Bus Bar Design

BUS-BAR DESIGN INTRODUCTION : Bus-bars are current carrying conductors. They are designed to carry certain normal

current continuously. In this note OBJECTIVE is

(i) to design the bus-bars on the basis of treated normal current and permissible temperature-rise,

(ii) to be able to check the suitability of a given bus-bar of the power panel.

BUS-BAR DESIGN : The bus-bars are designed to carry certain rated normal current continuously within the

permissible rise in temperature. The value of cross-section so obtained is verified for temperature rise under short time short-circuit current. This figure is further used from the protection point of view while considering the relay setting. In fact, the relay should be set such that under fault condition no damage is caused to these bus-bars.

Bus-bar conductors are separated on post insulators or strain insulators. These insulators

experience electrodynamics forces depending upon the current being handled. these forces produce bending moments on the insulators. These bending moments, therefore, become the ruling factor to decide the spacing between insulators (span).

The following factors are taken into consideration while designing a bus-bar :-

(i) Bus-bar material, (iii) Cross-section of conductors, (iii) Temperature rise during continuous normal current, (iv) Temperature rise during short circuit current of 1 sec. of 3 sec. (v) Design of insulator creepage distance and clearance, (vi) Distance between phase conductors, (vii) Force on insulators during peak short circuit current, (viii) Span of insulators support, and (ix) Enclosure design.

Copper and Aluminum are two preferred materials for electrical conductors. Considering

certain advantages of lightness, cost, resistance to corrosion, temperature coefficient of resistance etc. Aluminum has got an edge cover copper. As such, most of the bus-bar conductors now-a-days are of Aluminum (E 91-EWP Class).

Though a table has been prepared keeping certain parameter fixed (sec table-3) but to

select the cross-section of the bus-bar we may first have to find out the derating factors. Derating factors depend on :

(i) Allowable bar temperature over an ambient of 35 C, (ii) Enclosure of the bus-bar.

R. Sharma (BSNL Electrical Zone Patna)

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Derations for these two factors are obtained from the table 1 & 2 given below and

multiplied to get the overall derating factors.

TABLE - 1

Temperature rise Rating factors 1. Bar temperature rise 40 C over ambient of 35 C 0.88

2. Bar temperature rise 30 C over ambient of 35 C 0.75

TABLE-2

Enclosure

(Cross sectional area of bus-bar)/ (Total cross sectional area of Enclosure)

Derating factor

1. Outdoors < 1 % < 5 % < 10 %

0.95 0.90 0.85

2. Indoors where the enclosure itself is in a well - ventilated room

< 1 % < 5 % < 10 %

0.85 0.75 0.65

3. Indoors where the enclosure itself is poorly ventilated ant the room temperature is high.

< 1 % < 5 % < 10 %

0.65 0.60 0.50

After calculating the overall derating factor, the required current rating is obtained by

dividing the normal current rating by the overall derating factor. According to the requires current rating we note down the cross-section of the conductor. Suitability of this conductor depends upon the minimum size of the conductor required to withstand the short circuit current within permissible temperature rise for the given duration. Suppose the cross-sectional area obtained on the basis of derating factor is a Sq. cms. and obtained on the basis of derating factor is a Sq. cms. and that on the basis of short circuit withstand capacity be A Sq. cms. Obviously we are to select the cross-sectional area which is nearer to the greater of the two (i.e. between a and A). Now we are required to calculate the cross-sectional area of the conductor to withstand short circuit current for a given duration within permissible temperature rise. Basic assumption made here is that the heat produced due to a through short circuit current raises the temperature of the conductor and none escapes by convection or radiation in the very short time of the fault. By equating the heat produced and the heat stored it follows that for aluminium alloy busbar.

