AMPTEC bond resistanceground continuityairframe bondingfour-wire Kelvin measurement

Airframe Bonding and Ground Continuity Acceptance Testing

At milliohm bond resistances a two-wire meter mostly reads its own probes. Four-wire Kelvin measurement, test current, checking for stray voltage before you connect, and building an acceptance record that holds up.

In short

Structural bonding, fuel-system static bonding and ground-support equipment continuity are accepted at values from milliohms to a few ohms, where the lead and contact resistance of a two-wire ohmmeter alone can swamp the reading. The AMPTEC 620LK makes a four-wire measurement at 100 mA with 0.01 mΩ resolution on its 200 mΩ range; the 620LK-HR drives 1 A on its 20 mΩ range for .001 mΩ resolution and ± .025% of range accuracy; the 620LK-P5 grades ground continuity automatically on 20 Ω and 200 Ω ranges at 1 mA / 0.1 mA. All three are UL-913 intrinsically safe designs. On the bench, the 720A covers 200 mΩ to 200 kΩ in 7 ranges with 10 µΩ resolution, and the 601ES confirms the airframe carries no stray voltage before anything is connected.

Why milliohm readings demand a four-wire measurement

The bond between skin panels and structure is often accepted at a few milliohms. A set of test leads plus the contact between probe tip and surface can easily add tens of milliohms. A two-wire ohmmeter folds all of that into the reading, so what you measure is the probes and the leads, not the joint. A four-wire Kelvin connection separates the pair of leads that injects current from the pair that senses voltage; almost no current flows in the sense leads, so lead and contact resistance drop out of the result.

The second requirement is enough test current. A 1 mΩ joint drops only 100 µV at 100 mA and 1 mV at 1 A. With too little current the signal is buried in thermal EMF and noise, and the reading wanders and repeats poorly. The 620LK measures at a nominal 100 mA on its 200 mΩ and 2 Ω ranges; the 620LK-HR switches to 1 A on its 20 mΩ range and reaches .001 mΩ resolution, precisely so that the lowest range still has a usable signal.

Three acceptance tasks, three resistance levels

Flight deck instruments of a commercial airliner — Airframe and avionics

Bonding and grounding acceptance on aircraft falls into three groups. Structural bonding and lightning-protection paths are the most demanding, accepted at the milliohm level so that strike current follows the structure instead of discharging through harnesses or riveted seams. Static bonding of fuel systems and plumbing is far looser: it only has to bleed off charge, and limits are typically in the ohm range. Continuity checks on ground-support equipment, tow tractors and grounding leads sit between the two, where the point is to confirm the ground path is neither open nor corroded.

Each group calls for a different instrument range. Milliohm work needs the 20 mΩ to 2 Ω ranges of the 620LK or 620LK-HR and a test current of 100 mA or more. Static bonding and ground continuity are covered by the 20 Ω and 200 Ω ranges of the 620LK-P5, which grades resistance to ground automatically with 0.001 Ω resolution and ±0.05% of range accuracy. The actual acceptance limits depend on the aircraft type and each organization's engineering specification, so instrument selection starts by confirming the range and resolution the specification requires.

Before you connect: confirm there is no stray voltage on the airframe

The measurement terminals of a bonding tester are rated to a maximum input of 250 VDC; anything above that damages the instrument. An airframe on the ramp, a ground power cart and damp GSE can carry induced voltage or equipment leakage. The safe practice is to check with a voltmeter that there is no potential between the two test points before the bonding meter is connected. This matters most in fuel-handling areas, where no spark is acceptable.

The 601ES is a weatherproof safety digital multimeter built for exactly this step. It measures AC/DC voltage on 2 to 200 V and 600 V ranges, with a fail-safe current limit of 8 mA and an open-circuit voltage of only 1.6 VDC, and it meets UL-913 and MIL-STD-810-511 for explosive atmospheres. A waterproof housing and an all-metal Faraday cage let it read in rain and in electrically noisy surroundings, and its 4-wire Kelvin terminals measure resistance from 2 Ω to 20 kΩ, making it a first-line check tool on the flight line.

Field instruments: 620LK, 620LK-HR and 620LK-P5

All three are handheld, with a 4½-digit LCD, a reading update every 3 seconds, an operating range of −20 to 50 °C and a weight of about 3 lbs, and all three carry UL-913 intrinsic safety certification for use in fuel environments. The 620LK is the base model: two ranges, 200 mΩ and 2 Ω, a 100 mA test current, ±0.05% accuracy and 4 AA cells, suited to most structural bonding acceptance. The 620LK-P5 is dedicated to ground continuity grading at low currents of 1 mA / 0.1 mA.

The 620LK-HR adds 20 mΩ and 20 Ω ranges to the 620LK, holds ± .025% of range across all four ranges with a temperature coefficient of ± .0025% per 1 °C, and runs from an internal rechargeable battery pack. It uses Trident connectors and ships with the 630-406S 5 ft heavy-duty sharp-tip Kelvin probes; the 630-405K 10 ft needle-point probes and the 630-408J Kelvin clip probes are optional. On anodized or coated surfaces, needle-point probes pierce the film for a stable contact, and that often affects the reading more than the instrument's own accuracy.

Bench and production line: the 720A micro-ohmmeter

Incoming inspection of components and production measurement of harnesses and ground straps call for a bench instrument with wide range, speed and a computer interface. The 720A uses a 4-wire Kelvin connection across 7 ranges from 200 mΩ to 200 kΩ. Resolution on the 200 mΩ range is 10 µΩ with accuracy of ± .02% of reading plus .05% of range; the other ranges hold ± .02% of reading plus .03% of range. The 200 mΩ and 2 Ω ranges are driven at 100 mA, and the update rate of 3 readings per second is much faster than the handheld units.

With the optional OP-232 RS232C interface, readings go straight into a test program for automatic pass/fail and logging. The OP-300 gold-plated 4-wire Kelvin clip leads and the OP-403 handheld probes are included, and the 720-BAT battery-powered version serves stations without mains power. Note that the 720A has an open-circuit voltage of about 15 VDC; it is a general low-resistance instrument and must not be used on igniters or any circuit containing electro-explosive devices. That work belongs to the intrinsically safe models.

Turning a reading into an auditable acceptance record

For a bond resistance reading to survive an audit, the record needs at least the measurement location, the range and test current used, the probe type, the ambient temperature, and the instrument serial number and calibration due date. A temperature coefficient of ± .0025% per 1 °C looks negligible on the 20 mΩ range, but the temperature difference between hangar and ramp is enough to push a marginal reading across the limit, so temperature is recorded. AMPTEC instruments ship with an N.I.S.T. traceable calibration certificate, and the recalibration interval belongs in the organization's gauge control program.

Evencat is the sole distributor for AMPTEC Research in Taiwan. We help select the model against the range and current your acceptance specification calls for, recommend probe and lead combinations, and handle calibration certificates and recalibration scheduling. For short-term fleet inspections or MRO projects, bonding testers are also available for rental.