Short-circuit thermal stability is a concept every electrical engineer should know well. When a fault current flows through a conductor, it generates heat. Thermal stability measures the ability of conductors and circuit elements to withstand the enormous heat produced during a very brief short-circuit event. Per code, the circuit must be interrupted before the conductor reaches its maximum allowable temperature.

Fig.1 — Substation distribution panel installation

Part 1 — Short-Circuit Current Calculation at Panel Incoming Terminal

The following example is based on an actual residential building electrical design. The project has an underground substation with two 800 kVA transformers. A self-use distribution panel (for fans, lighting, and maintenance outlets) is installed inside the substation: capacity 10 kW, calculated load current 16 A, incoming cable NH-YJV-1kV 5×16, 5 m long, connected directly to the low-voltage transformer bus.

Step 1 — Transformer Impedance

S = 800 kVA
RT = 1.90 mΩ
XT = 13.27 mΩ

Step 2 — Busbar Impedance (LV Main Breaker → Panel Feeder)

Busbar: TMY-3(100×8)+(80×8),  L₁ = 6 m
R₁ = 0.04 × 6 = 0.24 mΩ
X₁ = 0.182 × 5 = 1.092 mΩ

Step 3 — Cable Impedance (LV Switchboard → Panel)

Cable: NH-YJV-1kV 5×16,  L₂ = 5 m
R₂ = 1.097 × 5 = 5.485 mΩ
X₂ = 0.087 × 5 = 0.435 mΩ

Step 4 — Three-Phase Short-Circuit Current at K1

RK1 = RT + R1 + R2 = 1.90 + 0.24 + 5.485 = 7.625 mΩ
XK1 = XT + X1 + X2 = 13.27 + 1.092 + 0.435 = 14.797 mΩ
ZK1 = sqrt(R² + X²) = 16.65 mΩ

IK1 = 1.05 × 380 / (√3 × 16.65) = 13.84 kA

Fig.2 — Short-circuit impedance calculation diagram

Part 2 — Minimum Cable Cross-Section & Thermal Stability Verification

Per GB 50054-2011 (Low-Voltage Electrical Installations Design Code), Clause 3.2.14, the conductor cross-section must satisfy:

S ≥ I × √t / k

S ≥ 13840 × √0.02 / 143 = 13.7 mm²

Minimum required cross-section: 13.7 mm². The original design specifies NH-YJV-1kV 5×16 (16 mm²) — larger than 13.7 mm², so it passes thermal stability verification.

Key takeaway: Although the self-use panel only draws 16 A in normal operation (meaning a 6 mm² cable would suffice by load current alone), the panel's proximity to the transformer forces a cable upgrade to 16 mm² to satisfy thermal stability. If you're close to the source, always run the thermal stability check.

Part 3 — Summary & Design Guidance

In civil building electrical design, the closer the load is to the substation (and thus to the transformer), the lower the circuit impedance — and the higher the prospective short-circuit current. Incoming cable selection must account for both normal load current and short-circuit thermal stability.

· Always perform a thermal stability check for distribution panels located inside or immediately adjacent to a substation.

· Panels far from the transformer will naturally have higher impedance, which reduces the fault current.

· The instantaneous trip time of the upstream MCCB (typically 20 ms) is used as the fault clearing time t.

· The material constant k = 143 applies to copper conductors with XLPE insulation; consult GB50054 Annex A for other types.