Want to know about the new hand-held test instruments currently available to you? These new breeds actually produce better results than most of the heavy, tabletop, and laboratory-grade equipment available just a few years ago.
Most of our diagnostic and setup work involves portable, handheld test equipment. Small and portable is no longer an automatic indication of significant limitation on measurement performance.
In fact, you can successfully do most servicing and diagnosis with just three major items: The true-rms digital multimeter (TRMS DMM), the power/harmonic meter (PHM), and the dual-channel digital storage oscilloscope (DSO). You may want to have the digital graphic multimeter (DGM) in your toolkit requirements, depending on what features you need. But, if you're like this author, you won't be satisfied unless you have all four instruments in your toolkit!
Sure, these wonderful tools are available to us. But, how best to use them is something not often discussed. Many believe the required usage skills are just picked up along the way, basically through experience. Nothing is further from the truth, however!
The fact is, we all can benefit when the methods for using this equipment are written down and available to study and review. This should be done before we try to use the devices in real-world situations, where time is money. Let's look at how to use (and not use) some of the more commonly available items of modern, handheld test equipment.
The digital multimeter. The modern DMM replaces the venerable volt-ohmmeter (VOM). It's exceptional because it incorporates many more features than ever available on the VOM; like frequency, counting, voltage and current peak hold and averaging, dB measurements, data recording, memories, and current transformer (CT) input, to name a few. Interface capability is available on some models, allowing you to connect the DMM to a computer. Here, you can use some advanced software packages to further "crunch" and display the accumulated data.
Remote control of the DMM from the linked computer is also possible. All these are important features, and you should know how to use them if you want to use the DMM to its fullest.
The most important feature on the modern DMM is its ability to process the input voltage and current waveforms as true-rms (TRMS) signals. This is opposed to average-actuated, rms-calibrated ones. Hands down, this one capability sets a good DMM apart from one that can get you into trouble. Here's why.
Many modern loads are of the nonlinear type and can cause harmonic voltage waveform distortion on the electrical supply circuits. This will result in significant reading errors when you use a DMM not of the TRMS style to measure voltage. Typically, all your readings will be lower than what they actually are. With the TRMS type, your readings will be correct.
Getting an erroneous low voltage reading on a circuit can cause you to misdiagnose a problem with the involved equipment. It may even lead to misguided attempts at upping the voltage by resetting transformer taps, etc. If you do raise the voltage due to this measurement error, then the involved (and maybe other) equipment will really have a problem with input voltage!
On current waveforms, where a CT is used, you can easily have a low reading if you're not using a TRMS device (where the load is of the nonlinear type). By definition, unless equipped with special current waveform circuitry, the typical nonlinear load will create a non-sinusoidal current waveform with less area under its curve than for an equivalent sinusoid. Hence, a low-current reading, unless the DMM is TRMS. Improper low-current readings make it impossible for you to know if a conductor, fuse, breaker, or transformer, etc. is being current overloaded or not.
The most important place to use a TRMS DMM is probably on the neutral conductor of a 4-wire, WYE connected circuit that's feeding line-to-neutral connected nonlinear loads. The neutral path, in this kind of arrangement, not only carries fundamental frequency current, but triplen and other harmonic currents as well. (See sidebar, "Triplens"). Making any current measurements on a neutral conductor or pathway while using anything other than a TRMS DMM is a certain way to get into trouble. It's the TRMS condition of a waveform (voltage and current) that directly relates to heat production in a conductor; this is what causes connections to burnout, insulationto fail, and fires to get started. Pwatts=I x R is the problem here, and expressing the current in TRMS terms is necessary to get an accurate indication of how much heat the current is going to produce in a particular resistance (e.g., an electrical joint, termination, conductor, etc.).
Using Peak-Hold feature on UPS equipment. Most uninterruptible power supply (UPS) equipment has a peak-current limit that establishes the point at which the UPS will either suffer voltage waveform distortion or collapse problems. If equipped with a synchronous static-switch, a UPS will automatically transfer to the bypass circuit until the peak-current overload condition passes. In either case, the UPS and load are not properly matched if these kinds of things occur during normal operation of the UPS and its supported load(s). Yes, you can sometimes observe these UPS-load compatibility problems by simply watching the UPS to see what it's doing. Yet, the underlying question is, "Why is the UPS doing what it's doing under the observed conditions?" The TRMS DMM and its peak-hold feature can answer this question.
If you set your TRMS CT-equipped DMM to record the maximum inrush current seen by the CT, you can then see if UPS-load compatibility is a problem and get a handle on how severe it is. Suppose an OEM specifies its UPS to be capable of a 150% current overload on its output before it exhibits either a voltage waveform problem or a transfer to its bypass. With a peak-hold TRMS DMM and a CT, you can actually see the current during the load equipment's various start-up and operating conditions. With this information, you can easily determine if it is approaching the 150% level and how closely, or if it is exceeding that level. This is especially useful to know if the load shows an unacceptably high inrush current only once in awhile, such as when it's first turned-on and where there's an intermittent, but very high input transformer core premagnetizing current produced.
Without a peak-hold feature, you can't do this diagnosis with any degree of certainty.
Sizing the UPS to its load. The TRMS DMM's peak-current hold feature is extremely useful when you want to know the peak-current capability of a UPS that's going to be purchased and used in a particular load situation. Knowing the UPS's peak current capability requirements in advance clearly beats trying various models and kVA ratings of UPS on the load until one combination gives satisfactory performance. Use your TRMS DMM to take inrush current readings and the "normal" operating peak-current readings; then, provide this data to the UPS manufacturer. It can then ensure the correct model and kVA rating of UPS on the first attempt and without experimentation. This is an obvious advantage to all parties concerned.
