* THE NEED FOR PROTECTION

BEFORE PRCEEDING TO DESIGN A DETAILED LIGHTNING PROTECTION SYSTEM,FIRST CAREFULLY CONSIDER IF THE STRUCTURE NEEDS PROTECTION.IN MANY CASES, IT IS OBVIOUS THAT SOME FORM OF PROTECTION IS REQUIRED.HIGH RISK STRUCTURES AS EXPLOSIVE FACTORIES, OIL REFINERIES, ETC. WILLREQUIRE THE HIGHEST POSSIBLE CLASS OF LIGHTNING PROTECTION TO BE PROVIDED. IN MANY OTHER CASES THE NEED OF PROTECTION IS NOT SO EVIDENT.BS 6651 PROVIDE A SIMPLE MATHMATICAL OVERALL RISK FACTOR ANALYSIS FOR ASSESSING WHETHER STRUCTURE NEEDS PROTECTION. IT SUGGESTS THAT AN ACCEPTABLE LIGHTING STRIKE RISK FACTOR 10^(-5) PER YEAR .ie,1 IN 100,000 PER YEAR THEREFORE, HAVING APPLIED THE MATHEMATICAL ANALYSIS TO A PARTICULAR SET OF PARAMETER, THE SCHEME DESIGNER WILL ACHIEVE ANUMERICAL SOLUTION. IF THE RISK FACTOR IS LESS THAN 10^(-5) A SUITABLE ANALOGY COULD BE MADE WITH THE ODDS IN HORSE RACING. THE SHORTER THE(EG 5 TO 1) THE MORE LIKELY THE HORSE WILL WIN THE RACE.THE LONGER THE ODDS(EG 100TO1) LIKELY THE HORSE WILL WIN THE SHORTERTHE RISK FACTOR (EG 1 IN 10,000) THE GREATER THE RISK THAT A STRUCTURE WILL BY LIGHTING.THE LONGER THE RISK FACTOR (EG 1 IN 1,000,000) THE LESS LIKELY THE STRUCTURE HIT BY THE LIGHTING IT IS ACKNOWLEDGED THAT CERTAIN FACTORS CAN NOT BE ASSESSED IN THIS WAY AND THESE RIDE ALL OTHER CONSIDERATION. FOR EXAMPLE,IF THERE IS A REQUIREMENT THAT THERE SHOULD AVOIDABLE RISK TO LIFE OR REQUIREMENT FOR OCCUPANTS OF A BUILDING TO ALWAYS FEEL SAFE, TO WILL FAVOUR THE ISTALLATION OF PROTECTION , EVEN THOUGH IT WOULD NORMALLY BE ACCEPTED PROTECTION IS UNNECESSARY.THESE ARE DECISIONS CONSULTANT /ARCH CLIENT CAN MAKE.THAT PREHAPS ONLY THETHE FACTORS WHICH SHOULD BE CONSIDERED FOR DETERMINING AN OVERALL RISK FACTOR CAN BE SUMMARISED AS FOLLOWS.A) THE GEOGRAPHICAL LOCATION OF THE STRUCTURE. B) THE EFFECTIVE COLLECTION AREA OF THE STURCTURE. C) WEIGHTING FACTOR A ( USE OF THE STRUCTURE )D) WEIGHTING FACTOR B ( TYPE OF CONSTRUCTION )E) WEIGHTING FACTOR C ( CONTENTS OR CONSEQUENTIAL EFFECTS )F) WEIGHTING FACTOR D ( DEGREE OF ISOLATION )G) WEIGHTING FACTOR E ( TYPE OF COUNTRY )

THE EFFECTIVE COLLECTION AREA OF STRUCTURE ( Ac ) : Ac = LW + 2LH + 2WH + H^2WHERE

L = LENGTH OF THE STRUCTUREW = WIDTH OF THE STRUCTURH = HIGHT OF THE STRUCTUR

1- PROBABILITY OF LIGHTNING PROTECTION SYSTEM CALCULATIONACCORDING TO BS 6651

L

W

L

H

W

1 Contract No. PID N-075106

* THE PROBABLE NUMBER OF THE STRIKE TO THE STRUCTURE PER YEAR (SYMBOL P) ISTHE PRODUCT OF THE FLASH DENSITY AND THE COLLECTION AREAP = Ac * Ng * 10^(-6)WHEREP = THE PROBABLE NUMBER OF THE STRIKE TO THE STRUCTURE PER YEARNg = THE FLASH DENSITY( 10^ - 6 ) IS INCLUDED BECAUSE ( Ac ) IS IN METERS SQUARED WHEREAS ( Ng ) IS PER KILOMETER SQUARED.

