Contact resistance testing with a micro-ohmmeter (DLRO) — verifying joints that thermography can miss

The thermographic inspection guide covers detecting poor connections via the heat they generate under load. This article covers a complementary, more direct method: actually measuring the resistance of a connection with a micro-ohmmeter, also known as a DLRO (digital low resistance ohmmeter).

Why an ordinary multimeter is not adequate here

The resistance of a healthy bolted or crimped connection is typically in the order of a few tens to a few hundred micro-ohms — far below the measurement range and accuracy of an ordinary multimeter. A micro-ohmmeter is specifically designed to reliably measure this kind of extremely low resistance, with two features an ordinary resistance measurement lacks:

4-wire Kelvin measurement: two separate leads carry the test current, two other, separate leads measure the voltage drop directly across the connection itself. This eliminates the resistance of the test leads and the contact resistance of the measuring clips themselves from the result — with an ordinary 2-wire measurement these would be included in the result, potentially causing a connection to be wrongly rejected or wrongly passed.
Relatively high test current (up to 100 A DC on many instruments): this breaks through any thin oxide layers on the contact surface that would show an artificially high resistance at a low test current, giving a more realistic picture of the resistance the connection has under actual operating current.

What a micro-ohmmeter detects

Busbar joints (busbar couplings, see the busbar trunking systems guide)
Cable-lug connections (see the cable lugs & crimp connections guide)
Bolted connections in switchgear and distribution assemblies
Contacts of circuit-breakers and switch-disconnectors

Why this can flag a problem earlier than thermography

Thermography only makes a problem visible once the load draws enough current to generate noticeable heat — for a circuit that is lightly loaded at the time of inspection, or a connection that only becomes problematic under peak load, a degrading connection can go unnoticed for a long time. A micro-ohmmeter measures the resistance directly, independent of the actual operating load at the time of measurement — an elevated resistance is thereby demonstrated even before the connection generates enough heat under normal operating conditions to become visible with thermography.

Note: this makes the two methods complementary, not interchangeable. Thermography has the advantage that a whole installation can be scanned relatively quickly, without contact, and under normal operating load; a micro-ohmmeter requires a de-energised connection and a targeted measurement per connection point, but reveals a degrading connection independent of the actual load situation.

Practical application

There is no universal limit value applicable to every connection — the acceptable resistance depends on the type of connection, the cross-section and the make. In practice, comparison is therefore often used:

Measure the resistance of all comparable connections in the same installation (for example all busbar couplings of the same type).
A connection that clearly deviates from the other, comparable connections (for example a multiple of the average value) is suspect, even without exceeding any universal absolute limit value.
Record the measured values at commissioning or first inspection, so later periodic measurements can be compared against a baseline — a gradual increase in the resistance of the same connection across multiple inspection rounds is an early warning, even if the absolute value on its own does not yet seem concerning.

Practical relevance

During the periodic NEN 3140 inspection of busbar joints, bolted couplings or cable-lug connections in a distribution assembly, a micro-ohm measurement is a valuable addition to thermographic inspection — particularly for installations that are not running at peak load at the time of inspection, where thermography would not yet show a degrading connection.

Common mistakes

Using an ordinary multimeter for this measurement — its accuracy and measurement range are inadequate for resistances in the micro-ohm range.
Using a 2-wire measurement instead of the 4-wire Kelvin method — this causes the resistance of the test leads and clips to be included in the result, which can lead to a connection being wrongly rejected or wrongly passed.
Relying solely on thermography and skipping a micro-ohm measurement on an installation that is lightly loaded at the time of inspection — a degrading connection can then go unnoticed until the next peak load.
Not recording a baseline measurement at commissioning, so later periodic measurements cannot be compared against a starting value.