Safety Issues in House wiring

Prepared by SHAN.B.S, AEE, KSEB

1.???? Effect of high atmospheric Temperature on wiring insulation

During summer season when atmospheric temperature rises to high Celsius, concrete, due to its material property, tends to absorb more heat and retends it to a very longer period. This sustained higher heat, even though is much lesser than the temperature withstanding capacity of PVC insulation, is inadvertently hastening the withering of PVC insulation. This inturn leads to the loss of insulation property and increasing the chances of short circuit. Even before an actual short circuit occurance, leakage current flow between conductors increases, shooting up the power loss and increased consumption.

??????????? Solutions to these problems is to ensure is that the building is constructed complying ECBC (Energy conservation Building code) specifications, giving adequate importance for building orientation, passive cooling methods etc. Another solution is to introduce a new insulating material or developing a better PVC insulating material.

2.???? Effect of harmonics on wiring insulation – increasing chances of short circuit and wiring loss

Nowadays majority of the home appliances, like inverter fridge, inverter AC, computers, LED TVs etc, are sources of harmonics as these are using dc currents at its load side. The power rating of these equipments does not usually include harmonics power portion. Meters measure only the fundamental component of the currents while harmonics components are left unmeasured. For wiring purposes, generally, the load current is calculated based on the rated power only and accordingly the wiring size is selected. But due to the presence of high current harmonics, the magnitude of the actual power loss( I2R loss) is not only limited to fundamental current components but to the square of the total True RMS current. This produces excess heating in the wiring cable, affecting the insulation property of the wiring leading to the deterioration of insulation and eventual breakdown (short circuit).

Presently there is no cost effective solution for harmonics. So only alternative solution is to limit or to avoid the use of nonlinear equipments. Or to use higher rating cables but higher rated cables are much costlier. But as a major player in this field, KSEB can insist consumer product manufactures, to provide suitable passive filters within the equipments. But passive filters generally provide a low impendence path for the harmonics currents to flow, generally to earth or to neutral. This necessitates additional precautions in the wiring.

3.???? Effect of harmonics on neutral/earth current

Harmonics currents tend to flow in the neutral and earth wires. If the harmonic components are much higher, then the neutral current increases to such high proportions that the neutral current magnitude may become double the current flowing through phases. So it is technically wrong to assume that the neutral current shall be very less and the practice of using lower rated cables for neutral with respect to phase should be avoided. This means instead of using 3.5 core cable, either 4 core or 5 core cable should be used. It may be needed to use double run for neutral wires.

This also is a reason for the lowering in insulation levels and eventual short circuits.

4.???? Blasting of capacitors due to harmonics

Harmonics essentially consists of high frequency components. The capacitive reactance of capacitors is inversely proportional to frequency.

Thus for higher frequencies, capacitors offers a lower impedance path. So if capacitors are used in high harmonic circuits, it may get blasted due to very high currents passing through it. Conventional practice of recommending capacitors to be used with pumps and motors for power factor correction is technically wrong.

Solution for the above problem is to use(recommend) detuned filters. Detuned filters are essentially capacitors connected in series with an inductor designed with resonance frequency shifted away from the integer multiple of the harmonics levels. Eg 183hz, 318hz etc. Due to the presence of inductor whose reactance is proportional to frequency, the impendence offered by the detuned filter shall not fall below dangerous levels at frequency harmonics. This arrangement draws leading currents for fundamental currents and voltages offering good power factor correction but at the same time shall limit the magnitude of current flowing through the capacitor in the presence of high frequency components.

In majority of the cases, it is very difficult to catch sight of whether the capacitors got failed or not in the field. Each time a capacitor fails the magnitude of the current increases due to lagging load. This higher current magnitude increases the power loss and heating of cables.

?