NSEU,
From Boeing AFM:
The design provides indication of low fuel quantity in the center tank based on input from the Fuel Quantity Processor Unit (FQPU) for a crew controlled wet-shutoff. If the crew fails to respond, or there is an error in the fuel gauging system, the ELMS system will de-power each center tank pump after 15 seconds of continuous low pressure based on input from the pump pressure switch.
So it looks like the pumps will shut themselves off and not run 'dry', based on pump outlet pressure. The
FUEL LOW CENTER EICAS appears once the centre tank is <=900Kg.