Grounding & Bonding
Bond strap impedance vs frequency
Calculate how bond strap impedance varies with frequency, length, width and geometry. A strap with 2.5 mΩ DC resistance can present tens of ohms at RF.
DC resistance and RF impedance
A DC bond test only measures resistance. But every conductor also has inductance — it resists changes in current — and the opposition that inductance creates grows in direct proportion to frequency:
A typical strap may have an inductance of a few hundred nanohenries. At 100 MHz, 300 nH produces approximately 190 Ω of inductive reactance, far exceeding a DC resistance of 2.5 mΩ. Geometry is therefore critical to RF bonding performance.
Short, wide straps generally have lower inductance than long, narrow conductors. A common guideline limits strap length to approximately five times its width. Compare a 30 cm × 6 mm strap with a 30 cm × 60 mm strap to assess the effect of width.
At higher frequencies still, skin effect pushes the current into the conductor's surface, raising the effective resistance — the tool includes this, though inductive reactance usually dominates long before skin effect matters.
Inputs
The cyan line shows total modelled impedance from 1 kHz to 1 GHz. The grey dashed line shows DC resistance, and the amber dashed line shows inductive reactance, 2πfL. Where inductance dominates, impedance rises at approximately 20 dB per decade in this model.
Inductance here uses the standard partial-inductance approximations for a straight strap or wire in free space. A strap bonded close to a large metal surface will behave somewhat better than shown, and bends, hardware and joint quality all add impedance in practice. For facility earthing and bonding design, IEC 61000-5-2 is the reference standard.