SCP and SCRP control panels use zero cross (burst firing) solid state relays (SSR's) in conjuction with external heat sinks to remove heat from the panel enclosure. These SSR's are manufactured with direct copper bonding technology and a built in varistor for surge protection. The zero switching relay switches ON when the sinusoidal curve crosses the zero and switches OFF when the current crossed zero. Zero cross SSR's create very low harmonics and RFI and are much more reliable.