genset voltage sag when raining

[gegeum]

Kindly help me with this one, our genset are located outdoors and is equipped with a bonnet/rain proof type of canopy. The problem is when it rains we experienced voltage sag of around 10 V. Is this a grounding problem considering that it only occurs when it rains?
Thanks!

 

[jbartos]

Suggestion: It may be caused by the higher humidity in the air which impacts the genset voltage regulators.

 

[peterb]

More questions rather than answers -
What size genset are we talking about?
What loads does it drive?
Is the control wiring or power cabling exposed to moisture?