The short answer is the breakdown voltage of the insulation. The insulation in the cable keeps the wires from shorting out.High-voltage and low-voltage are relative terms. The power service to your home, up on the poles, typically has no insulation at all, but the individual wires do not short out because the wires are separated air between them. The voltage on the pole, above the local transformer, is typically 13,000 volts. The power company calls these lines “low” voltage because the distribution lines (on the tall steel towers) can be from 250,000 volts to 500,000 volts.Most folks think of the power wiring in their house as “high voltage” and other things (stereo speakers, lawn lights, door bells, etc) as “low voltage”. Typical house wiring cable has insulation rated at 600V. The stereo and lawn lights’ cables’ insulation is not rated that high.
Courtesy:-http://www.electrotechnik.net/20...
I want to know all the IEC standards for low voltage switchgear?
The only IEC Standard for Low Voltage Switchgear is IEC 60947 - Specification for LV Switchgear and it is available in 7 parts. If you really want to gain practical working knowledge on LV switchgear attend a 4-days training programme at our Switchgear Training Centre. You may contact me at "SIVAKUMARK@LNTEBG.COM" for further details. I assure that after this training programme you will gain what all you wnated to know about LV switchgear.
High voltage current & cable size calculation?
hi, i am working as electrical engineer for building service division, i know how to calculate the rated current with the known power for low voltage. please someone explain me, 1.how to calculate the rated current for the know power for the voltages like 3.3 kv ,11kv,22kv? can u make one model calculation and send to me? 2. how to find the transformer rating for high voltage& capacity of the transformer? 3. how to select the suitable cable, please forward me cable selection chart/catalog. Thank u.
The operating voltage is the voltage at which any device or machine is intended to be used.The rated voltage is the maximum voltage that the device can bear without damaging.The operating voltage must of course be lower than the rated voltage. This gives a safety margin i.e. the difference between operating and rated voltage. This safety margin must be large enough to take account of variations or fluctuations in the nominal voltage on the power lines.
"The percentage impedance of a transformer is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage." "It is also the percentage of the normal terminal voltage at one side required to circulate full-load current under short circuit conditions on other side." The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions.It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is: 2 MVA x 100/5 = 40 MVA and from this figure, the equivalent primary and secondary fault currents can be calculated. A transformer with a lower impedance will lead to a higher fault level (and vice versa).The figure calculated above is a maximum. In practice, the actual fault level will be reduced by the source impedance, the impedance of cables and overhead lines between the transformer and the fault, and the fault impedance itself.
A Milliken conductor is laid up in segments as shown in the drawing (reference 1) with each segment being lightly insulated from adjacent segments (reference 2). The technique is used on large cross section conductors, and is usually applied to high voltage cables.This insulation considerably reduces eddy currents in the cable, which reduces skin effect. This effectively increases the carrying capacity over a cable of the same cross sectional area that does not use the Milliken effect. For a cable of 3000 sqmm CSA, the increase in capacity can be greater than 25%.Although the technique may be applied to low voltage cables, it is expensive to manufacture so is only used where the cost is offset by the reduced insulation cost of using one cable rather than two or more parallel smaller conductors.
In most cases the reason for separating different voltage conductors in construction is to prevent induced current from the high voltage conductor interfering with the low voltage conductor if there is a large difference (120 or 277v Ac lines running parallel with 12v DC fire alarm circuits or data cabling for several feet will cause numerous issues with operation, and is difficult to locate & fix if it becomes a problem, the field generated around the AC wire will induce small amounts of electricity into conductive substances nearby, eg. another copper wire or random pieces of steel, several types of low voltage wire have foil shields wrapped around them under the jackets to prevent this from happening) - this is mostly an issue because in many cases these wires are not carrying power for a load but are instead being used to transmit information via small changes in observed voltage at the termination point