "Insulators No. 2" have been made obsolescent and, when
stocks are exhausted, "Insulators No. 3" will be used instead.
16. The principal advantages which will result from the change
are as follows:-
-
insulators will not require changing to accommodate double
terminations (see E 3080) (5) surveys and stores transactions will be
simplified.
-
stocks of insulators held by gangs and wire-men can be
reduced.
17. Binding-in
The number of points at which wires are terminated will not be affected by the
change. At "through" positions the line wire should be bound-in to the upper
groove of the insulator (see E 3090).
18. Requisitions should still be forwarded for "Insulators No.
2", as a considerable stock of this-item is held. When the stock is exhausted,
requisitions will automatically be met by the issue of "Insulators No. 3".
FIG. 1
P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
LINES
OVERHEAD
F 3035
Issue 4, 20.9.61
WIRING
Insulators
1. General
This Instruction describes the standard types of line insulator and their
conditions of use.
2. Material and colour
Insulators are made of white porcelain, white stoneware or black composition.
White porcelain and stoneware insulators are, for all practical purposes,
identical as regards electrical and physical properties, and both are superior
to black composition as regards electrical characteristics.
Black composition weathers more rapidly than porcelain and
stoneware but is much more resistant to impact, and therefore has greater
immunity from damage by stone throwing.
3. For the sake of appearance, a pole should be fitted with
insulators entirely of one colour.
4. Types
Insulators used by the Department are Nos. 1, 3, 7, 10, 16 and 21.
5. Conditions of use.
As a long leakage path is needed to provide satisfactory insulation between
conductors, a separate insulator should be used for every conductor. The use of
a two-groove insulator to carry two separate conductors results in an inadequate
insulation resistance between them, and is therefore prohibited except as
detailed for shared lines in E 3139. The conditions of use are given in Table 1.
-
White porcelain insulators should normally be used on trunk
and junction circuits, and on subscribers' lines in localities where there
is difficulty in maintaining adequate insulation resistance with black
insulators. White insulators should normally be used also on open-wire
ring-type D.P.s (see E 3130).
-
Black insulators should normally be used for subscribers'
lines except for ring-type D.P.s and where there is difficulty in
maintaining adequate insulation. Black insulators should also be used in
sections of line where heavy damage from stone throwing is experienced or
anticipated. In other sections, the use of black insulators for trunk and
junction circuits should be avoided as far as possible.
-
Brown insulators are used in National Parks and Areas of Outstanding
Natural Beauty (see 3901) where there are objections to white insulators on
aesthetic grounds and there is difficulty in maintaining adequate insulation
with black insulators, brown insulators may be fitted. Recovered brown
insulators should be used as far as possible, but when this source of supply
is inadequate they may be obtained by local purchase from Messrs. J. Bourne,
Denby Pottery, Derby.
TABLE 1
DETAILS OF INSULATORS
6. Fitting
Before insulators are fixed in position, all traces of dirt, dust or packing
material adhering to them should be removed, otherwise insulation will be
impaired.
7. Rings, Insulator
A rubber washer should be placed on the insulator thread shoulder of every
spindle before the insulator is screwed on. The washer provides a resilient
seating for the insulator and enables the insulator to be screwed down tightly
without risk of damage to the thread and tends to prevent unscrewing due to
vibration. The washer also minimizes the effects of unequal expansion between
the spindle and insulator material. The washers, of 5/32 in. square section, are
designated Rings, Insulator No. 1.
8. Cleaning
The cleaning of insulators in situ should rarely be necessary. It may
occasionally be found desirable to remove the accumulation of cobwebs, etc. from
the spindle cavity and from between the sheds; Brushes, Insulator are available
for this purpose. In the event of trouble arising through severe fouling of
insulators - for example, by tarry or sooty deposits in city areas - new
insulators should be substituted and the dirty ones cleaned in bulk or scrapped
(see E 5035 for detailed instructions).
9. Recoveries
The following should be scrapped upon recovery:-
-
Black composition insulators, all types, having no date mark
or having been in position for more than five years under average
atmospheric conditions.
-
White or brown insulators of obsolete types which are
surplus to local requirements for renewals and which are not required as in
5(c).
-
Any insulators which are cracked or otherwise defective.
10. Recovered porcelain and stoneware insulators not included in
the above categories may, if necessary, be cleaned as described in E 5035 and
re-used.
11. Other instructions concerning the method of disposal of
insulators are given in STORES, Workmen's Procedure, Section J.
P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
LINES
0VERHEAD
E 3039
Issue 1, 30.8.38
REEL INSULATORS
1. General
A new type of insulator, termed an "Insulator, Reel", has been introduced to
facilitate the wiring work involved in the conversion of subsidiary poles to
D.P.s.
2. The insulator is made of composition, with a metal bush which enables it to
be screwed on a "Spindle No. 8, or No. 10" above the spindle nut.
3. Conditions of Use
When the overhead part of conversion work is ahead of the underground, the
spindles and insulators may be changed and the wires left working back to the
existing D.P. via reel insulators, as indicated in Fig. 1.
4. As soon as the underground plant is available, the leads may
be run to the "Insulators No. 16" and the diversions made. Recovery of the wires
made spare, and of the reel insulators on which they were terminated, may then
be effected without loosening the spindle nuts.
5. Requisition
"Insulators, Reel" can be obtained from the Stores Department, Birmingham.
Fig. 1
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