Sparks Electrical News July 2021

CABLES AND CABLE ACCESSORIES

6

Oscilloscope vs. multimeter: Choosing the right tool for testing and troubleshooting

Selecting your electrical cable S electing your electrical cable is one of the most important steps in ensuring the longevity of your electrical installation and the correct running thereof. SANS 10142 carries an intense array of tables that will be used in this selection process. Step 1: Load determination The first step is to decide exactly what the required load is and its characteris- tics are. That is to say, as an example, 100 A motor load that is to run daily for two eight-hour shifts, for instance. The motor is also 3-phase DOL preferred. Location 25 m away from the DB. Step 2: Ambient derating The anticipated local ambient temperature should also be stated at this time as the cable would require derating, possibly if the daily ambient temp ex- ceeded 30°C. Step 3: Possible cable type preferred From copper types available, we could use 3/4 core or single cores. The 3/4 core is probably more popular but the method from the supply DB to the load must be chosen. This is also a major point of selection as the cable method does determine the load capability of your cable. Step 4: Voltage-drop maintained There appears to be a general perception that a 5% voltage drop is mostly allowable as per SANS 10142, so at this time, any cable selected as probable must either be checked by calculation or maximum cable length tables. Step 5: Method of delivery Depending on the cable routing/method of delivery, additional derating may be required. Many of the above steps may have been done previously and do not have to be done continually from these first principal steps. The supply side of the many municipalities in South Africa have recognised standards as the loads are standardised in many ways. This can obviously make life far easier for the electrical staff. Many of these supply authorities have also introduced aluminium cables due to the theft risk of copper. Aluminium brings with it a select range of challenges to ensure that corrosion on site does not reduce reliability, especially in the humid coastal areas. On no account must Al/Cu be coupled together directly. Even if the cable has been correctly chosen and specified, it has to be care- fully ended and connected in order to control the localised heating that can easily result in a connection burnout, the so-called hot spot which will wreck the joint and potentially also the protection breaker. In both images below, you can see an electrical connection being made via a steel bolt. This will result in hot connections. Steel is not to be considered an electrical conductor from the power current perspective. This shows a to- tal lack of knowledge of what is to be achieved with a low resistance power connection. Similarly, a misplaced steel washer can cause a major incident if incorrectly placed between contacts. By Kevin Flack Flack has recently retired from industry and believes he still has valuable in- depth knowledge of the South African electrical landscape due to wide experi- ences gained in his 45+ year working career. He is keen to explore the training arena so that this can be passed on to the youth so as to benefit their practical ability in the electrical field. In these trying COVID-19 times, Flack can provide online training to keep your staff up to date on all aspects of circuit breaker deployment.

T he difference between an oscillo- scope and a digital multimeter is sim- ply stated as ‘pictures vs. numbers.’ A digital multimeter is a tool for making pre- cise measurements of discrete signals, ena- bling readings of up to eight digits of resolu- tion for the voltage, current or resistance of a signal. An oscilloscope is designed to depict waveforms visually to show signal strength, wave shape and the value of a signal. But how does the difference between numbers and pictures translate to real-world testing or troubleshooting? Why use a digital multimeter? Handheld digital multimeters typically have 3,5 to 4,5 digits of resolution and good ac- curacy. They are portable and lightweight, used typically for front-line testing and general-purpose measurements. They also contain advanced functions for special- purpose testing, such as: • Min/max • Conductance • Relative reference • Duty cycle/pulse width Logging You can also find high-accuracy (5 to 8 digits resolution), bench-type, line-pow- ered digital multimeters that are not in- tended for field use. These digital multime- ters are used in the lab, mostly for research and development or for production sys- tems. An advanced bench precision digital multimeter can cost as much as a portable oscilloscope. Why use an oscilloscope? Oscilloscopes are designed for engineer- ing work or troubleshooting systems that might contain complex signals that send at speeds much faster than a digi- tal multimeter can capture. Scopes have much faster measurement engines and much wider measurement bandwidths than digital multimeters, but typically do not have the same accuracy and resolu- tion strength as a multimeter. Oscillo- scopes generally have a resolution equiva-

Fluke 87V MAX Industrial heavy-duty multimeter

lent to a 3,5 to 4-digit digital multimeter. One of the advantages of the oscillo- scope over the multimeter is that oscillo- scopes can also display complex signals (the ‘picture’ part in the adage) visually. A transient signal that may pose a threat to a system can be displayed, measured, and isolated with an oscilloscope. It will also graphically show distortion and noise that may be present in the signal. Oscilloscopes can be either line or bat- tery powered; they can be big or small. Battery power and smaller size, for port- ability, are typically required for field use. Some oscilloscopes have built-in multim- eters like the Fluke 120B ScopeMeter, giv- ing the user both numbers and pictures. In many cases, these kinds of oscilloscope can replace a multimeter. Oscilloscope vs. multimeter Be sure to keep your digital multimeter with you for any electrical work. Grab one to make high-precision checks of voltage, current, resistance, frequency, and other electrical parameters. If a red flag is raised with your multimeter, you may need to grab an oscilloscope or more powerful tool to diagnose further. Reach for an oscilloscope if you want to make both quantitative and qualita- tive measurements. For general-pur- pose maintenance or general electronic testing, a digital multimeter is fine, but

Fluke 120B Industrial ScopeMeter handheld Oscilloscope

when testing or troubleshooting machine controls or other complex systems, or when doing electronic design work, an oscilloscope is needed. Industrial electronic applications like automa- tion and process control: an oscilloscope with two isolated inputs and 60, 100, or 200 MHz bandwidth is the choice. Industrial machine applications measuring three-phase power electronics, or three-axis control systems comparing and contrasting multiple signals: an oscilloscope with four iso- lated input channels and 100 or 200 MHz bandwidth is ideal. Industrial network applica- tions: some oscilloscopes add industrial net- work physical layer analogue measurement algorithms to validate the network health.

Enquiries: +27 (0)10 595 1821

Email kevin.flack@outlook.com

SPARKS ELECTRICAL NEWS

JULY 2021