Grounding & Bonding
Bond strap impedance vs frequency
A bonding strap that measures a beautiful 2.5 milliohms on a bond meter can present tens of ohms to an RF current. This tool shows why — and lets you see how length, width and geometry change the picture.
Why a "good" bond can fail at RF
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:
For a typical strap the inductance is a few hundred nanohenries. That sounds like nothing, but at 100 MHz even 300 nH presents about 190 Ω. The 2.5 mΩ you measured with the bond meter is irrelevant a thousand times over — at RF, the strap's geometry is what matters, not its resistance.
Two practical rules fall out of the numbers. First, short and wide beats long and thin: inductance is driven mostly by length, and a wide flat strap has less inductance than a round wire of the same length. Second, the old workshop guideline that a strap's length should be no more than about 5 times its width exists precisely to keep the inductance down. Try a 30 cm × 6 mm strap in the tool, then the same length at 60 mm wide, and watch the chart.
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 is the strap's total impedance from 1 kHz to 1 GHz. At low frequency it sits on the flat resistance floor (grey dashes); above a few kilohertz the inductive reactance 2πfL (amber dashes) takes over and the impedance climbs 20 dB per decade, forever. This is the chart every bond meter is silently not showing you.
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.