A line concentrator is a piece of relay equipment that uses one common T&R, plus one control lead per button, (like an "A" lead, but nastier) plus a lamp pair per button, between the set and the equipment. The control lead operates a relay that applies T&R of the associated control circuit's talk path onto the common T&R to the station, and lights the line lamp.
They were used in installations where many multi-line Call Directors or answering turrets were used in a large "square" wiring format, in order to save on copper cabling.
In the list of nightmarish repair assignments, working on a concentrator is up there near the top. Not quite as bad as a crossbar ACD (automatic call distributor.)
Mathematically, the conventional installation would use, as a best case, 2.5X pairs plus a ground, where X is the number of CO lines.
The concentrator uses 1 pair plus 1.5X pairs where X is the number of CO lines.
A 29 button Call Director would therefore use, as a minimum, 72.5 pairs plus a ground.
The same 29 lines on a concentrated tel set would require 43.5 pairs plus a ground.
You can see the how the copper savings would acrue in a large system that had sets with 100 lines or more.
The reason that lamp grounds cannot generally be concentrated is that with long runs of cable, and many lamps lit at the same time, you get a condition known as "cross-modulation" the effect of which is lit lamps will go dim or almost out when other lamps turn on. That makes lit steady lamps look like they're flashing, but in an opposite cadence to the ones that are really flashing. It makes operation of the set difficult. Some systems allow a few lamps per lamp ground lead, to save a few pairs. You can see this condition on the highway at night, when the driver of an old Peugeot turns on the left turn signal, and the rear running light goes out. They were notorious for having a bad ground connection in their tail light assemblies.
But I digress.
