Sorry I couldn’t reply sooner, but thank you all for your kind responses to my question. This is far more complicated than I envisaged and find myself more confused than ever.
I began by drawing out Gavin’s irrigation system on a piece of paper and realized that the analogy didn’t seem to be quite right, since the pumps were feeding the main supply pipe from both ends of the field. In the case of a PV installation, wouldn’t this be different, because the pumps, like the inverter/mains, are effectively connected at the same point on the system? (someone called it a henley block?). This raises the problem of the pumps delivering unequal pressure – if one is higher than the other, then the water might be forced backwards through the weaker pump. I suppose this would be the same as the inverter forcing current back into the mains.
If the supply pipe burst near the pump connections, then I suppose that since there would be no resistance, water would flow from both pumps simultaneously, which is what I didn’t understand about the inverter and mains supplies both feeding the household circuits at the same time. But this would be enough to blow the fuses wouldn’t it?
Maybe a spark of understanding was prompted by yellowvanman’s explanation – when electricians say that the inverter always tries to supply a higher voltage than the mains, does this mean that it has to do so in order to make the voltage which appears at the henley block the same as that from the mains and not higher? If this is the case, then since the two are equal, it sort of makes sense that current could flow from both sources at the same time. But then, I’ve just thought of another problem – if both voltages are equal, how can the inverter drive current into the mains (when exporting)?
Sorry if I sound a bit dim. Perhaps I should stick to gardening and not ask silly questions.
