When measuring very low resistance the four-wire test method has for many years been the standard method. The main advantage being that it simply and easily removes the resistance of the test leads from the measurement. However, what happens when those test leads need to be very long?
Many modern applications require long test leads, but test instruments do not always cater for this. The limiting factor is how muchresistance the instrument can have between the current terminals(C1 and C2), which for many low resistance ohmmeters is somewhere between 400mΩ and 1Ω.
Example application case study: Testing wind turbine lighting protection resistance challenges.
Applications such as measuring the resistance from wing tip to the ground connection at the base of a wind turbine regularly require test leads 100m long. Let’s take a look at the specific challenges that arise when testing in this application.The biggest issue is that the thick cable required to keep the resistance down makes the cable reels big and heavy, and the instruments used are often not compact and are fairly heavy too. This results in the following challenges to the user:
- Most instruments capable to measuring with a high level of test lead resistance are more expensive and tend to be fairly large and heavy•Difficult to carry around and store everything required, especially with limited storage in a van.
- The cable reels and instruments are hoisted up the turbine, so any additional weight increases the safety risk.
- The thick cable and large reel make them expensive to buy, so many users choose to make their own, which is time consuming.
- Difficult to maintain a minimum 1 A test current requirement during the test while using extra-long test leads.
- Accidental auto-ranging down of test current.
How Megger can help: Introducing the new DLRO2 and the new dedicated Long Test Lead Mode.
Long leads resistance test mode has been designed for use with test leads of very long length. The Difference Meter is disabled in long leads mode. All readings are displayed in mΩ.Minimum current is 1 A.Restricted to single tests.In long leads mode, all soft keys are inactive. The test is pre-set to single measurement, 1 A with previous 2 results displayed.
Megger has addressed all of the above problems with the new DLRO2. Equipped with a specially dedicated 1 A-only test range called the Fast mΩ/Long Test Lead Mode, you can now have a handheld, lightweight instrument that can measure with over 3Ωof test lead resistance.The result is an instrument that can provide the following benefits:
- Cost effective – providing a solution at a great price.
- Compact handheld solution –easy to carry and transport.
- Enables lighter cable reels –with the DLRO2s ability to test with much higher test lead resistance, it means smaller wire gauge can be used.
- Enhanced safety when working from height: Less instrument weight to use at height, Lower weight cable reels.
- The Long Test Lead Mode features a fixed 1 A test current with a positive tolerance only –guaranteed 1 Aminimum test current, no auto-ranging to lower test currents.
- The Long Test Lead Mode has a fixed mΩ only display –ideal for non-technical staff as there are no unit changes to be concerned with.
- IP54 rated for all weather outdoor use.
- More flexible test solution for engineers –with the DLRO2 test modes, the instrument is able to cater for all low current testing applications, such as those found inside a wind turbine.
- Power supply flexibility.
- New innovative Difference Meter.
- Inductive mode for testing motors and small power transformers.
- CATIV 300 V and CATIII 600 V high safety rating.
- 600 V protection without blowing a fuse, 1000 V maximum protection.