What is Surge Impedance Loading (or) SIL?

Surge Impedance Loading (SIL)

Surge impedance, also referred to as characteristic impedance, is a term utilized in the analysis of the electrical transmission lines. 

It is defined as the voltage/current ratio of a moving wave on a line. Surge impedance is especially relevant in the presence of high-frequency signals (or) transient phenomena like as lightning strikes or switching surges.

The formula for surge impedance (Z0)

(Z0) = √(L/C)


L – Inductance/Length &

C – Capacitance/Length

of transmission line.

Thus, from the given formula, we clearly observe two key conclusions. 

  • The surge impedance of a transmission line is determined by its inductance and capacitance.
  • The surge impedance is independent of transmission line’s length.

In this post, we will explore the concept of surge impedance loading. 

What is Surge Impedance?
  • The surge impedance for transmission line (overhead) is 400Ω (Z0=400Ω).
  • The surge impedance for underground cables is 40Ω (Z0=40Ω).
  • Pure inductors have infinite surge impedance (Z0=∞) since their capacitance is zero.
  • A pure capacitor has no surge impedance (Z0=0) since its inductance is zero.
  • Transformers typically have a surge impedance of around (Z0=5000Ω).

Surge impedance loading (SIL) can be described as the amount of power delivered when the load connected to transmission line equals the surge impedance.

Surge Impedance Loading (SIL) = Vs2/Z0 (W)


Vs = source voltage (or) operating voltage, and 

Z0 = surge impedance.

In other terms, SIL is the megawatt power delivered by a load with an impedance equal to the transmission line’s surge impedance (Z0).

So we can express it as,

SIL = 3 Vph Iphcosφ

The load is resistive, therefore cosφ=1

IPh = Vph/R = Vph/Z0 = VL/√3 Z0

SIL = 3×VL/√3×VL//√3Z0 = VL2/Z0

SIL = VR2/Z0= VS2/Z0=VS2√C/L


VS and VR represent line voltage

C & L represent phase value

What is Surge Impedance? Graph

Surge impedance loading (SIL) is proportional to the square of source (or) line voltage.


SIL is proportional to square root of capacitance.

SIL ∞ √C

SIL is inversely proportional to square root of inductance.

SIL ∞ 1/√C

The different methods to reduce L include:

  • By utilizing parallel lines,
  • Utilizing series capacitance,
  • Using bundled conductor.

It is square root of the ratio of the series impedance to the shunt admittance. It indicates that characteristic is the transmission line’s impedance at any given point.

Zc = √Z/Y


Zc – Characteristic Impedance

Z – Series Impedance

Y – Shunt Admittance

The surge impedance of a transmission line is an important element in determining wave propagation over the line. 

It controls how voltage & current waves react as they move along the transmission line.

Transmission line surge impedance ranges from 200-400Ω based on voltage class, with a phase angle of 0-15 degrees. The table below provides typical HV & EHV overhead transmission line surge impedance values.

For overhead lines, apply the following rule of thumb to calculate series inductance & shunt capacitance.

  • SIL is essential for establishing transmission lines that maintain voltage stability and provide efficient power delivery.