▁serve ▁several ▁purposes . ▁They ▁are ▁used ▁to ▁track ▁tr ends ▁in ▁a ▁single ▁phot ov olta ic ▁( P V ) ▁system , ▁to ▁identify ▁fault s ▁in ▁or ▁damage ▁to ▁solar ▁pan els , ▁to ▁compare ▁the ▁performance ▁of ▁a ▁system ▁to ▁design ▁specific ations ▁or ▁to ▁compare ▁P V ▁systems ▁at ▁different ▁locations . ▁This ▁range ▁of ▁applications ▁requires ▁various ▁sens ors ▁and ▁monitoring ▁systems , ▁adapted ▁to ▁the ▁intended ▁purpose . ▁S ens ors ▁and ▁monitoring ▁systems ▁are ▁standard ized ▁in ▁IE C ▁ 6 1 7 2 4 - 1 ▁and ▁class ified ▁into ▁three ▁levels ▁of ▁accuracy , ▁denoted ▁by ▁the ▁letters ▁“ A ”, ▁“ B ” ▁or ▁“ C ”, ▁or ▁by ▁the ▁labels ▁“ High ▁accuracy ”, ▁“ Med ium ▁accuracy ” ▁and ▁“ Basic ▁accuracy ”. ▁ ▁Ir rad iance ▁sens ors ▁ ▁On - site ▁ir rad iance ▁measurements ▁are ▁an ▁important ▁part ▁of ▁P V ▁performance ▁monitoring ▁systems . ▁Ir rad iance ▁can ▁be ▁measured ▁in ▁the ▁same ▁orientation ▁as ▁the ▁P V ▁pan els , ▁so - called ▁plane ▁of ▁array ▁( PO A ) ▁measurements , ▁or ▁hor izont ally , ▁so - called ▁global ▁horizontal ▁ir rad iance ▁( G HI ) ▁measurements . ▁Typ ical ▁sens ors ▁used ▁for ▁such ▁ir rad iance ▁measurements ▁include ▁therm op ile ▁py ran om eters , ▁P V ▁reference ▁devices ▁and ▁phot odi ode ▁sens ors . ▁To ▁conform ▁to ▁a ▁specific ▁accuracy ▁class , ▁each ▁sensor ▁type ▁must ▁meet