A = 4.276 T

log + 258 + 258

SC

10

θθ

F

I


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Where, A = Cross-sectional area of conductor (Sq. mm.) T = Duration of fault (Sec) θF = Final conductor temperature after the fault (C) θI = Initial conductor temperature before the fault (C) ISC = Short circuit current in kilo - Amp. The above formula can further be simplified as follows :- A = 13.88 ISC √T Sq. mm; upto 6.6 KV 13.7 ISC √T Sq. mm. ; for 11 KV

The insulators are selected by considering mechanical bending load occurring at that

instant of peak short circuit current. During short circuit the insulators supporting the bus-bars experience a bending force and unless these insulators have enough cantilever strength to withstand the dynamic force occurring during short circuits they will fail.

Let ISC be the short circuit current in KA & Ipeak be the peak short circuit current in KA,

Ipeak = 2.55 ISC If L be the length of the bus-bars in metres and r be the phase to phase spacing (in

metres) then total force F developed in Kgf is

F = 2.04 I2peak x L/r X 10-5 Kgf.

Force per metre length on bus-bars,

F = 2.04 X 10 I

r

-5(peak)2

Let H be the height of insulator (in metre), Sk the cantilever strength insulator and L, the

span of insulators then

F X H X L = Sk

(Factor of safety)

Bolting and clamping should be done by proper grades of steel or of alumimium alloy.

Typical properties for recommended material are Tensile strength = 71 Kg/Sq. mm. 0.2 % Proof stress (minimum) = 52 Kg/Sq. mm. Hardness = Brinell 200 minimum Rockwell B 93

minimum Also absolute minimum contact pressure should be taken as 28 Kg/Sq. cm. and a

desirable minimum should be 56 Kg/Sq. cms.


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The washers (Plain and spring) should also be used. They should be aluminized or

electrogalvanised. The following sizes are recommended :-

Bolt size (mm) Washer Dimension (mm) O.D. I.D. Tickness 9.53 25.4 11.11 2.34 12./7 28.58 14.239 9.64


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TABLE-3

Cross sectional area in Sq. mm

Size of AL bus bar (mm x mm)

Current Ratings Short circuit with stand capacity

Weight of bus bar/length (Kg./m)

Typical maximum d.c. resist. at 20 C

Typical reactanc Xa, at 305 mm spacing at 50 c/s

Single bar

Double bar

Tripple bar

2 Sec. in (KA)

1 Sec. (in KA)

161.29 25.4 X 6.35 355 705 970 8.22 11.6 0.435 194.35 236.22

241.30 38.1 X 6.35 520 1020 1350 12.30 17.38 0.652 129.56 215.22

322.58 50.8 X 6.35 670 1290 1705 16.43 23.24 0.871 97.18 199.14

403.23 63.5 X 6.35 810 1510 2000 20.54 29.05 1.089 77.82 186.35

483.87 76.2 X 6.35 960 1740 2310 24.65 34.86 1.306 64.80 175.85

645.16 101.6 X 6.35 123 2140 2800 32.87 46.48 1.742 48.59 159.45

806.45 127.0 X 6.35 1505 2510 3240 41.08 58.10 2.177 38.88 146.00

967.74 152.4 X 6.35 1780 2860 3680 49.30 69.72 2.613 32.38 134.51

484.12 50.8 X 9.53 830 1500 1970 24.66 34.88 1.307 64.80 195.86

726.19 76.2 X 9.53 1180 2050 2660 37.00 52.32 1.961 43.18 174.21

968.25 101.6 X 9.53 1495 2480 3150 49.33 69.76 2.614 32.38 157.48

1210.31 127.0 X 9.53 1860 2930 3660 61.66 87.20 3.268 26.21 144.36

1452.37 152.4 X 9.53 2120 3340 4080 74.00 104.64 3.921 21.59 134.18

1936.50 203.2 X 9.53 2750 4150 4900 98.65 139.52 5.228 16.21 117.45

967.74 76.2 x 12.7 1355 2240 2830 49.30 69.72 2.613 32.38 170.93

1290.32 101.6 x 12.7 1740 2720 3360 65.73 92.96 3.848 24.28 155.84

1612.90 127.0 x 12.7 2080 3120 3900 82.17 116.20 8.355 19.42 143.04

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Bus Bar Design