Wiring technology: easy bare wire testing

Bare wire testing is essential

With the advancement of technology, you might think that simple test equipment based on bare wire (two or more conductors in a sheath that carry power and/or low-speed signals) will become obsolete. A sample survey of the wiring installation industry shows that the bare wire installation business is likely to grow as distributed audio, security, room control and automation systems are more widely used.

Bare wire test - verify end-to-end connectivity

One of the most basic tests for bare wire testing is continuity testing, which searches for a continuous path through each conductor. To perform a continuity test, you can use a resistance-based multimeter and short the ends of the wires to ensure a complete loop. When connecting a device such as a speaker, you may need to test the resistivity. When connecting a safety device, you may want to test the switching condition of a switch. Verifying the switching condition is usually supplemented by an audible indication of a closed circuit, and many multimeters, such as Fluke multimeters, have this function. This most basic test can also be performed by many of the inductive tone generators currently available, such as Fluke Networks' MicroNet Blink?. This tone generator is equipped with a simple LED display, making general continuity testing a breeze.

Bare wire test--polarity test

Similar to the continuity test is the polarity test. Polarity testing is usually done with a multimeter, but it can also be done easily with an inductive tone generator that has this feature. Fluke Networks' MicroNet Blink can clearly display polarity with colored LED lights. Typically, low-voltage polarity testing is mainly used for telephone cables and power or signal lines of security surveillance cameras.

Bare wire test--length measurement

The distance to the short or open point is important information for fault detection and is also important information for material management and installer billing. Length can be measured electronically in three ways. The first method is to short the two wires together, then measure the resistance and convert the measured value into length. Unfortunately this method requires the use of a shorting cable and is rarely used.

You can also measure length using capacitance. Just like resistance, the capacitance between two wires will vary depending on their length, and the benefit of measuring length using capacitance is that you don't have to short the two wires together. Fluke Networks products such as the 620 LAN Cable Tester, NetTool Network Multimeter, and LinkRunner can use capacitance to measure cable length.

The third method is to use time domain reflectometry (TDR), which is similar to using a wave detector to test copper wire. Waves are sent down the cable, and when they encounter an open or short circuit between the wires, the waves are reflected back. The time it takes for the waves to be sent and returned is recorded to determine the length of the cable.

The transmission time varies due to the propagation rate constant of different types of cables. This constant is usually marked on the cable box, but a TDR with a reference length of the cable can also be used to calculate a constant with reasonable accuracy. Products such as MicroScannerPro cable tester, DSP-4300 and OMNIscanner2 digital cable analyzer all have TDR functions.

Bare wire testing - fault detection and material management

Basic management of material usage is a daily task. At the beginning of each workday, check the length of cable in the box or on the spool to be used. If you use a TDR or capacitive tester, there is no need to short the ends of the cable. Write down the value and then confirm that it is enough for the length of cable you need. At the end of the day's work, check the length of the cable in the box or on the spool again. Subtract the length measured this time from the number measured before the work started, and you will get the length of cable used that day.

Another example of length testing is troubleshooting before you finish a job. Let's say you're doing a TDR for wiring continuity and suddenly discover a short 10 meters away, but your records say the link should be 50 meters long. It could be that the drywall installer just happened to drive a nail into a spot, and you might be able to find the short and fix it (unlike with twisted pair, where you'd need to replace the cable).

Bare Wire Testing - Locating Cable Faults Using an Audio Generator and Probe

How can you find the point in the cable where the nail is? Does your TDR show it's ten feet away? You can use an inductive tone generator and probe such as Fluke Networks' MicroNet BlinkKit to test it. Connect the tone generator to the two wires and apply power to them. The cable will transmit a signal through the drywall. Then use a Fluke Networks MicroProbe tone probe to capture this 1000 Hz signal and convert it into an audio signal to quickly find the fault point.

Capturing such a signal can be both an advantage and a disadvantage to your work. You can use this signal to detect wires in a wall, but you may run into trouble if you encounter two wires running side by side. One of the unpowered wires may capture the signal of the powered wire, making it difficult to distinguish them. To avoid this trouble, leave a certain distance between the two bare wires when running the wires. In addition, the wires will collect noise from fluorescent lights or running machines, which should be minimized to ensure the speed of wire tracing.

The bare wire test methods have been introduced. For your work, their significance is not only beyond simple connectivity testing, but also can help you reduce the time of fault detection and handling. I hope readers can master them.