The M200-PLL & PLD are synchronizing check relays, also known as paralleling relays. They are used to ensure at two AC supplies are synchronized. For a system to be synchronized, frequency, phase angle and voltage have to within pre-set limits.The M200-PLL can monitor either mains bus bar and incoming generator or two generators.The PLL has customer adjustment of the differential voltage between 10 to 30%. This voltage corresponds to 6 to 20 electrical degrees. The unit compares the input voltage and phase relationship of the bus bar to that of the generator when the signal is within the pre-set limits, the relay energizes. The M200-PLD operates as the M200-PLL but has the additional feature of the dead bus facility. This enables the relay to energize with a generator supply only, which is a common requirement when mains failure occurs.