只有一个能量为零的信号，即信号$x(t)=0$，这不是一个非常有趣的信号。所有能量信号，即所有具有有限能量的信号，都具有零功率。这就是为什么将它们称为“功率信号”是没有意义的。所以“功率信号”都是具有有限但非零功率的信号。

“有限”确实会引起混淆。我们也将它用作“一组有限的值”：$\{2,7,13,17\}$. From a mathematical point-of-view, finite means "a non infinite scalar value", and zero would be allowed. Bounded periodic functions like sines are typically with "infinite energy": they are not $L_2$-norm integrable, but they have finite power. A function with zero-energy could be not-zero (on a set of null-measure), but treated equivalently as the null function, as in a class of equivalence.

In DSP though, it is traditional to rule out zero-energy and zero-power signals: an energy signal has zero average power, and a power signal has infinite energy. There are more details here: for the continuous How to check if a signal is power signal or energy signal? and the discrete case Power or Energy Signal?