# Electrical Thumb Rules (Part-2)

August 6, 2013 16 Comments

**Useful Equations:**

- For Sinusoidal Current :
**Form Factor =**RMS Value/Average Value=1.11 - For Sinusoidal Current :
**Peak Factor =**Max Value/RMS Value =1.414 **Average Value of Sinusoidal Current(Iav**)=0.637xIm (Im= Max.Value)**RMS Value of Sinusoidal Current(Irms)=**0.707xIm (Im= Max.Value)**A.C Current=**D.C Current/0.636.**Phase Difference between Phase=**360/ No of Phase (1 Phase=230/1=360°,2Phase=360/2=180°)**Short Circuit Level of Cable in KA (Isc)=(**0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec)**Max.Cross Section Area of Earthing Strip(mm2)**=√(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K- K=Material Factor, K for Cu=159, K for Alu=105, K for steel=58 , K for GI=80
**Most Economical Voltage at given Distance=**5.5x√ ((km/1.6)+(kw/100))**Cable Voltage Drop(%)=(**1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength (km)x100)/(Volt(L-L)x Cable Run.**Spacing of Conductor in Transmission Line (mm) =**500 + 18x (P-P Volt) + (2x (Span in Length)/50).**Protection radius of Lighting****Arrestor**= √hx (2D-h) + (2D+L). Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time).**Size of Lighting Arrestor=**1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).**Maximum Voltage of the System=**1.1xRated Voltage (Ex. 66KV=1.1×66=72.6KV)**Load Factor=**Average Power/Peak Power- If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
- If Load Factor is Low (0 or 25%) =you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.
**Demand Factor=**Maximum Demand / Total Connected Load (Demand Factor <1)- Demand factor should be applied for Group Load
**Diversity Factor=**Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)- Diversity factor should be consider for individual Load
**Plant Factor(Plant Capacity)=**Average Load / Capacity of Plant**Fusing Factor=**Minimum Fusing Current / Current Rating (Fusing Factor>1).**Voltage Variation(1 to 1.5%)=**((Average Voltage-Min Voltage)x100)/Average Voltage- Ex: 462V, 463V, 455V, Voltage Variation= ((460-455) x100)/455=1.1%.
**Current Variation(10%)=**((Average Current-Min Current)x100)/Average Current- Ex:30A,35A,30A, Current Variation=((35-31.7)x100)/31.7=10.4%
**Fault Level at TC Secondary**=TC (VA) x100 / Transformer Secondary (V) x Impedance (%)**Motor Full Load Current=**Kw /1.732xKVxP.FxEfficiency

Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec)……….what and why is 0.094?.. thanks in advance

i so grateful the information,PLEASE I WILL like to know how to calculate surge circuit breaker

One small issue in our plant to find the neutral line

all phases are giving correct voltge level with phase to neutral also if we give supply to appliance it getting off 4 days we are tried to find the problem finally we got the answer that neutral line is disconnected earth line acting neutral line

in this case how can judge and find the neutral line fault

Thanks for sharing the knowledge. Keep sending.

Regards

Vipul Kadakia

Manager ( Dist – NZ).

Torrent Power Ltd

9276055878

Very Informative………. Keep on going guys… very good job…

Sir, can you explain how A.C. =D.C./0.636 .

11kv system na current nu KWh,KW,MW,MWh karvu hoy to ? 66 kv system na current nu KWh,KW,MW,MWh karvu hoy to ? Same as. 132kv,220kv,400kv,765kv

JE Getco

as a site engineer , it is great collection of formulae for me, bt regarding (spacing of conductor in transmission line how the value (500 ,18,50)in *500+18*p-pv+2* span length /50 came from?

What is P in above equation? (P-PV)

Phase to Phase Voltage !!!!

(Line Voltage)

what is thumb rule for voltage drop?

sir ,please answer it

Dear Sir,

I have small doubt about your earth bar calculation, this is which standard because i am getting correct sq.mm that only ask. In case of any customer asking means how to i explain.

Mohan K

Hw is measured by dyn1 nd dyn11 vector connection in transformer. plz send it sirji…..

Thanks sir

what are all the standard fault level as per IS?

Dear Jignesh sir,

In power transformers can we get value of Inductor (L) .