§434 — Short-circuit protection & the adiabatic equation

A cable that is correctly sized for its normal operating current (§523) is not automatically also resistant to a short-circuit current. §434 imposes a separate, additional requirement: the cable must be able to withstand the thermal stress of a short circuit until the protective device disconnects — this is a different check from the current-carrying capacity calculation of §523 and from the discrimination assessment of §536.

The adiabatic equation

$$S = \frac{\sqrt{I^2 t}}{k}$$

• S — minimum required cross-section (mm²)
• I — short-circuit current (A)
• t — disconnection time (s) — the equation is valid for disconnection times up to 5 seconds; for shorter, faster faults the heat generation is assumed to occur adiabatically (without heat dissipation to the surroundings), hence the name.
• k — material-/insulation-dependent constant

k-factors

These values come from table 43A (IEC 60364-5-54) and are internationally harmonised — the same k-factors are also used in, for example, BS 7671.

Two separate checks

In practice §434 requires two independent verifications, both of which must pass:

1. Thermal cable check (the adiabatic equation above): can the cable withstand the I²t energy of the short circuit without damaging the insulation?
2. Breaking capacity of the protective device: is the rated breaking capacity (Icu/Icn) of the circuit breaker or fuse greater than or equal to the prospective short-circuit current at the point of installation?

One check says nothing about the other: a protective device with sufficient breaking capacity protects itself, but does not guarantee that the cable behind it survives the let-through energy thermally — and vice versa.

Why this is relevant alongside §523

A cable can be fully correctly sized according to the current-carrying capacity tables of §523 (Iz ≥ Ib, see the correction-factors guide) and still be undersized for short-circuit conditions, if the upstream protective device disconnects too slowly: the let-through energy (I²t) increases as the disconnection time gets longer. §434 is therefore an additional, non-replacing check on top of the ampacity calculation.

Common mistakes

1. Checking only §523 (operating current) and assuming that this also guarantees short-circuit resistance — these are two separate sizing requirements.
2. Using the wrong k-factor — PVC- and XLPE-insulated cables have a different maximum final temperature and therefore a different k-value; mixing them up leads to an under- or oversized cross-section.
3. Checking only the breaking capacity (Icu/Icn) and skipping the thermal cable check — both checks are required, not just one of the two.
4. Ignoring the 5-second limit — for longer disconnection times the adiabatic assumption (no heat dissipation) is no longer valid and a different calculation method must be used.

Related

• §523 — Cable cross-section & current-carrying capacity
• §536 — Distribution boards & discrimination
• Calculation questions: short-circuit current & voltage drop