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Many time we require a quick calculation as to how much current a 3 phase motor of 15 HP draws... Its easy once you have a calculator in hand and also if you know the formula. But many times you many not remember formula correctly or dont have a calculator in Hand.


Here are few thumb rules you may follow if you cant remember formula correctly or dont hace a calculator in hand:



Cable Capacity

· For Cu Wire Current Capacity (Up to 30 Sq.mm) = 6X Size of Wire in Sq.mm
Ex. For 2.5 Sq.mm = 6×2.5 = 15 Amp, For 1 Sq.mm = 6×1 = 6 Amp, For 1.5 Sq.mm = 6×1.5 = 9 Amp

· For Cable Current Capacity = 4X Size of Cable in Sq.mm, Ex. For 2.5 Sq.mm = 4×2.5 = 9 Amp.

· Nomenclature for cable Rating = Uo/U

· where Uo = Phase-Ground Voltage, U = Phase-Phase Voltage, Um = Highest Permissible Voltage
1HP = 0.746
KW1KWH = 3.6 MJ

Current Capacity of Equipment

· 1 Phase Motor draws Current = 7Amp per HP.

· 3 Phase Motor draws Current = 1.25Amp per HP.

· Full Load Current of 3 Phase Motor = HPx1.5

· Full Load Current of 1 Phase Motor = HPx6

· No Load Current of 3 Phase Motor = 30% of FLC

· KW Rating of Motor = HPx0.75

· Full Load Current of equipment = 1.39xKVA (for 3 Phase 415Volt)

· Full Load Current of equipment = 1.74xKw (for 3 Phase 415Volt)

 

Earthing Resistance

· Earthing Resistance for Single Pit = 5Ω, Earthing Grid = 0.5Ω

· As per NEC 1985 Earthing Resistance should be < 5Ω.

 

Minimum Bending Radius

· Minimum Bending Radius for LT Power Cable = 12 x Dia of Cable.

· Minimum Bending Radius for HT Power Cable = 20 x Dia of Cable.

· Minimum Bending Radius for Control Cable = 10 x Dia of Cable.

Transformer

· Current Rating of Transformer = KVA x 1.4

· Short Circuit Current of T.C /Generator = Current Rating / % Impedance

· No Load Current of Transformer =< 2% of Transformer Rated current

· 4No. earth pits per transformer (2No. for body and 2No. for neutral earthing),

 

 

Diesel Generator

· Diesel Generator Set Produces = 3.87 Units (KWH) in 1 Litter of Diesel.

· Requirement Area of Diesel Generator = for 25KW to 48KW = 56 Sq.meter, 100KW = 65 Sq.meter.

· DG greater 1000kVA can either be in a canopy or skid mounted in an acoustically treated room

 

Electricity is Good Servant but a bad master.
We can not imagine our life without electricity. Be it comfort cooling or heating or light or all sorts of machines, electricity serves us at its best.
But a small mistake upon electricity ...it do not tolerate. Its a very bad master. You dont have ignorance at your rescue to avoid any punishment. Punishment can be a mild shock to a major fire or burns or even death.
Now, what to do to check electricity?? What to do to avoid electricity to be on enemy side??
Here I discuss few mistakes people do due to ignorance and end up paying heavy costs
1. Improper design - You cant run heavy current in a small conductor. Insulation may break under certain conditions and create dangerous path for current. There are certain principles electricity behaves and if properly taken care, risk of electrical accidents can be significantly reduced.
2. Casual Choice of materials : When we buy a jeans or T shirts, we look for two parameters, it should look great and it should also have a good life. We need a friends company who help us in buying. But when we buy Electrical accessories, we sometimes depend upon our electrician or a contractor. Believe it, this practice is most dangerous because they have their own interests. All the companies do marketing for their products and tempt these electricians or contractors. Worse the quality, better are margins. Better margins means good incentives to the electricians and contractors. And definitely, good incentives keep them moving, making efforts and victimize us. Depend upon the advice of professionals, if possible.
3. Ensure required protection : The earthing is essential in all installtions but at most places, it is either not considered necessary or installed inappropriately. The salt used to eat the metal of earth electrode or terminal joint and become sensitive to water. The Gel earthing or Chemical earthings used are solution if the back fill compound i.e. the conditioned mineral soil like compound enriched with chemicals is good. Again we should depend upon qualified Electrical engineer as there are many fake products available on this and results are even worse than conventional earthing for these.
Disclaimer : 3E Solutions also manufacture Chemical Earthings ( Gel Earthings ) in Jaipur.
Same ways the Earth Leakage Circuit breakers ( ELCB or RCCB ) and Surge arresters are still not considered which reduces the risk to minimal. Lightning protection is also essential but often ignored.
4. Qualified technician for execution of task necessary.
5. If possible, a dependable third party inspection agency be hired for periodic checking of the quality of work and materials.
6. The results of checking of insulation resistance and earthing resistance be recorded and kept ready for future use. These must also be checked every year and results compared from history for any degradation.
( Author has worked in state electricity board and ABB and has 25 years of experience in industries)

