If a bypass diode fails short for any reason, the effect will be like shading of that corresponding cell string. You’ll see Voc of that string drop, representing the loss of that cell string. If you’re measuring more than one string in parallel, you’ll see a step in the downward leg of the curve.     If the bypass diode fails open, it’s a dangerous condition because that leaves the cells in that cell string unprotected. The worst case is where only one of those cells is fully shaded, and the rest of the string forces it into reverse breakdown. Then it’s dissipating maybe 5A x 18V or 90W. The temperature of the shaded cell rises dramatically, and can be damaged.     The PV Analyzer does not detect failed-open bypass diodes directly, because in non-shaded conditions, the bypass diodes are effectively open, they are reverse biased and not conducting. However, you may be able to indirectly test them using the comparative shading method. You can measure the string I-V curve multiple times, successively shading each of the cell strings. Where the bypass diode is working normally, you’d see a step representing the kicking in of the respective bypass diode. If the bypass diode is failed open, you’d see a different response.   

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