A current-collecting plate of a PEM fuel cell has a reaction gas flow-field comprising a plurality of flow-channels each having an inlet leg communicating with a fuel supply manifold, an exit leg communicating with a fuel exhaust manifold, and a branched midsection intermediate the inlet and exit legs. The cholesteric compound layer transmits elliptically polarized light having an amount of absorbance which is related to the angular rotation of the polarization. The invention further relates to a power plant installation for the execution of such a method. Hereto, the coating agent is extensible in wet state and comprises thereto a wet-extensible material that has a wet-elongation of at least 400% or even at least 500%, and preferably a tensile stress at break in the wet state of at least 1 MPa. A further signaling device according to the invention is characterized in that a drop in the capacity of the or each source of current below a given threshold value can be signaled by optical means.