When it is necessary to reduce demand on the power system, {TAIL ~ the power system} sends {HEAD ~ a load control signal} to a controller at the consumer indicating that supply of power to the first load is to be interrupted 
When it is necessary to reduce demand on the power system , { TAIL ~ the power system } sends { HEAD ~ a load control signal } to a controller at the consumer indicating that supply of power to the first load is to be interrupted
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH