DLRO10HD and DLRO10HDX digital low resistance ohmmeters

The applications for low resistance testing are varied, but some of the most common are:

• Testing switches, connectors, and relays - to ensure contact resistance is within specified values.
• Cable resistance - too low shows too much copper in the cable (higher costs), and too high means insufficient copper, so the cable’s current carrying capacity is compromised.
• Motors and generators - to determine heat rise under load, measure winding resistance, and check for short or open circuits.
• Fuses - to ensure resistance is within specified values.
• Cable looms - to check the bonding and interconnections when installing equipment, racks, etc.
• UPS/car batteries - carrier to plate weld resistance where a high resistance indicates poor weld quality that will restrict the battery’s ability to carry current.

The application and the asset under test will determine whether low or high power is required. Here are three examples:

• Contamination – The application of high power will result in the test piece heating. Many tests are performed on bonds, connections, and contacts in low-current applications. If you have contamination between surfaces, a higher test current and power will ‘blast’ through the contamination resulting in a good test result, even though the connection will be unreliable in use. Testing with low current and power will reveal the problem much more readily.
• Rough surfaces – An example where high power is an advantage is testing connections or bonds with rough surfaces. In some such cases, you will obtain a good test result with a low test current and power, with the contact points between contact surfaces being low enough resistance. However, applying a higher test current and power will heat these small points of contact. The result is a changing test result as the heating takes place, highlighting the problem.
• Frayed wires – On lower current carrying systems (typically less than 10 A), testing with higher power will cause heating on weaknesses such as frayed wired, with the remaining wires presenting a higher resistance.

The 25 W power output can be supplied continuously for at least 60 seconds, meaning you can measure resistance with inductance. However, the DLRO10HD/HDX is unsuitable for testing large inductive circuits, such as power transformers.

Applying too much current during a test will result in power dissipation in the test piece, which results in heating. The heating alters the resistance of the test piece. However, there are some applications where having a higher output is useful, which is why you can select measurement ranges of either low (0.2 W) or high (25 W) power.

The applications for low resistance testing are varied, but some of the most common in the railway industry are:

• Strap and wire bond between rail segments - for maintaining the performance of control and telephone systems and minimising power loss.
• Cable joints - for power system efficiency.
• Earth/ground bonds - to ensure lightning protection on structures and limit step and touch potential on metal floors, handrails, ground mats, metal cladding, platform edge doors, and more.