ABSTRACTS OF PAPERS 321

STREET LIGHTING

By E. C. LENNOX, Member.*(ABSTRACT of an Installations Section paper which'was published in April, 1946, in Part II of the Journal.)

THE NEED FOR ADEQUATE STREET LIGHTINGStreet lighting fulfils a public necessity in providing for the

convenience, and increasing the safety, of all road users; inassisting police in their work of preventing crime; and in en-hancing the attractiveness of shopping areas, and giving emphasisto, and creating pride in, civic centres.

The author firmly believes that adequate street-lightingfacilities will be a major factor in minimizing road accidentsduring the hours of darkness and that the cost of such lightingwill be but a small premium to pay for reducing the high costto the country of those accidents which result from bad lighting.

ILLUMINATION REQUIREMENTS

The principal requirement of street lighting is to provide ade-quate means whereby the road-user can see accurately andquickly any object in his line of vision. From Jhe point of viewof the road-user, whether a pedestrian or a driver of a fast-moving vehicle, it is clear that he must see objects on the roadwayagainst a background made up of the road surface and its en-virons, i.e. footpaths, fences, buildings, etc., lining the roadway.The street-lighting installation must create a contrast in bright-ness between the object and its background.

Method of Obtaining Brightness Contrasts.There are two obvious ways of obtaining brightness contrasts:

(a) by illuminating the object to a higher level of brightness thanthe background, or (b) by creating a "background brightness,"i.e. a brightness of road surface and its environs, against whichthe object can be seen in silhouette.

Method (a) is not practicable with conventional lightingsystems owing to the characteristic reflection properties of roadsand of objects. To achieve any degree of contrast a system re-sembling a series of fixed head-lamps would be necessary; evenso, the results would be unsatisfactory and except in one-wayroads glare might be severe.

Method (Jb) calls for the light to be directed along a roadwaysurface rather than on to it, in order to utilize the selective-reflecting properties of road materials.

Light Distribution from Lanterns.There are two principal types of distribution from lanterns

which produce this result—"cut-off" and "non-cut-off.""Cut-off" distribution may be represented by a curve of the

type shown in Fig. 1, the light output being cut off at angles

Candles4000 3000 2000

70

60° 50° 4 0 3 0 ° 20° 10° 0

Fig. 1.—Curve of "cut-off" distribution.

above 70° to the downward vertical. A fitting to which such acurve applies would produce a bright area at right angles to theroadway, but one of short spread along the roadway. This

• North-Eastem Electric Supply Co., Ltd.

spread appears shorter still as the observer views the result atgreater distances from the lamp. Fig. 2 shows photographically

270 ft 450 ftFig. 2.—Source: 250-watt h.p.m.v. lamps with "cut-off" lanterns.

Distances: 90 to 450 ft.Mounting height: 25 ft.Road surface: Concrete, very matt.

the results obtained with a single lamp source with cut-off dis-tribution.

It has been shown that better reflection from road surfaces isobtained when the incident light strikes the surface at veryoblique angles, but with cut-off distribution advantage cannot betaken of this factor as the light is cut-off above 70°. The re-sulting brightness is therefore very much less than in systemslighted by "non-cut-off" type lanterns, but some compensationis gained by the relative lack of glare. The short spacing neces-

322 ABSTRACTS OF PAPERS

sary with them is not economical, and a type of distributioncapable of more reasonable spacing must be considered.

In "non-cut-off" distribution, of which a typical curve is givenin Fig. 3, the maximum candle-power is emitted at 75° or above

CandlesZ000 1000

5 0 ' 4 0 5 3 C T 2 0 ' 10" 0

Fig. 3.—Curve of "non-cut-off" distribution.

to the downward vertical and is well maintained up to about 86°.Below 75° the distribution must provide good local illuminationaround the lamp standard. With such distribution a certainamount of glare is inevitable, but practical results show thatprovided it is kept within reasonable limits the road-user suffersno disability.

Fig. 4 shows the distribution on a worn asphalt surface atpoints from 100 to 500 ft from the source.

The reduction of mounting height gives a result similar tothat of extending the distance, namely the brightness areabecomes narrower and shorter, central areas become brighter,outer areas darker, and the general effect is more patchy.

Building-up a Complete Installation.The complete lighting installation must, in effect, be such a

build-up of the bright areas produced by the various lanternsthat these areas merge and present a complete pattern of back-ground brightness to the observer. This requires careful sitingof each lighting unit so that the maximum value of backgroundbrightness will be observed by road-users.

In traffic-route lighting, the spacing of lamp standards, whichshould normally be an average of 150 ft (180 ft maximum in anyspan), may on bends be reduced to as low as 90 ft with lanternsmounted 25 ft high. The overhang of the lantern over theroadway must be considered in relation to the road width.

The optimum effect which can be produced by lanterns appro-priately designed can easily be lost by giving insufficient con-sideration to these details. Recommendations as to spacing,mounting height, layout, etc., are fully given in the Final Report,1937, of the Ministry of Transport's Departmental Committeeon Street Lighting.

The responsibility for careful siting cannot be too stronglyemphasized, and attention must be given to backgrounds(buildings, palings, etc.), bends, road intersections, trafficroundabouts, etc.

Special Unidirectional Unit.A unidirectional distribution of light may be used on double

carriage-ways or one-way roads where traffic proceeds in onedirection only. Owing to the reflection properties of road sur-faces, light directed away from a driver does not add to thebrightness of the road in front of him; by lighting objects itreduces their contrast with the background.

