OVERHEAD CONSTRUCTION
INSULATORS FOR OVERHEAD OPEN WIRING


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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.

  1. White porcelain insulators should normally be used on main lines, and for short trunk and junction circuits on minor lines.

  2. Brown insulators may be used in lieu of white where it is desired to render a line inconspicuous.

  3. 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.

  1. Black composition insulators, all types, having no date mark or having been in position for more than five years under average atmospheric conditions.

  2. 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).

  3. 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:-

  1. insulators will not require changing to accommodate double terminations (see E 3080) (5) surveys and stores transactions will be simplified.

  2. 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.

  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).

  2. 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.

  3. 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:-

  1. Black composition insulators, all types, having no date mark or having been in position for more than five years under average atmospheric conditions.

  2. White or brown insulators of obsolete types which are surplus to local requirements for renewals and which are not required as in 5(c).

  3. 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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Last revised: January 05, 2022

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