How to select Earthing Electrodes

1. Life : The life depends upon the pipe, its thickness and quality of hot dip galvanizing The best pipes are Tata as they have best life due to there best Hot Dip Galvanizing Plant. Alternatively, the GI pipes should be used with minimum thickness 2.0 mm.
2. Termination : Should be 30 X 6 mm minimum in GI or Copper. GI Terminations are used where Earthing is to be connected to Equipment or board or panel or machine by GI strip or GI wire. Copper termination is used in case connection is made by Copper Strip 25X3 mm or higher or 8 SWG ( also called 8 No ) which is 4 mm dia.
3. Strip in Pipe or Pipe in Pipe : This increase the life of earthing by always providing alternate path to fault current to bottom of electrode even when the main electrode corrodes. Pipe in Pipe is stronger and has better life but in special cases single strip extended upto termination is manufactured on order which is expensive but most stable under very heavy fault currents above 25000 Amp.
4. Back fill compound : The back fill compound should not be corrosive and in this way sulpher content not above 2%. The conductivity should be as much as 2 mS/cm or better to get better results.

Difference Between Grounding and Earthing

It confuses many times when people speak Grounding or Earthing or Bonding. Let us discuss about it..

Earthing : The objective of earthing is to minimize the danger of electrocution, fire due to leakage of current through undesired path and to ensure that the potential of a current carrying conductor does not rise with respect to the earth than its predicted range.

Earthing and Grounding is the same terms used for connecting equipment or terminal. Grounding is the commonly word used for earthing in the North American standards like IEEE, NEC, ANSI and UL etc while, Earthing is used in European, Common wealth countries and British standards like IEC etc. India earthing word is more common.

The word Bonding used for jointing two wires as well as conductors, pipes or appliances together. Bonding is known as connecting the metallic parts of different machines which is not considered to be carrying electric current during normal operation of the machines to bring them at the same level of voltage.

Need of Earthing or Grounding. Why Earthing is Important?

When the metallic part of electrical appliances (parts that can conduct or allow passage of electric current) comes in contact with a live wire, maybe due to failure of installations or failure in cable insulation, the metal become charged and static charge accumulates on it. If a person touches such a charged metal, the result is a severe shock.

To avoid such instances, the power supply systems and parts of appliances have to be earthed so as to transfer the charge directly to the earth.

Below are the basic needs of Earthing.

To protect human lives as well as provide safety to electrical devices and appliances from leakage current.
To keep voltage as constant in the healthy phase (If fault occurs on any one phase).
To Protect Electric system and buildings form lightning.
To serve as a return conductor in electric traction system and communication.
To avoid the risk of fire in electrical installation systems.

Everyone knows that monsoon bring happiness in our lives and we wait for this season desperately for picnics with family and outings to a watersides.
But there is a hazard in monsoon too... The wetness everywhere give rise to old and new houses who have not taken due care of electrical safety of their houses. Its true that maximum accidents due to electrocution happens in this season only major reason to that is loose wiring or cuts in wire length or unsafe electrical joints. An electrical joint is not safe if it is accessible to children or pets but even such safe joints can become unsafe in monsoon.

Another reason for electrical accidents is equipments becoming shocking!!! Many times our washing machines or microwave or geysers start giving electrical shocks in monsoon season. Actually they donot give shocks if the house has a safe earthing connected to meter or MCB boxes and the earth wire runs the whole house to sockets and in turn to all such equipments. But many times, in the effort to start dwelling in house soon we ignore few basic safety tips.... The house seems ok from all sides i.e. architecture or civil or basic electrical or look or comfort and we delay earthing for some later date that donot coe till a monor shock is observed...

Its true that more than 25% of residential buildings donot have an earthing connection... another 50% have an old salt charcoal earthing that has almost lost life. It has life as long as we water it and then for few days its ok. But accident can not be safely avoided by such earthing. Good thing is, such earthing works in monsoon season and mostly fails in summer. At least 10-15% purchase good quality 3 pin sockets, connect them all the way to a common earthing point in MCB box near meter but dont get an earthing installed and in the end for a mere 5000/ to 10000/ expenditure there huge invenstment become useless...