If this light is re-directed towards the observer the power re-quired can be greatly decreased; several advantages accrue fromthis, namely: (1) object contrasts are increased, especially with

100 ft

200 ft

300 ft 500ft

Fig. 4.—Source: 250-watt h.p.in.v. lamps with totally-enclosed lanterns.Mounting height: 25 ft.Distance: 100 to 500 ft.Road surface: Asphalt, worn.

light objects; (2) one carriage-way of a double carriage-way roadappears dark to the user of the other carriage-way, and vice-versa. The lanterns providing the lighting for that carriage-way

ABSTRACTS OF PAPERS 323

are quite invisible; and (3) greater flexibility in planning ispossible, as lighting in one direction only has to be considered.

B.S. SPECIFICATIONThe recommendations in the Final Report of the Ministry of

Transport's Departmental Committee on Street Lighting set outthe best code of practice for street lighting that has yet appeared.A new specification, which implements these recommendations,is expected to be issued shortly.

There are two important differences between B.S. 307—1931and the Final Report of the M.O.T. Departmental Committee.The latter recommends that Group B roadways, or non-trafficroutes, should have only one mounting height and a limitedstandard of lighting. The lighting is not expected to be such asto warrant headlights being dispensed with—the decision to usethem or not being lefYjto the driver. In Group A roadways, highmounting height and higher-power lanterns should warrant theuse of the roadway without headlights.

The views of the Committee were that (a) if traffic routes areto be lighted at all, they must be lighted to full standard to bereally safe, and (b) there is no half-way stage to adequate streetlighting.

STREET LIGHTING APPARATUS

Poles and columns used for street lighting are to be chosen forappearance by day as well as to serve their useful purposesduring lighting hours.

Lanterns should be designed for easy maintenance as well asefficiency; the new side-entry type provides a rigid and simpledesign. Care in choice of lanterns will minimize the annual costof maintenance.

Lighting sources are available in many sizes, shapes and types.Regard must be had to the full annual cost of energy plus main-tenance before direct comparisons can be made. Colour is notof primary importance in roadway lighting. The future willdemand improved colour output in town centres and shoppingareas. There is always reasonable hope that mercury-vapourlamps will be colour-corrected. The present form of mercury-vapour fluorescent tube is not expected to play much part instreet lighting, owing to the difficulties and cost of fixing andmaintaining a tube 5 ft in length.

Control of street lighting is now the major problem; timeswitches with synchronous motor-driven clocks fitted with solardials to set and re-set time of operation are a reliable and anexcellent method of control.

High-frequency impulses over distribution networks presentanother method, of which several types are now available, some ofwhich are capable of easy extension to undertake control ofwater heating, space heating, etc.

Injection of d.c. impulses on l.v. networks is another methodof control fast gaining popularity in various areas.

Estimates of costs of equipment are given, together with fullrunning costs including capital amortization and interest, supplyof energy and maintenance of street lighting plant. These costsare applied to a proposed scheme for lighting a "through trafficroute," to Group A standard. The annual costs, reduced to afigure of annual rate in pence per £ required by each of the22 lighting authorities involved, show figures averaging from0-08d. to 148d., thereby indicating the impossibility of carryingout the lighting of such a traffic route without an amendment ofthe present method of administering street lighting. A pool ofthe total annual cost spread over the 22 lighting authorities wouldnecessitate a rate of only 0*95d. per £.

THE OPERATION OF LARGE TURBO-ALTERNATORS TO REDUCEROTOR WINDING DEFORMATION

By R. H. COATES, B.Sc.(Eng.), Member,* and B. C. PYLE, Associate Member.f

(ABSTRACT of a paper which was published in April, 1946, in Part II of the Journal.)

The heavy incidence of alternator outages during the waryears reached a peak in 1944, with some 110 000 kW of plantout of commission simultaneously with rotor faults.

The majority of such faults have been attributable to accu-mulated deformation of the rotor windings leading to doubleearth faults, which deformation resulted from high operatingtemperatures and the frequent starting and stopping of largehigh-speed 3 000-r.p.m. machines.

Design limitations have led manufacturers in many cases toadopt the full B.S. temperature rise of 90 deg C for 3 000-r.p.m.sets of 20 MW and above, and it is believed that the probabilityof failure is accentuated by the method of operation necessitatedby present-day working under Grid conditions, calling for fre-quent starting and stopping of these large sets, and habitualoperation at full load, conditions which will increase in effectwith the continued installation of large plant.

The trouble will be most prevalent in stations depending uponcooling towers, with the resulting high cooling-air temperatures,and where industrial loads of low power factor have to becatered for, requiring high excitation.

The paper gives details of tests carried out on a number ofalternators of the type in question, with the object of evolving apalliative for the apparently recurrent failures.

• City of Portsmouth Electricity Undertaking.t Sheffield Corporation Electricity Department.

Rotor winding temperatures were measured for each set atseveral loads, making it possible to estimate temperatures atany other load, and full details were obtained of past operatingconditions. By means of specially designed extensometers,movements of copper windings relative to the steel body of therotor were measured at various temperatures with the rotorstationary. Fig. 1 summarizes the results of several tests,together with the stress curves for the windings calculated there-from. The temperature base used in this Figure is:

Differential degrees =115

(temperature of copper) — — (temperature of iron), deg C.

The basic cause of winding deformation, given in a paper byJuhlin,* is briefly as follows: Part of the copper is mechanicallyrigidly held by centrifugal force when at full speed. When heatedwhile thus held it is unable to expand and is thereby in a stateof stress, and if the stress is beyond the elastic limit, plasticdeformation of the copper is produced.

The phenomenon is cumulative, each operating cycle—runup, heat up, run down, cool off—producing further deformation,shortening the winding until the copper is work-hardened suffi-ciently to raise the elastic limit to the maximum stress occurring.

• Journal I.E.E., 1939, 85, p. 544.