AResidual Current Device (RCD) is amechanical switching device or association of devices intended to cause the opening of the contacts when the residual current* attains a given value under specified conditions. *Residual Current is the algebraic sum of the currents in the live conductors of a circuit at apoint in the electrical installation.
Rating factors are divided into the nominal rating of the circuit protective device (In) to determine theminimum current-carrying capacity (Iz) of a cable in a circuit. For example:Iz = In/(Ca x Cg x Cc x Ci x Ch)Rating factors:Ca –ambient temperatureCc –buried installation methods 70 to 73 (Cc = 0.9)Cd –for depth of burialCf –BS3036 […]
Value of current used for specification purposes,established for a specified set of operating conditions.
Power factor is the ratio of the resistance (R) and impedance (Z) in a circuit, that is:Power factor = cos ∅ =R/Z
Themaximum value of earth fault loop impedance (Zs) that will allow the protective device in a circuit to disconnect an earth fault within the prescribed time. Example: if themaximum earth fault disconnection time for a circuit is 0.4 s and a fault current Ia = 320 A is required to operate the circuit protective device […]
Factors for conductor materials from BS7671:2018,Table43.1. Different values are given for different types of conductor and insulatingmaterials. k2s2 used in adiabatic calculations gives the short-time withstand of a conductor in A2s.
Iz The current carrying capacity of a cable under the defined installation conditions. This is the effective rating of the cable after the relevant rating factors have been applied, i.e.,Ca, Cg,etc.
Ir The adjusted overload rating of an overcurrent protective device, in Amperes.
Prospective Fault Current (Ipf) is the value of overcurrent at a given point in a circuit resulting from a fault of negligible impedance between live conductors having a difference of potential under normal operating conditions,or between a live conductor and an exposed-conductive-part. Or, the calculated fault current at a particular point in the network. The […]
I2t The energy let-through of a device in A2s. The total amount of energy let through by a protective device for a given level of fault current at a stated voltage.
