difference between earthed and unearthed cables

Introduction:

• In HT electrical distribution, the system can be earthed or unearthed. The selection of earthed/unearthed cable will depend on system. If distribution system is earthed then we have to use cable which is manufactured for earthed system. (Which the manufacturer specifies). If the system is unearthed then we need to use cable which is manufactured for unearthed system. The unearthed system requires high insulation level compared to earthed System.
• For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.

Earthed System:         

• Earlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.
• In Three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.
• Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.
• In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth. Examples: 1.9/3.3 KV, 3.8/6.6 KV system

 Unearthed System:

• Today generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance. Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times. If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.
• In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it. Examples : 6.6/6.6 KV, 3.3/3.3 KV system.
• Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is √3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture. So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.

 Nomenclature:

• In simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same. The size of cable will depend on current rating and voltage level.
• Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage.
• Earthed system has insulation grade of KV / 1.75 x KV.
• For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV.
• Unearthed system has insulation grade of KV / KV.
• For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV.
• 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.

Thumb Rule:

• As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e. 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system. because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kv+6.6kv = 11kv
• For transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables are not economical but insulation will be good.
• Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications.  Such a case is similar to an unearthed application but with earthed shield (some times called solid bonding).  In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables. But it is always better to mention the type of system earthing in the cable specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.

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What is the difference between a battery and a charged capacitor?

  main difference: "Capacitor is a device that is used to store an electric charge. It is basically an arrangement of conductors. A capacitor stores electrical energy directly due to the creation of an electrostatic field which is created between two metal "plates". Battery is a device that consists of electrochemical cells that convert stored chemical energy into electrical energy."

 

CAPACITOR

 A capacitor consists of two conductors which are separated by a dielectric medium. It stores electrical charge and is capable of discharging it whenever required. It blocks Direct Current (DC) and allows Alternating Current (AC) to pass through it. Therefore, they are widely used to extract AC components from DC + AC components. It is basically an arrangement of conductors. Capacitors are widely used in electrical circuits as energy-storage devices.

Capacitors are capable of storing electrical energy directly as an electrostatic field is created between two metal "plates". A capacitor is generally constructed using two metal plates or metal foils separated by an insulator called a dielectric material. This insulator prevents charges to move across the plates. Any non-conducting substance can be used as a dielectric material. However, porcelain, mylar, teflon, mica, cellulose are generally preferred. Thus, a capacitor can be defined as an arrangement of two conducting surfaces separated by a dielectric medium. One of the common applications of a capacitor can be found in a flash camera. A capacitor in a flash camera charges up and stores the electric charge. It releases the charge to a light bulb, and thus flash is created whenever the picture is taken.

BATTERY

 Battery is a device that consists of electrochemical cells that convert stored chemical energy into electrical energy. The battery was created in 1799. Originally, it was called the Voltaic battery. The first battery was created by using copper and zinc rings. The set up was placed in an acid solution that is known as the electrolyte. A battery has evolved since then. However, the basic mechanism remains the same till now. A battery consists of two ends namely cathode and anode. In a battery, the positive terminal is referred to as a cathode. On the other hand, anode is the negative terminal. The two terminals are connected in order to form a circuit. Electrons move through the wire and electricity is produced.


A modern battery zinc carbon is packaged in zinc that serves as a container as well as an anode. A carbon rod is used as a positive terminal. An electrolyte is used as a paste of zinc chloride and ammonium chloride dissolved in water. The electrons that move from anode are collected by the carbon and then are returned to cathode portion of the battery. A battery can also be explained as an assembly comprising of 2 or more cells connected in series to generate a voltage which is equivalent to the sum of the voltages of the cells.

Comparison between Capacitor and Battery:

	Capacitor	Battery
Definition	In capacitors, energy is stored in their electric field.	Battery is a device that consists of electrochemical cells that convert stored chemical energy into electrical energy.
Types	Three major types of capacitors are ceramic, electrolytic, and tantalum : Electrolytic capacitors - They resemble small cylinders and range in value from 1 µF to several Farads. Ceramic capacitors - They are quite smaller in size and value, ranging from a few Pico Farads to 1 µF. Tantalum capacitors – They are quite similar in size to ceramic. However, they can hold more charge, up to several hundred µF. They tend to be accurate and stable.	Alkaline battery Lead-acid battery Lithium battery Lithium-ion battery Nickel-cadmium or NiCad battery Zinc-carbon battery or standard carbon battery
Uses	High Voltage Electrolytic used in power supplies. Axial Electrolytic - lower voltage smaller size for general purpose where large capacitance values are needed. High Voltage disk ceramic - small size and capacitance value, excellent tolerance characteristics. Metalised Polypropylene; small size for values up to around 2µF good reliability. Sub−miniature Multi layer ceramic chip (surface mount) capacitor. Relatively high capacitance for size achieved by multiple layers. Effectively several capacitors in parallel.	Wet-cell - Lead acid batteries to power vehicles; also used by industry. Dry-cell non-rechargeable - these are the most common types of household battery. Dry-cell rechargeable batteries - these are widely used in power tools, cordless appliances, mobile phones etc.
Charging & Discharging	Quickly	Comparatively slow
Size (Equivalent charge)	Comparatively large	Comparatively small
Composition	Thin metal sheets held very close together but separated by an insulator	Chemicals and metals

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