Electrical Engineering

A sheath voltage limiter is an arrester that is used to clamp the voltage induced on the sheath of an underground high or medium voltage cable during a fault on the system. It is common practice when applying underground cable to only ground the cable sheath at one end of the cable, and leave the other end open. Leaving one ungrounded reduces circulating currents and losses during steady state operations. IEEE standard 575-1988 indicates that typically distribution arresters can be used for this function. IEC Application guide 60099-5 soon to be published offers a formula to use to determine the voltage rating of the arrester, but does not offer any guidance regarding the energy handling rating.

Between the two plates of a capacitor lies a medium with a specific dielectric constant. Placing a fish between these plates changes to dielectric constant. If the fish is fresh it will be strong and tight, thus possessing a higher dielectric constant. If the fish is older, it's dielectric constant, or body, will be weaker due to decay.

The unit of Current is Ampere. Abbreviation of Current is 'I' and that of Ampere is 'A'.

The unit of Resistance is ohm. Abbreviation of Resistance is 'R' and that of ohm is the symbol of 'omega' or the symbol of 'ohm'.

The reading of 14.65 on the 20k setting means 14.65K Ohms, and you probably need to go back and re-read the instructions for the meter. If, for instance, you used a 2M scale, if you had one, you would get 0.014, but the result would not be very accurate, as you need to pick a range that is close to maximum reading for that range.

It is normal procedure to start with a high range and come down in range as you approach an on scale reading.

8,000 watts = 8 kVA / (the power factor)

If the power factor is 1, then 8 kVa = 8,000 watts.

Assume you are talking about a boat and a water skier. Yes it will work. I have been towed by a 35 HP outboard. It depends on the skill of the skier to be able to get up quickly. It also depends on the skis and the drag they cause while being pulled from the water.

The figure 1.73 is the results of the square root of 3. 1.73 is used in three phase calculations. The number is also used on three phase four wire systems. To find the coil voltages of a wye system 1.73 is divided by the phase voltage. An example, three phase 480 wye system. 480/1.73 = 277 volts

Suppression of harmonics

When low inductive currents, such as magnetiing current of transformer, shunt reactors are interrupted by breaker, there is rapid deionization of contact space and may cause the current to be interrupted before its neutral zero. This phenomena is called current chopping. This generally occurs during opening small inductive currents by vacuum ckt brkr or air blast circuit brkr. The transient voltage having high restriking voltage RRRV appears across the contacts, unless the arc restrikes. If it restrikes a further, chop may occur or several chops may occur before the current is finally interrupted. Circuit breaker may fail to clear the current.

Circuit breaker in which dielectric strength of contact space grow at slow rat, the problem of current chopping is less severe as the gap breaks down and absorbs successive magnetic energy by restriking. Oil circuit breakers are therefore suitable to such application. This is the reason that sometimes in air blast circuit breakers resistance switching is adopted.

mw/mva=power factor

reactive power(Q)=I2XL or E2/XL where XL= REACTANCE

apparent power = square root of (MW2 + MVAR2 )

If the filament really was made from a material that has a negative temperature coefficient (as temperature increases, resistance decreases) then the decreasing resistance would cause more and more current to be taken as the lamp heated up and the temperature would get higher and higher in a runaway manner until either the power supply's breaker would trip or (more likely) the light bulb's filament would simply burn open.

In fact the filament has to be made from a material that has a positive temperature coefficient. (As temperature increases, resistance increases.) Then, as the bulb's temperature rises, its filament's increasing resistance causes less current to be taken than when it was cold. Quite quickly a stable "steady-state" temperature and "running" resistance is reached so that the bulb simply continues to give out a steady amount of light according to the current it is taking from the electricity supply.

+ and - voltages not connected to earth

There are 3 cases in C.B. paralleling:

1. Both Circuit breakers of same current ratings with same time settings.

2. Both circuit breakers of different current ratings

3. Both Circuit breakers of same current ratings with different time settings.

Case1: Any of the C.B.trips (depending on CT characteristics as no two CTs' can have same characteristics)

Case 2. The one with lower current ratings would trip first.

Case 3: The one with lower time settings would trip earlier.

Ground fault breakers monitor the current on the neutral wire flowing back to the neutral bar, a 4 to 6mA difference will trip the breaker, Shunt trip breakers on the other hand can be tripped remotely from some other type of switch or location. In industrial applications it is used to shut power off in an emergency situation, when access to an electrical panel may denied.

The most common voltage is 600 Volts DC. However, various voltages have been used. Most newly built light rail lines use 750 Volts DC. Some have used 1200 VDC or 1500 VDC. A few have used AC current.

There are two diode ratings. One is the forward current rating, and the other is the reverse breakdown voltage rating. The forward current rating is the maximum current that the diode can conduct before failing, and the reverse breakdown voltage rating is the maximum reverse voltage that can be applied before failing.

Of course, the reverse voltage rating has a different meaning in a zener diode, but this answer applies to ordinary diodes.

-Yes, the transformer draw current when secondary open. Because, the secondary of the

transformer is open, the primary draws only no-load current, which will have some copper loss.

Therefore, power is no longer transferred from primary to secondary in this approximation, and

negligible current goes through the secondary windings.Some current will flow through the primary as

well as secondary to magnetise the core to maintain the internet relaship between primary &secondary

winding as the winding consist of small resistance it is neglected practically hence the current will at at

no load also which is small in magnitude compared to the full load current.Thta is why we says that

transformer draws current when secondary is open.

water and gas pipes services

Earth wire is meant to protect the user in case there is earth fault in the device or circuit. Only earth wire alone is not sufficient. It needs to be provided with suitable circuit breaker that breaks the circuit automatically.

It is another way of saying that the circuit is capacitive reactive circuit. Look up the mnemonic ELI the ICE man. ELI, voltage leads the current in an inductive circuit. ICE, current leads the voltage in a capacitive circuit.

insulation resistance can be measured by means of megger, with built in d.c generator. It is measured between the conductor and the earth.

earth resistance can be measured by means of earth resistance tester. For good effective earthing, th earth resistance should be of order 0.5 ohm

The thermistor motor protection relays CM-MSE, CM-MSS and CM-MSN are used to control motors equipped with PTC temperature sensors. The PTC temperature sensors are incorporated in the motor windings to measure the motor heating. This enables direct control and evaluation of various operating conditions. Depending on the products also ATEX approvals for use in hazardous areas are available.

.The magnitude of the voltage and current of both the armature and shunt field coil.

To decrease the speed when the load is increasing then increase the shunt field current while decreasing the armature voltage or current.

To increase the speed while the load is increasing then increase the armature current while decreasing the shunt field current. The decreasing and increasing of these currents and voltages can be done by connecting a variable resistor in series or parallel with each of the armature and/or shunt field coil.