This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
These should be rigidly enforced, as all tend to the benefit of the consumer. The contractor aims to get his work done with a minimum of cost to himself, and is quite satisfied if he "gets through the test" As it is quite possible to get a satisfactory test with faulty material on a dry building it will be seen that vigilance is necessary. On the other hand, damp in a new building will sometimes entirely spoil the test even with the best of work, by connecting the exposed parts of distribution boards, etc., to earth with a film or sweat of moisture; yet, unfortunately, damp does not of necessity expose the weak points of faulty workmanship. In a case of this sort it is necessary to get rid of the moisture from the parts affected in the best way possible with the means at hand.
It is advisable, before commencing work, to submit a sample of all fittings, cable, etc., to the undertaker's engineer. This course may save a great deal of trouble and loss of time and money, while the engineer of to-day will be found, without exception, courteous and willing to advise.
The following regulations have been compiled by Mr. H. Tomlinson-Lee, Chief Engineer of the Wimbledon Electricity Works, who has kindly permitted their publication. They may be taken as representative, and contain much that should be embodied in specifications of wiring for electric light.
The first clause is the usual "Declaration of pressure," beyond which a maximum variation of 4 per cent, only is allowed by B.O.T. regulations. This clause will vary according to locality, and in the case of direct current no "cycles" will be specified.
Alternating current will be supplied at a pressure of 220 volts, 50 cycles.
A double-pole switch of approval construction must be supplied and fixed by the consumer in an accessible position, and as near as possible to the Council's fuse.
The necessary loops for the meter connections must be provided by the consumer in the position selected. A shunt wire of 1/18 gauge should also be left for the meter connections.
All installations must be fully completed before they will be inspected and connected with the supply mains. No work of a temporary nature will be allowed, and no current will under any circumstances be turned on to any installation until the work is entirely finished.
The Council's engineer will inspect and test each installation free of charge on completion, but if through defective work or other causes it is necessary to inspect again, a fee of 10s. 6d. will be charged for each and every subsequent test.
All work must be carried out to the satisfaction of the Fire Insurance Company, or if not insured to the rules issued by the Institution of Electrical Engineers, and to the approval of the Council's engineer.
All installations requiring more than 25 amperes may be required to be divided into separate circuits from the Council's meter or meters.
No double-pole switch, except the main switch, shall control more than 10 amperes, and no single-pole switch more than fifteen 8 candle-power lamps or their equivalent.
All single-pole switches must be on the same pole.
Each circuit to be protected with a single-pole cut-out on both poles and having a clear break of 1 1/2 inch. Cut-outs should be of an improved high voltage type; those having small unventilated close-fitting covers are not approved, and no double-pole cut-outs of any pattern are to be used. Cut-outs to be arranged on distributing boards of non-combustible mate~ial. No cut-outs are to be fitted anywhere except on distributing boards, nor are fuses to be fitted in ceiling roses or wall plugs.
The insulation resistance of all wires shall not be less than 600 megohms per statute mile. All wires greater than No. 16 must be stranded, and no wire smaller than No. 18 will be allowed.
The area of conductors should be so proportioned to their length that the drop in pressure does not exceed 2 volts with all lamps alight simultaneously, and they should be of such sufficient section that with the maximum current passing continuously there is no appreciable electrical heating.
To be carefully made, soldered with resin flux only, and insulated with at least six layers of pure rubber strip, protected by waterproofed tape ; no joints to be made in damp places or drawn into pipes. As far as practicable, joints should be avoided, leads being run back to terminals of fittings, etc.
To be carefully and efficiently insulated from gas and water pipes and from earth.
Wall plugs shall be preferably of the 2-pin type. Key socket holders where employed to be submitted for approval before being used.
Combination gas and electric fittings are greatly objected to, and if such are proposed they must first be effectively insulated from the gas supply pipe and approved by the Council's engineer.
No exposed metal carrying current will be allowed, and all switches, etc., must have insulated handles and covers, and must be so screened as to prevent risk of contact with charged metal by persons using the same.
Forty-eight hours' notice must be given before any installation can be tested.
All installations will be tested at a pressure of 400 volts, and after one minute's electrification they must have an insulation resistance to earth of not less than 100 megohms, divided by the number of lamps installed. All fittings, switches, lamps, etc., must be fixed and connected when this test is made.
All outside leads to arc lamps should be protected by lead sheathing, or else special thickly braided outer covering, thoroughly coated with good insulating enamel. Single conductors must never be enclosed in metal tubes, and single lead-covered wire will not be allowed.
Where incandescent lamps are used in outside brackets an insulating piece of ebonite or porcelain should be inserted between the holder and adjacent metal to guard against each leakage.
Arc lamps should be provided with suitable inductive choking coils, or properly constructed resistance coils, and must be hung on insulators in all cases. Exposed terminals will not be allowed. They must be so constructed that the starting current is not more than 75 per cent, in excess of the normal working current.
Proposals for use of these should in all cases be first submitted to the Council's engineer. The starting current is not to be more than 25 per cent, in excess of the maximum working current, and they must be provided with suitable resistance and starting switches so constructed that the starting and stopping of the motor shall not cause any disturbance of the lights on its own or neighbouring circuits. The terminals must be guarded against the possibility of accidental contact.
Motors, arc lamps, and special consuming devices must be controlled by double-pole switches, and be protected by an approved cut-out on each pole.
This term includes electric stoves, cooking and heating apparatus, hot plates, etc. These must be provided with a stand of incombustible material, and should be placed at least 18 inches clear of any woodwork or inflammable material. The terminals must be protected against accidental contact.
Cut-outs, switches, etc., should be provided with proper covers, and be arranged where they will not be liable to knocks or other injury, as this causes heating, and possibly extinctions, or danger of fire.
Care should be exercised in arranging lamp bulbs in windows or show-cases so that they do not come in contact with flimsy paper, cotton, or similar goods, from which there might be a risk of fire originating. In crowded shop windows the lamps should be guarded by shades or other means.
Resistance coils, etc., should be specially inspected and guarded so that they will not endanger their surroundings if hot.
If these and similar precautions are adopted, and wiring work entrusted to thoroughly competent men, the electric light will be found to be much safer than any other illuminant.
The engineer will be pleased to advise in any case where special wiring may be required, such as cold stores, breweries, and damp buildings, drapery warehouses, wood-working shops, organ chambers, printing and bookbinding works, etc.
A few words upon the various systems of charging for current may not be entirely out of place.
There are four systems - the Flat Rate, Flat Rate with Rebate, Flat Rate with Sliding Scale, and the Maximum Demand System.
The first is the original method of charging a fixed sum per unit (maximum fixed at eightpence by Board of Trade) for any quantity.
The Rebate system provides a discount according to quantity beyond a minimum.
The Sliding Scale system charges full rate for a fixed minimum, but reduces the rate from that to another minimum beyond which the amount of current used is charged at a still lower rate, so that the total will be made up of three items where the consumption is large enough to take full advantage of this arrangement.
The maximum demand system regulates the charge according to the largest amount (or demand) of current taken at any one time during the quarter.