line tap
tee point
point on the multi-terminal electric line where portions leading, directly or indirectly, to three or more terminations are joined

In fact, with the available context, the original does not make much sense, because "descarga" in general, refers to an unwanted (and usually dangerous) event in the case of a power grid or circuit and if a "subestación" is involved it seems that medium or high voltage is involved, which means that a "descarga" entails arcing, and I do not see any normal "puntos de descarga para clientes libres" under such conditions.

I believe that the PROPER Spanish word here is "derivación" instead.

If I am right (that's why I say "tentative" above), that's the translation

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Note added at 2 hrs (2012-09-29 16:37:37 GMT)
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Ah, I forgot to add the source:

http://www.electropedia.org/iev/iev.nsf/display?openform&iev...

Point of discharge