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Apparent Power Monitoring

Choosing the right Data Logging Tool for Job

There are many relevant terms used to describe the methods for gathering information about electricity – load, demand, kVA, kW, apparent or real power, etc. With the advent of computer technology, the data can be interpreted in many different ways, but today’s economy demands that jobs be accomplished with the least amount of effort in order to remain competitive. There will always be more technologically advanced devices on the market, but do they make the task easier or more complicated? Separating these measuring tools into skill categories helps make the buying decision easier. 

SKILL LEVELS

Looking at the measurement products in the market place, they can be associated with levels of expertise.

Level Measurement Tool
Novice Handheld Clamp-on Meter
Intermediate Self-Contained Data Logger
Expert 3-Phase Power Analyzer

Novice

First look at the novice or entry-level approach. The analog clamp-on style Amp meter gives the electrician a relatively simple tool to check instantaneous load currents. Its needle is naturally dampened by a slow response to fast changing Amperage, making it simple to read and record at a glance. Next is the digital version with its many features which mask the fact that it acts differently than its predecessor – it digitizes at a fixed rate and has a jerky response to the eye as the digits update and finally settle on a value. No, it flips another couple of digits. So which reading should be recorded? The next logical step is to log this information automatically in order to get rid of the indecision.

Intermediate

Suitable for the intermediate level, Data Loggers using split-core current transducers have filled an industry need for portable test tools that give long-term data while being reliable and cost effective. Like its former counterpart the Analog Clamp Meter, a 10-year battery powered data logger does not require battery changes in the field. This is important when recording data because there is no wasted effort caused by dead batteries and the data is always available at the end of the session. A passive probe means there is no need to tie into any power source whether in the panel or outside. Less reliable recorders depend on rechargeable batteries; others require a wall adapter to be plugged in.

Expert

Experts in the field use a 3-Phase Power Analyzer with no less than 3 current probes and 3 or 4 voltage clips, with a display and power supply. These analyzers usually take considerable time to setup and use depending on the technician’s level of expertise. This is adequate for special applications, but for routine work like determining existing loads, the analyzers are over-kill. Considering the higher cost and lack of usability in tight spaces, they are not meant to be left in the field. However, Power Analyzers do give a lot of useful information when troubleshooting and is the tool of choice when looking at complex wave forms.   

Breaking it down, you get what you pay for, but do you really need all the extra features outlined in the promotion material? Not if you want to save money and keep the job simple.

REQUIREMENTS

The requirements are related to job function and can be broken down by project phases. 

Phase Electrician Technician Engineer Concern
1 - Plan Data Study  VA
2 - Build Installation Design  VA
3 - Commission Troubleshooting

Verification 

VA
4 - Operate Maintenance Performance  VA-W-k-kWh

Plan

The best example is the NEC requirement to Determine Existing Loads before a project begins. The electrical code states that in lieu of a year’s worth of energy demand data, Amperage recorded for one month at 15min intervals is required. In addition the peak value must be adjusted for seasonal loads. What is actually required? The peak Electric Current and the name plate data for either the heating or cooling system.

The Standards Code is asking that the electrical components be sized for FLA, with the appropriate design factor applied and that everything be accounted for in the calculation. The Code is not asking for real power applied to the load in kW because the system needs to handle the maximum peak current seen by the circuit. This is where the requirements and the features get blurred.   

Given that incoming power is usually the purest sine wave and the electrical code does not stipulate otherwise, if you measure True RMS you are actually running the risk of sizing the conductor too large. In other words as:

Average RMS = True RMS for a perfect sine wave.

Most loads are inductive, or worst case, are switch mode conduct only at the peak of the waveform, the True RMS current reading will be higher than the Average RMS measurement for a given load. 

There are two schools of thought here, one is to size everything larger based on True RMS, driving up material costs, or to pay more attention to mating more surface area in connections and splices where the failures occur.

The world has come full circle and we are back to requiring the equivalent of the Analog Clamp Meter with the addition of recording readings at a preset interval - either time stamped readings, or better yet, a printed graph. This is the premise of a Portable Data Logger; it records into its memory for backup later. By loading software onto a laptop or computer in the office, the logger is set to take readings either continuously or to stop when it is full. Data Loggers can use Average RMS Split Core CT(s) which are non-invasively attached around the current carrying conductor(s).

Build

The design Technicians and Engineers are interested in apparent power in kVA for equipment sizing, so again, Average RMS is more relevant for the reasons stated above. The real power applied to the load is considered when selecting the equipment used to meet Energy conservation targets – this effective power or Watts is a concern for energy audits. Power factor comes into play only when adding capacitor banks and is already provided by most equip. manufacturers in order to meet Energy efficient standards.

Measuring voltage adds unnecessary cost and setup time to data recording when considering the above facts. During the design phase voltage can be assumed as it will not be known for sure until the project is complete. So if tests are necessary to assure that devices meet specification, it is prudent to use the rated voltage from the name plate rather than the voltage applied. With Data Loggers this is accomplished by keying the value into an equation that calculates kVA. 

Commission

On the practical side, all these instruments can be used for a load balance test, however the steps involved require the Electrician to make changes immediately.  Using a Clamp-on Meter and moving it between phases after switching load combinations on/off and writing down the readings is a pain, especially as a one-person job. The Data Logger has the ability to record all phases simultaneously and can display them in real time, as can the Power Analyzer.    

How much time has actually been saved? For the electrician, probably none, as changes need to be made at the distribution panel in order to correct imbalances. However, during the commissioning phase, recording can be invaluable as systems are tested and brought on line without the need for an electrician. This is especially true for other trades such as HVAC-R, when they are working independently and are responsible for significant loads such as large fans and cooling.

Power Analyzers are invaluable for troubleshooting anomalies, as their manufacturers point out. However, the trick is getting the bulky instrument inside the distribution panel when it has to be left on site. Not to mention, leaving an expensive piece of gear in the field and hoping nobody steals it. Longer term 3-phase recording can be accomplished by sub-metering, but when tracking down offending loads it helps to have a recording device to move around the plant. 

Operate

Operation and maintenance depend on gathering similar information previously described for upgrades and troubleshooting. Verification and performance on the other hand, look for proof of meeting the project specifications (which are designed from the requirements). This is generally a high level task that amalgamates data for reporting purposes.

Verification is done in the field, making sure each sub-system performs to spec. The easiest way to prove, or disprove, whether requirements are met, is to show a picture – full featured trending software plots dissimilar data on a graph, showing performance over hours, days, weeks, months, or even years. An example is plotting temperature against power consumption to account for the conditions that exist at the time the data was captured.       

Next the question arises – can an Average RMS Data Logger be used to record kW or effective power? The answer is yes, if consideration is given to where the data is collected, the benefits of using a data logger can outweigh any inaccuracy. Power from the utility should be pure; otherwise penalties that show on the billing have to be paid. This means PF is close to 1 and voltage and current waveforms are sinusoidal. If this holds true throughout the physical plant, which should be the case with quality equipment, a portable data logger is advantageous. The Power Factor can also be keyed into the software to approximate billing and applying a compound equation can integrate into kWh.

CONCLUSION

Each of the tools outlined have a role to play in the job functions and project phases. Handheld meters are quick and easy to use. Power Analyzers are ultimately useful instrument. Portable Data Loggers are the best of both worlds when it comes to gathering data from the field - they are relatively simple to operate and their compact size makes them easy to deploy. An interface cable and software is all that is needed to complete the kit when using an estimating computer as the viewer and printer.

What does this all mean to the contractor? Looking for the simplest solution does contribute to the bottom line both from a capital cost and manpower perspective. The job gets easier with the right tool.

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