Grounding Point-to-Point Continuity Testing of Bonding Paths

What it is

Grounding point-to-point continuity testing is a measurement of the electrical path between two points that are supposed to be bonded together, typically a known reference point on the grounding system and the metal frame, enclosure, structure or conductor that is meant to be connected to it. A low-resistance ohmmeter passes a test current through the path and reports its resistance, which is normally very small when the bonding is sound. The test is used on grounding systems throughout a facility, from the main ground bus out to switchgear lineups, transformer tanks, motor frames, cable tray, structural steel and fences, and on the bonding and grounding conductors of adjustable speed drives and their enclosures. It is different from the ground electrode test, which measures the connection of the grounding system to the earth. Here the question is internal: is every piece of equipment that must be grounded actually connected, with a path of low enough resistance, to the rest of the grounding network? The measurement approach follows the field techniques described in the guide IEEE 81-2025 for continuity and bonding checks.

Why it is done

Grounding protects people and equipment only if it is continuous. A frame that is not bonded to the grounding system can become energized by an insulation failure and stay at a dangerous voltage until someone touches it, because a fault current has no path back to the source that would let the protective device trip. A bonding conductor with a loose, corroded or painted-over connection may look fine and still have a resistance high enough to limit fault current, to delay protective devices or to raise voltage differences between nearby metal parts. In drives and other electronic equipment, a poor bonding path also compromises noise control and shielding, causing nuisance faults and failures of sensitive components. Connections degrade over time through corrosion, vibration, thermal cycling and mechanical damage, and construction or modification work can disturb or disconnect them. A continuity test finds those defects before a fault does, and it is a basic acceptance check for new installations, where missed connections are common.

The principle

Resistance across a bonding path is the sum of the resistances of the conductor and of every connection along it. A healthy path made of copper or aluminum conductors and clean, tight connections has a very small resistance, and the instrument needs to be able to resolve small differences from it. A low-resistance ohmmeter, which injects a substantial direct current and reads the voltage drop through separate leads, can distinguish a sound connection from one that has become a little worse. Because the resistance of the leads and clamps would otherwise be significant compared with the quantity being measured, the four-wire arrangement or a lead-nulling procedure keeps them out of the result. The measured value depends on the length and cross-section of the path as well as on the quality of its connections, so a reading is judged against what similar paths show, not against a single universal figure. Paint, rust and oxide at a contact point add resistance that has nothing to do with the bond, which is why probe contact is made on cleaned bare metal. Another subtle point is the possibility of a parallel path: the instrument reports the combination of every route between its two probes, so a hidden second route can hide a defect in the primary one.

How it is performed

The equipment and the grounding system are de-energized and, where the work requires it, isolated under the owner's lockout procedures, because current from the test set should travel only through the grounding network and not through connected loads. A reference point is chosen on the grounding system, usually the main ground bus or a well-established ground connection. The low-resistance ohmmeter is connected with one lead on the reference point and the other on the item to be checked, with the contact surfaces cleaned to bare metal at both places. Long test leads on a reel are common because the points may be far apart, and the lead resistance is nulled or excluded according to the instrument. The reading is allowed to settle and then recorded, and reversing the current direction and averaging removes the effect of thermal voltages. The same procedure is repeated for each point of interest, such as every cabinet in a lineup, each transformer tank, the drive enclosure and its bonding conductors, and structural members. Readings that look odd are checked by re-cleaning the contact and repeating the measurement, and, where a second path may exist, by considering how the measured value could be affected by it. All findings are written down on the spot, and any loose connection found is corrected and retested before the equipment is released.

What is recorded

The record identifies the reference point and each point tested, the resistance measured between them, the instrument used with its calibration status and the way lead resistance was handled. It notes the condition of the connection and of the contact surfaces, for instance clean, painted, corroded or loose, along with any correction made and the retest result. Where useful, the path length or conductor size is noted so that similar points can be compared fairly. A sketch or list of the measured route lets future testers repeat the same measurements, which is essential for trending because a different route gives a different resistance.

How results are judged

This page states no numeric limits. Results are judged against several criteria taken together. The first is the project or owner requirement for the maximum resistance of bonding paths, which comes from the specification and the applicable codes, and in some cases from the manufacturer of the equipment, particularly for drives. The second is comparison between like points, such as the cabinets of one lineup or the frames of identical machines, where a reading that is out of line with its neighbors points to a poor connection. The third is trending against earlier results on the same path, using the same route and method, so that a gradual rise shows developing corrosion or loosening. The fourth is a sanity check on the measurement itself, that is, whether the contact was clean and whether a parallel route may be affecting the value. The methods are covered in IEEE 81-2025, and the test is called for in ANSI/NETA ATS-2025 for acceptance and in ANSI/NETA MTS-2023 for maintenance on the equipment named above. Use your licensed copies of these documents for the governing wording and values.

Variations

At acceptance the test verifies that a newly built grounding network is continuous throughout, which is the right time to find missed connections, and it records the baseline. In maintenance it is repeated on a schedule or after any modification, with attention to points that carry high fault current or are subject to vibration and weather. Large grounding grids and outdoor yards involve long distances and many points, so a sampling plan is often agreed on in advance. On adjustable speed drives the emphasis is on the protective earth and bonding conductors of the drive, its enclosure, the motor and the cable shields, where a good bond also controls noise. In equipment lineups, such as switchgear, continuity between sections and to the ground bus is checked, and doors and removable panels may be included if they are meant to be bonded. The test complements the ground electrode test and the bolted connection resistance test, which examine other parts of the same fault path.