Circuit-breaker manufacturers publish tables showing combinations of pairs of circuitbreakers, one upstream,one downstream,where the downstream device can be used at a location where the prospective fault current (Ipf) is higher than its breaking capacity (Icu). The Icu of the upstream devicemust equal or exceed Ipf at its point of installation.
k2s2 its an Energy withstand of cable in A2s. For example,a cable with a withstand of 12 x 104 A2s, can sustain without permanentdamage,a fault current of 1,000 A for a time of 0.12 s (1,0002 x 0.12 = 12 x 104 A2s).
Ambient temperature rating factor used for calculating theminimum required cable size. For cables installed in air, values for this factor are given in BS7671:2018 Table4B1.For ground installations, values are given in Table4B2. Elevating the ambient temperature will result in a lower rating factor Ca.This may result in an increased cable size.
The breaking capacity of a circuit-breaker is themaximum level of fault current (kA) it cansafely interrupt. Each circuit-breaker has two breaking capacities: Ultimate BreakingCapacity (Icu) Icu is defined as the ‘Ultimate Breaking Capacity’ of a circuit-breaker – Icn is the ‘Normal Breaking Capacity of an MCB, with the same meaning as Icu. This is the […]
Protection against electric shock under fault-free conditions;use of barriers,placing out of reach,and so forth.
The temperature of the free air surrounding a cable,or in the case of cables installed in the ground, the temperature of the surrounding soil.For an enclosed method such as ‘In conduit’ this will be the temperature of the air surrounding the conduit, not the temperature inside it.
Adiabatic checks are required to determine whether conductorsmight be damaged by the thermal effects resulting from the level of calculated fault conditions.
