Panelboards, as well as other power distribution assemblies like switchboards and switchgear, carry a short-circuit current rating that defines the amount of fault current that the equipment is designed to withstand. There are three protection systems used to protect low voltage power distribution systems. They are:
Fully rated protection — where all overcurrent devices are rated for the full prospective short-circuit current at their line side terminals throughout the system. In a fully rated system, the short-circuit current rating of the panelboard is determined by the short-circuit current rating of the lowest rated overcurrent device in the panelboard
Selectively coordinated protection — A fully rated system where the overcurrent device closest to the fault will open first, thus isolating the faulty circuit.
Series rated protection — A short-circuit interrupting rating assigned to a combination of two or more overcurrent protective devices that are connected in series and in which the rating of the downstream device(s) in the combination is less than the series rating. Series ratings are also known in the industry as integrated ratings, series combination ratings and series connected ratings.
In a series rated system, the overcurrent devices in series in the protective scheme must have been tested and listed by Underwriters Laboratories (UL), for series combination use in the system.
Eaton’s currently published series ratings can be found in the current version of the panelboard and switchboard series rating information manual.