As stated previously, these systems work by including transformers at the input to each speaker and directly after the amplifier output. The transformers are used to convert the impedance of each speaker to higher value, and to convert the amplifier output impedance to a correspondingly high value. (See image below) In a 100 volt line system, speaker impedances (with transformers) may range from below 20 ohms to as high as 10,000 ohms or more. But you won't need to calculate the load impedance in ohms, because of how the high impedance approach works.

High impedance (100 volt and 70 volt (USA) line) systems have three major advantages over low-impedance systems:

1) System impedance-matching is made much easier — it is simply a matter of adding up speaker power taps and selecting an amplifier rated for at least that much power plus an allowance for headroom.

2) Line loss is greatly reduced, especially over long cable runs, resulting in better performance and reduced cost compared to long low impedance lines.

3) The amplifier output is electrically isolated from the speaker line by the output transformer, protecting the output stage against a grounded line and thus eliminating a potential source of system failure.