High Feed Impedances

We all know a cable has a loss. Anyone who has tried to do arc welding on a long extension lead will know how futile this can often be. But there is this belief that it is only external connections, such as extension leads, that has this loss. Not so. The internal wiring of the house/building, and even the feed to the house or building suffers loss. Granted, the loss on the feed and internal building wiring is far lower than that of a typical extension lead.

High feed impedances manifest themselves differently in buildings and houses. In buildings, where the overall load is relatively constant but fluctuates by time of day, the general issue is one of low voltage during peak demand and above normal voltage when the demand is low. In domestic houses the problem is more one of a highly fluctuating load which gives rise to flicker.

The common issue with both is the loss, as a result of voltage drop on the wiring, which adds to the power bill but never constructively does anything. Over time this starts costing more than what it would take to re-wire with higher rated cable.

Many supply companies find themselves in positions where they only test the feed impedance into a property and not beyond. This gives rise to continual complaints until someone is called in to measure the rest of the property. What is needed is to do a "differential measurement" to ascertain whether or not, and by how much, the internal wiring is contributing to the problem (remembering that according to the IEC868 flicker model the problem is exacerbated by Vē).

High feed impedances can also be masked by certain loads, or a number of them. Everything on a feed must be regarded as being in parallel with the feed, which could dramatically reduce the real impedance and thus mask the reason for flicker.

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