POWERMAT

UNIT III — LOW VOLTAGE SYSTEM

SOME BOOKS & STANDARDS

SHORT CIRCUIT

POWERMAT Some relevant Books & Standards on Low-Voltage Switchgear Systems :

• The J & P Switchgear Book. • Siemens Electrical Engineering Handbook.

• NFF 61-016 : Insulators for connections and insulating supports of switchgears.

• IEC 60664-1 : Insulation coordination for equipment with in Low-Voltage Systems.

• IEC 60243-1 : Electrical strength of insulating materials-Test methods.

• IEC 60947-1 : Low-voltage switchgear and control gear.

• IEC 865-1 : Short-circuit currents-Calculation of effects.

POWERMAT

Some guidelines on low voltage A.C. three phase system short circuit test from J&P Switchgear handbook. Minimum Creepage Distance Required

Rated Voltage Upto and including

( kV ) Minimum Creepage Distance in Air, Phase to Earth

Inch. mm 0.415 3/4 19 0.6 1 26 3.3 2 51 6.6 3 ½ 89 11 5 127 15 6 153 22 8 204 33 12 304

Notes: 1) Based on Table 11 of BS 159 :1957 2) mm dimensions rounded up to whole figures 3) Increase by at least 50% when applied to insulation between Phases.

Selection of Bus Bar Material

Aluminium and Copper are the two materials are commonly used as Bus Bar. There are some properties of Aluminium and Copper.

Properties

Aluminium Copper

Specific weight (gm/cc) 2.7 8.9 Melting point ( °C) 660 1083 Co-efficient of linear Expansion ( α / °) 0.000023 0.000017 Hardness (B.H.N) 21 45 Tensile Strength (kgf/mm2) 25 40 Thermal Conductivity (at 20 °C) kCal X 10-3 / deg.Cm.S 0.570 0.941

The above table shows that Aluminium is lighter than Copper. But, Aluminium conductors must have about 60% greater

sectional area than Copper for passing same current rating. So, Copper is prefer where space is a valuable factor. In polluted atmosphere Aluminium is more preferable because its oxide film is continuous, very hard, adherent, and stable. Thus sealing the metal from further oxidation.

Sometimes a alloy of Aluminium (E91E) is also be used. It has almost same strength as Copper. But, it has current

rating of 3% lower than pure Aluminium.

For rough calculation the A.C. carrying capacity for Aluminium is 1A/ mm2 and for Copper is 1.7A/ mm2 . The Calculation based on approximations may be wrong in the case of multiple bars particularly carrying high current. So, it would be preferable to follow the following chart for a range of metric dimension rectangular Aluminium bars (assumed that the bars are mounted in still but unconfined air)

Size of Bus Bar (mm

x mm) Sectional Area

(mm2) 1 Bar 2 Bars 3 Bars 4 Bars 5 Bars 6 Bars

6 x 25 150 364 640 900 1120 1315 1485 6 x 40 240 545 935 1310 1630 1900 2130 6 x 50 300 660 1130 1580 1950 2255 2505 6 x 60 360 782 1350 1870 2200 2630 2885 6 x 80 480 995 1700 2310 2745 3070 3330 6 x 100 600 1215 2090 2770 3190 3490 3745 6 x 125 750 1467 2501 3289 3760 4115 4404 6 x 150 900 1726 2929 3833 4360 4765 5073

10 x 50 500 870 1500 2060 2505 2850 3100 10 x 80 800 1250 2215 2940 3355 3675 3950 10 x 100 1000 1565 2650 3465 3940 4315 4635 10 x 125 1250 1873 3161 4155 4727 5145 5456 10 x 150 1500 2185 3718 4880 5543 5970 6288 10 x 200 2000 2795 4750 6160 5932

POWERMAT

Size of Bus Bar (mm

x mm) Sectional Area

(mm2) 1 Bar 2 Bars 3 Bars 4 Bars 5 Bars 6 Bars

12 x 80 960 1344 2222 2807 3214 12 x 100 1200 1618 2530 3125 3627 12 x 125 1500 1934 2920 3627 4232 12 x 150 1800 2251 3255 4092 4743 12 x 200 2400 2846 4139 4929 5720 12 x 250 3000 3385 4650 5580 6371

Notes: 1) Based on bars mounted with the long side vertical and a gap between the bars equal to the bar thickness. 2) Current Rating for E 91E bars are about 3% lower.

