Overload protection
Ib £ In £ Iz
If £ 1,45 Iz
were Ib = Operating current of the circuit
In = rated current of the protective device
Iz = the current-carrying capacity of the cable
If = operating current of the protective device
Protection against Short Circuits
IccMax £ P.d.i.
I²t =< K²S²
dove IccMax = maximum short-circuit current
P.d.i. = power interruption protection equipment
I²t = Joule integral of the short-circuit current
K = coefficient that is used
115 for PVC insulated cables
135 for rubber insulated cables natural and butyl
143 for ethylene propylene rubber insulated cables and cross-linked polyethylene
S = section of the cable
Let-through energy
I²t £ K²S²
dove I²t = value read on the curve let-through energy I²t protection in
correspondence of the short circuit currents
K²S² = let-through energy of the cable
dove K = coefficient of cable type (115,135,143)
S = cable section
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