* THE OVERALL WEIGHTING FACTOR = A * B * C * D * EWHERE

* WEIGHTING FACTOR A ( USE OF THE STRUCTURE )* WEIGHTING FACTOR B ( TYPE OF CONSTRUCTION )* WEIGHTING FACTOR C ( CONTENTS OR CONSEQUENTIAL EFFECTS)* WEIGHTING FACTOR D ( DEGREE OF ISOLATION )* WEIGHTING FACTOR E ( TYPE OF COUNTRY )

THE OVERALL RISK FACTOR = PROBABILITY OF BEING STRUCK * OVERALL WEIGHTING FACTOR

AS THE 10^(- 5) ( 1 IN 100,000 ) IS THE CRITERIA FOR DETERMINING WHEATHER PROTECTION IS NECESSARYIF THE RISK FACTOR IS 10^- 4 ( 1 IN 10,000 ) THE PROTECTION IS NECESSARY IF THE RISK FACTOR IS 10^- 6 ( 1 IN 1000,000 ) THE PROTECTION IS NOT NECESSARY

Office blocks, hotels,blocks of flats and other residential building other than lose included below.

but with metal roofing.

VALUE OF FACTOR AHousing and other building of comfortable size.Housing and other building of comfortable size with

USE TO WHICH STRUCTURE IS PUT0.3

1

metal roof.

WEIGHTING FACTOR A ( USE OF STRUCTURE )

Steel framed encased or rein force concrete with

out side aerial.Factories,work shop and laboratories.

Any building with thacthed roof.

Chimber framed or clad with any roof other than metal or othacth.

Strick,plain concrete or masonary with any roof other than metal or thacth.

TYPE OF CONSTRUCTION

Strick,plain concrete,masonary,timber framed

stations and stadium srtucture.

Steel framed encasedwith any roof otherthan metal* Rein force concrete with any roof other than metal .

1.7

VALUE OF FACTOR B

Schools,hospitals,children's and other homes.

0.20.4

2

Places of assembly,eg churches,hall,theaterexhibition,department,stores,post office,air ports

0.8

1

1.4

1.7

0.7

1.2

1.3

WEIGHTING FACTOR B (TYPE OF CONSTRUCTION)

2 Contract No. PID N-075106

Hill country 1

2

1.7Mountain country between 300m and 900m.

Flat country at any level. 0.3

Structere compeletly isolated or exceeding at least twice

Structure located in an area with few other structure or trees

WEIGHTING FACTOR D ( DEGREE OF ISOLATION )

TYPE OF COUNTRY VALUE OF FACTOR EWEIGHTING FACTOR E ( TYPE OF COUNTRY )

the height of surrouinding structure of trees.

or geater height,e.g. in a large town or forest.0.4

1

Power stations,gasi installation,telephone exchanges,radio stations.Key industrial plants ancient monuments and historic

1

1.3

valuable contents.

of the similar height.

Ordinary domestic or offices buildings,factories and work shops not cntaining valuable or specially susceptible contents.

0.3

(CONTENTS OR CONSEQUENTIAL EFFECTS)CONTENT OR CONSEQUENTIAL EFFECTS VALUE OF FACTOR C

WEIGHTING FACTOR C

0.8

buildings museums,artgalleries or other buildings with specially

School,hospitals,children's and other homes,

DEGREE OF ISOLATIONStructure located in a large area of structure or trees of the same

VALUE OF FACTOR D

Industrial and agriculatural buildings with speciallysusceptible* contents.

places of assembly.1.7

1.3Mountain country a bove 900m.

3 Contract No. PID N-075106

THE OVERALL RISK FACTOR = PROBABILITY OF BEING STRUCK * OVERALL WEIGHTING FACTOR

THUNDER DAYS PER YEAR = 50.1 TO 0.5 ( Ng )

L = 125 MW = 100 MH = 9 M

A = 1.7B = 0.4C = 1.7D = 1E = 0.3

Ac = 12500 + 2250 + 1800 + 254.34= 16804.34

P = 0.1 * 16804.34 * 0.000001 = 0.0016804

0.000583 =

AS THE 10^(- 5) ( 1 IN 100,000 ) IS THE CRITERIA FOR DETERMINING WHEATHER PROTECTION IS NECESSARY THE RISK FACTOR IS 10^- 4 ( 1 IN 10,000 ) THE PROTECTION IS NECESSARY

Number of Down Conductors. (According to art. 3-9.10)

the total perimeter of intermediate school = 625 mt

the No. of down conductor = 625 / 30 = 20.83333 = 21 down conductors (as shown in drawings)

FLASHES PER KM 2^ PER YEAR =

5.83 * 10 ^- 4OVERALL RISK FACTOR =

2- NUMBER OF DOWN CONDUCTORACCORDING TO NFPA 780

At least two down conductors shall be provided on any kind of structure, including steeples. Structures exceeding 250 ft (76 m) in perimeter shall have a down conductor for every 100 ft (30 m) of perimeter or fraction thereof. The total number of down conductors on structures having flat or gently sloping roofs shall be such that the average distance between all down conductors does not exceed 100 ft (30 m)

OVERALL WEIGHTING FACTOR = 0.3468

4 Contract No. PID N-075106