A.C. Rating for rectangular Copper Bars (assumed that the bars are mounted in still but unconfined air)

Size of Bus Bar (mm x mm) Sectional Area (mm2) Approximate Rating (A)

6 x 25 150 450 6 x 40 240 660 6 x 50 300 792 6 x 60 360 915 6 x 80 480 1162

6 x 100 600 1395 6 x 125 750 1671 6 x 150 900 1915

10 x 50 500 1060 10 x 80 800 1525 10 x 100 1000 1800 10 x 125 1250 2150 10 x 150 1500 2456 10 x 200 2000 3140

12 x 80 960 1332 12 x 100 1200 1665 12 x 125 1500 1980 12 x 150 1800 2235 12 x 200 2400 2820 12 x 250 3000 3375

Multiplying Factors for A.C. Copper Bars

Multiplying Factors Total area of Cross Section ( mm2 ) 2 Bars 3 Bars 4 Bars

500 1.78 2.45 3.13 1000 1.72 2.36 3.00 1500 1.65 2.24 2.84 2000 1.60 2.16 2.70 2500 1.55 2.10 2.60 3000 1.52 2.02 2.52 3500 1.48 1.98 2.48 4000 1.44 1.96 2.45

POWERMAT

During Short Circuit Test a high density current passes through the Bus Bar for a very short time. At this moment temperature rises and Electro-magnetic forces are generated. The Electro-magnetic forces of attraction and repulsion are set between adjacent conductors of different polarity or phases. To with the temp. rise and the Electro – magnetic forces there should be proper sectional area of Bus Bar , Bus Bar Material and Supporting Structure.

For better safety in case of temperature rise, if we consider 100°C rise for the initial temperature of 70°C , then we can

use the formula (I/ A) x √ t = 0.122 for Copper and (I/ A) x √ t = 0.0775 for Aluminium. Where , I = Short Circuit Current (k A) A = Sectional Area of Conductor (mm2 ) t = Time (sec.) For example, if during Short Circuit Test 50 k A current is passing through 1 sec. Then the minimum cross-sectional area

of Copper should be A = (50 x √ 1 ) / 0.122 = 410 mm2 and for Aluminium it would be 645 mm2. Temperature Factor

Normally Bus Bar System given are at 35°C while in practice the temperature is about 50°C . So this variation leads to certain deration in the carrying capacity of Bus Bar. So to arrive the current conductivity of the

particular Bus Bar a suitable deration factor has to be applied. The formula for getting the deration factor =( T 2 / T1 ) 0.588

Where T1= Temperature at the site of installation (i.e. 50°C ) T2= Ambient Temperature of the Bus Bar manufacture (i.e. 35°C )

Proximity Factor

Some deration in the current carrying capacity happens due to gap between the two phases . The following table should

be taken into account the proximity effect.

Distance between Phases The Deration Factor 3 x W 0.82 4 x W 0.89 5 x W 0.95 6 x W 0.99

> 6 x W 1.00 Enclosure Factor

The size of Enclosure also create some deration in the carrying current capacity. To derive the Enclosure Factor we use

the formula=(Area of Cross Section of Bus Bar / Area of Enclosure ) x 100% . After calculating % value , we can calculate deration due to Enclosure Factor from the below mentioned table.

Enclosure Cross-Sectional Area of Bus Bar / Cross sectional Area of Enclosure

Deration Factor

< 1 % 0.95 5 % 0.90

Outdoor

10 % 0.85 < 1 % 0.85 5 % 0.75

Indoor where the Enclosure itself is in a well ventilated room

10 % 0.65 < 1 % 0.65 5 % 0.60

Indoor where the Enclosure itself is poorly ventilated and the room

Temp. is high 10 % 0.50 The Final Capacity of the Bus Bars : Actual Capacity of the Bus Bar x Deration due to temperature x Deration due to Proximity x Deration due to Enclosure.

POWERMAT For POWERMAT Bus Bar Support, the main concern is to withstand the forces develop during Short Circuit Test. The maximum force develop at the time of Short Circuit :

16 x I2 x 10-4 x K

Fm = --------------------------- N/m S Where, I = The R.M.S. value of the current in Amp. K= Space factor for Rectangular Bars. S = Spacing between the conductors center (mm)

The value of Space Factor (K) is determined from the Graph firstly by determining the ratio (S-a) / (a+b) and then from the value on the base scale projecting upwards to the point of intersection with the curve appropriate to the ratio a/b. (a=thickness of cross sectional area of Bus Bar & b= Width of Bus Bar)