The power/harmonic meter. The PHM is a cross between an oscilloscope (but uses a LCD for its output instead of a cathode-ray tube), a spectrum analyzer (See sidebar, "Spectrum Analyzer") for 60 Hz power circuits, and a TRMS DMM. You can use the typical PHM as a replacement for many features of a TRMS DMM (and CT).
The PHM also provides direct access to percent of total harmonic distortion (%THD), crest-factor, apparent power (kVA), active power (watts), and power factor (PF) measurements. An interface to a computer and sophisticated software is also available. The typical handheld PHM is a single-phase device and will not directly handle three-phase measurements. Thus, you'll have to take single-phase measurements sequentially by moving the PHM's probes. Then you can log each reading, store them in the PHM's memory, or send the information to a connected computer such as a laptop.
Basic connection cautions. You can use the PHM with no complications as a TRMS or peak-reading voltmeter or ammeter. But, if you're using the spectrum analyzer feature for current measurements, you should note some precautions.
- Make sure you connect both the voltage and current inputs at the same time to the circuit you're examining.
A common error is to connect the PHM to the conductor by useof the CT input and then to set the PHM to the spectrum analyzer function. Sometimes this produces a usable display on the LCD, but usually it produces what appears to be an overload or out-of-range indication. In fairness to PHM manufacturers, their technical manuals typically show the PHM used with both the voltage and current inputs simultaneously connected. But, any clear instructions in this area are a bit hard to find. Here, experience becomes your teacher instead of the manual.
- Always connect the voltage probes in a consistent manner and always orient the arrow on the CT in the direction of the load
each time you make a simultaneous measurement involving voltage and current. If you don't, the PHM will produce erroneous PF readings. For example, flipping the CT's direction around during measurements makes the pf appear to lead or lag, depending on the CT's orientation, instead of its consistent lagging (or leading). This is also important when making dc measurements as the wrong CT orientation affects the polarity indication.
The handheld, digital-storage oscilloscope. The latest, and one of the most important instrument developments in recent years, is the handheld digital-storage oscilloscope, or DSO. Before the DSO was available, your only recourse was to haul around one of those well-built, but heavy bench-type units. Now, there are several well-built handheld equivalents for these venerable "boat-anchors." With more features, to boot.
Once you take the time to become proficient in its operation, you should get excellent results with your DSO. If you just take out a new DSO on an assignment before you really get familiar with its operation, you can waste a lot of valuable time and may not get results as good as you expect to obtain. So, take your time and get to know your DSO.
Then take it out on an assignment; you won't regret following this advice. There are many tricky things you can do with the DSO and all of its features, but they are worth nothing if you can't get the basics under control. And, you certainly don't want to damage the DSO through misuse.
Take note of the following probe attenuator advice.
- Make sure you use your DSO probes with the attenuator switches in the ON position.
This is especially important when you're making measurements on a circuit where loading by the DSO might produce a problem. Making measurements with the probes set to 1x instead of 10x, for example, may upset the operation of the circuit you're trying to check out. Also, the input to the DSO is more susceptible to noise coupling, which can affect your measurement, if the probe is unattenuated by having the switch in the 1x position. If you must use the 12xsetting, be certain the signal you're observing isn't significantly affected by the setting. You can do this by moving the switch back and forth between 1x and 10x, while watching the signal on the screen. Sometimes this will be difficult (such as when the signal is low-level), but you must do it to stay out of trouble.
- Be aware, the DSO will display higher frequency components on the signal when operating the probe in the attenuated (e.g., 10x) position, as opposed to the 1x position.
- Start out making your measurements with the probe set to 10x while you're getting the DSO's other settings properly arranged.
This will prevent damage to the input channel's amplifier, if you get too high a voltage connected to the probe. Once you determine the voltage on the probe and the channel you're using is safely within the range of the DSO's input, reset the attenuator switch on the probe to 1x, if required.
- Never connect a probe to a circuit unless you've checked the attenuator switch's setting and know it's properly set for the voltage level on the circuit.
Fixing a blown input on a DMM is expensive when you set it to the wrong voltage, but the cost to repair a blown channel on a DSO is guaranteed to get your attention!
Using differential input measurements. One of the most unsafe things to do is to use an ungrounded (e.g., floated) oscilloscope while making measurements on an AC (or DC) circuit of several hundreds of volts to "ground." Make sure you avoid this setup at all times. It is unsafe for the oscilloscope and you.
That said, how do you make safe voltage measurements between two points of a circuit where neither is at ground potential? The answer: Use a differential setup on a dual-channel DSO. It's a safe method, but only when you don't exceed the working voltage of the input channel probes to ground. Check your DSO's manual to determine this value and then don't exceed it for any reason. For DSOs that don't have differential input capability, or where the DSO doesn't have sufficient voltage rating, you can use available accessories to correct the problem. Some can extend the DSO's operating voltage, allowing you to take differential measurements safely.
The A-channel probe is normally connected to the higher voltage in relation to "ground" and the B-channel to the other. If both test points are equal in voltage to "ground," it makes no difference. Nevertheless, the A-channel is usually connected to the signal (voltage) designated as the phase or zero-crossing "reference," such as for the sweep trigger. You should set both of the DSO's channels to the same input attenuation, AC or DC setting, and V/cm levels.
The digital graphical multimeter. A recent addition to the handheld instrument toolkit is a sort of hybrid called a digital graphical multimeter, or DGM. This device incorporates some features of a DSO and a TRMS DMM. One standard feature includes an LCD that displays both digital information and waveshapes. Typical features are an AC voltmeter (300 kHz bandwidth) and ammeter, a 10 MHz frequency counter, a component test and logic test function, and a 1 MHz bandwidth oscilloscope type of waveform display.