Low Resistance Ohmmeter (DLRO): How It Works and How to Choose One

DLRO GUIDE

A low resistance ohmmeter measures resistances far below what a general-purpose meter can resolve, typically in the micro-ohm to milliohm range. Engineers use it to verify circuit breaker contacts, busbar joints, cable splices, bonding straps, and motor windings. This guide explains how the instrument works and which specifications matter when you compare models.

• Four-wire (Kelvin) connection removes lead resistance
• Test current separates 10 A, 100 A and bench classes
• Match the current to your standard and load

It is also called a DLRO (digital low resistance ohmmeter), micro-ohmmeter, or low ohm meter. What matters is the test current, range, and accuracy behind the label.

Megger describes the instrument as especially valuable for measurements below 1 Ω. At that level, a bad joint and a good one may differ by only a few micro-ohms, so resolution and measurement method decide whether the reading is useful.

Why a standard multimeter is not enough

A handheld multimeter is built for a different job. According to Megger, multimeters typically test at a few volts and 5 to 200 mA, and resolve resistance only to tenths or hundredths of an ohm.

Megger’s own example makes the point clearly. On a copper busbar, a multimeter would read 0.00 Ω, while a DLRO at 1 A read 4 µΩ across the same span. An over-tightened bolted termination on the same setup measured 1.131 mΩ on the DLRO, a fault the multimeter could not see.

Two problems limit a multimeter here:

• Low test current. A small current produces a voltage drop too small to resolve on a micro-ohm object.
• Two-wire connection. The resistance of the test leads and the contact points is added to the reading, and it is often larger than the value you are trying to measure.

How it works: four-wire Kelvin measurement

Every serious low resistance ohmmeter uses a four-terminal (Kelvin) connection:

• Current leads (C1, C2) push a constant DC current through the test object.
• Potential leads (P1, P2) measure the voltage drop directly at the object.

Because almost no current flows in the potential leads, the resistance of the leads and their clamps drops out of the result. Megger notes that this matters most at low values and that lead resistance is usually unimportant above a few tens of ohms.

Reversing the current

Where dissimilar metals meet, a small standing (thermal) EMF can appear and bias the reading. The standard remedy is to take one reading, reverse the current, take a second reading, and average the two. Many DLROs do this automatically; Megger’s DLRO10 range, for example, reverses the current and displays the corrected result without operator action.

Key specifications to compare

Choosing test current by application

Test current is the specification that most clearly separates one class of instrument from another. The table below groups typical applications with example models from our Microohmmeters / DLROs category.

Figures are from each manufacturer’s product page.

Circuit breakers: why current level matters

Healthy breaker contacts often measure in the tens of micro-ohms, so the instrument needs high resolution. Megger also points out that a 10 A test may not break through the carbonised layers left by arcing, which can make a good breaker look bad. A higher test current gives a more representative and more stable reading.

Breaker test standards set minimum test currents. Megger summarises ANSI C37.09 as calling for at least 100 A, and IEC 62271-100 as allowing any value from 50 A up to the breaker’s rated current. Always follow the standard and the procedure your site specifies.

For substations where both sides of the breaker stay grounded, look for DualGround capability. On the DLRO100 series it is available on the XB and HB versions with an optional DC clamp.

Inductive loads: windings and transformers

Motor and generator windings are inductive. When DC is applied, the current rises gradually, so the reading takes time to settle. Choose an instrument with a continuous or inductive mode that holds the current steady until the value stabilises.

There are limits. Megger states that the DLRO10HD family can supply 25 W continuously for at least 60 seconds in inductive mode, but it is not suited to large inductive loads such as power transformers. For those, a dedicated transformer ohmmeter is the right tool.

DLRO vs insulation tester: not the same test

A low resistance ohmmeter and an insulation tester (megohmmeter) measure opposite ends of the scale. The DLRO checks that a conductive path has very low resistance. An insulation tester applies a high DC voltage to confirm that insulation has very high resistance. Most maintenance programs need both; for insulation work, see our Megohmmeters and Insulation Testers.

Tips for repeatable readings

• Isolate first. Test only de-energised, isolated equipment, following your site’s lockout and grounding procedure.
• Clean the contact points. Oxide, paint, or grease under a clamp adds resistance and scatter.
• Use reversal or averaging whenever dissimilar metals are involved.
• Record the temperature. Conductor resistance changes with temperature, so note it with each result.
• Compare like with like. Megger observes that a faulty connection shows higher resistance than similar connections. Trending identical joints over time is often more revealing than a single absolute value.
• Keep the instrument in calibration. Micro-ohm readings are only as good as the meter behind them; see 7 signs your test equipment needs calibration.

Quick selection checklist

1. What is the lowest resistance you need to resolve, and the highest you need to measure?
2. Does a standard or customer procedure set a minimum test current?
3. Will you test inductive loads such as windings?
4. Field or bench? Battery, mains, or both?
5. What safety category does the work location require?
6. Do you need stored results and reports for audits?