What’s the D(eal)-LRO?

In case it’s unclear from the title, we are trying to say, “what’s the dealio?”, but with DLRO. It was supposed to be clever. It’s always a good sign when you have to explain your jokes, right? Right. 

Anyways, we are talking about the continuity test this week, one of the most popular tests performed by electricians around the globe. Traditional multimeters and most insulation testers are capable of performing this simple test; however, certain circumstances may warrant a more advanced instrument – a digital low resistance ohmmeter (DLRO).

How does the test work?

First, simply connect two leads across the opposite ends of the item under test, then hit the ‘TEST’ button, and BOOM, the instrument shoots out a resistance reading automatically. Very complicated. In general, a low reading is considered to be “good”, while a high reading is “bad”. Unfortunately, it’s so simple that many folks often overlook the fact that a more advanced instrument is often required to fulfill certain demanding applications.  

Just in case you need some background, we should also mention the purpose of a continuity test. As the name indicates, you are looking to establish that your circuit is continuous. See what they’ve done there? Like the title of this very blog, it’s quite clever! A continuity test will guarantee that there are no wiring mistakes and that all connections are correct (and tight). Just what we like to see.

I don’t want another piece of equipment.

Let’s chat about the general instrument background first. When it comes to low resistance testing, current, rather than voltage, is of primary concern. Both a low resistance ohmmeter and multimeter operate at only a few volts. However, a DLRO can provide anywhere from 10 to 600 amps, while a multimeter typically operates at 5 to 200 mA. That’s a pretty significant difference. Likewise, multimeters are only capable of giving resistance readings down to the tenth, or perhaps, hundredth of an ohm; DLRO’s will read in the micro-ohm range.

So, a low resistance ohmmeter is particularly valuable, if you are making measurements below 1 Ω. At this level, just checking the continuity is not enough. You need to take it a step further than the capabilities of a multimeter. For example, you’re going to want to be certain that the circuit or joint under test can perform without overheating or burning. No one likes a burnt circuit. In demanding applications, like grounding for lightning protection, fault clearance, or maintenance of bolted connections and solder joints, measuring in the micro-ohm range will allow you to identify and correct issues, before they cause significant damage.

Okay, give me an example.

If you’ve moved into a new apartment or house and had the unique pleasure of putting together Ikea (or Target or Walmart) furniture, then you know all about torqueing of bolts. You also know that putting together furniture can ruin relationships, so watch out - it’s not for everyone. Anyways, when it comes to building your bed frame, the phrase, “the tighter, the better” probably applies to the screws and bolts you are installing. We aren’t furniture experts though, so don’t hold us to that. 

Fortunately, we are electrical experts, and we know that the same is not true when electrical current is at bat. Over-tightening of bolts can distort the connections, reducing the surface contact and increasing the resistance. Over time, the heat generated by over-torqueing will stress out the circuit to the point of failure, eventually.

But how can a DLRO help you in this situation? Well, it will detect over-torqueing, as a higher resistance reading, compared to the resistance of similar connections – allowing you to take preventative action. Handy! See the visual below to get a better idea of what we are talking about.

Still not convinced? We have some data for you. Let’s say you are measuring connections on a copper bus bar. Great. Even your top of the line, high quality, go-to, trusty multimeter is going to read 0.00 Ω at 200 mA. Even if you spread the leads out and take another measurement, you’re still going to get 0.00 Ω. Worse, if you connect the leads from the buss to an over-tightened bolted termination, you’re still going to get a reading of 0.00 Ω. Don’t believe us? We did this little experiment ourselves and we have the photo evidence to prove it. See below.

Enter, our good friend, the DLRO. You’re never going to believe us, but even at just 1 A, the DLRO is reading 4 µΩ on the same span of copper bus bar. Uh oh. When you connect across a wider span, as before, the DLRO is reading 10 µΩ. Just wait, it gets worse. If you then connect a lead to the over-torqued bolted termination, the reading shoots up to 1.131 mΩ. Don’t know about you all, but we are sensing a bit of a discrepancy between these instruments. For all of you detectives out there, we've included the proof below too. 

Okay, I think I am convinced now.

Good. Let us not forget that digital multimeters (DMM) will always have a place on an electrician’s toolbelt. They are tried, trusted, and the perfect instrument for basic electric maintenance testing, like wiring mistakes and establishing electrical continuity. However, certain circumstances demand more advanced technology. Speaking of technology, we should probably mention why the DLRO is capable of making such precise measurements in the micro-ohm range.

It’s 4-wire Kelvin bridge technology, ladies and gents. While the DMM uses two leads to connect to the item under test, the DLRO functions entirely different. Follow along with us using the visual below. Basically, the DLRO injects a constant DC current into the system, using the two ‘C’ connections, shown in the figure. Meanwhile, it uses the two ‘P’ connections to measure the voltage drop and calculate the resistance between the two points, eliminating the potential for test lead or contact resistance from the reading. The resistance measurement you get from a DLRO is significantly more precise, allowing you to detect even the smallest problems with resistance.  

- Meredith Kenton, Digital Marketing Assistant