Liquidated damages vs penalty

Liquidated damages are applied by the customer if it suffers some damage/loss due to fault of the supplier. It operates under a no harm, no foul principle, principally it is not applied if the customer dosen't suffer any loss.
However penalty, generally seen in British civil code can be applied even if the customer had not suffered any loss due to default of supplier.
There is a very fine line between the two and it is upto the supplier to ensure there are proper and specific  implications given in LD clause, otherwise in some law systems it would be very easy to legally interpret it as a penalty.