Over the next few months, iGround will be presenting a series of blogs to address the methods of approaching the various levels of Power Quality site surveys, depending on the location to be investigated.
This installment focuses on Grounding Electrode System.
The range of testing with the Level 1-3 surveys are applicable at nearly every location within the electrical premises wiring system, with the exception of one…the Grounding Electrode System (GES). This area of the electrical distribution is no more complicated than a Level 1 survey. There are a number of reasons for this; some of them (the most important ones) will be discussed here.
Level 1 (Visual Investigation)
Recommended Tools and Equipment to have:
- Appropriate screwdrivers and wrenches
- Torque wrench to check tightness
- True RMS AC/DC clamp-on ammeter
- Earth Ground Tester (maybe…read below!)
The first step in evaluating the GES is knowing exactly WHERE it is. Sometimes, you can practically trip over it while walking around the area just outside the building’s electrical service entrance. In other cases, you have to do a lot of digging for that information…literally.
In some cases, you’ll have buildings with more than one grounding electrode connection. The biggest mistake I find people making here is prioritizing (in their mind) as to what connection is more important compared to another. For example, some may make sure that the connection to building steel is of high integrity and ignore a supplemental connection to a ground rod. Granted, the metal frame of the building will be crucial in nearly all aspects of grounding and bonding. But overlooking the possibility of loose connections in any path of the grounding electrode system is inviting disaster. When a lightning strike occurs, the voltage drop across the connection’s resistance can create transferred earth potential differences high enough to be destructive.
(See Figure 1 for example of electrode not bonded.)
In the same light, you might want to make sure that there are no separately driven rods for any equipment or electrical system that are not bonded to the main grounding electrode system. This may take some walking around inside the building, particularly peaking under raised floors in data centers. Separately driven electrodes are one of the most common causes of lightning damage.
Another visual cue to work on is investigating the presence of corrosion on the GES components. Corrosion can be caused by a number of factors; dissimilar metals that create a DC battery effect being one of them (more on that in a sec). A study done by the International Association of Electrical Inspectors (IAEI) twenty years ago found that the ‘life expectancy’ of a 10-foot ground rod may only be 12 years. Corrosive elements in the soil (naturally-occurring or man-made) and other contributors influence this but, in any respect, it’s worth taking corrosion into consideration.
(See Figure 2 for example of corrosion on an electrode.)
Level 1 – (Metering)
Let me start with the biggest mistake that most people make…if you are attempting to making an earth ground resistance measurement on an already-operational building…YOU ARE WASTING YOUR TIME AND EFFORT! The results are meaningless and you’ll have better success of ‘finding’ bonding & grounding problems if you hire a psychic. (Psychics cost less per hour, too…)
There is a lot of money to be made by equipment manufacturers of the earth ground test equipment, particularly those that sell the clamp-on earth ground testers. But earth ground resistance meters (of any variety) are only useful in extremely limited applications; many of those outside the realm of your premises wiring systems. Save your money and buy a…
…MICRO-OHMMETER! There are many manufacturers of these devices, some of whom make earth ground testers, too. It always perplexed me as to why they don’t market these more. Micro-ohmmeters can be useful when performing Two-Point Bonding testing between electrodes to make sure they are bonded together. Some testing groups recommend bonding resistances of 5 milliohms (0.005Ω). The more testing I perform in my career, the less I get hung up on a particular value. Though a low value is good (my personal recommended maximum would be 100 milliohms (0.1Ω)), I would just make sure that all readings are within a certain percentage of each other.
(See Figure 3 for Two-Point Measurement)
One very good application of the Two-Point Bonding testing is to measure the resistance between the Main Service Entrance Panelboard and the nearest metal frame of the building (or other electrode). This will provide a good indication of the integrity of the connection between the panelboard and the electrode…WITHOUT having to open up the panelboard and endanger yourself or others.
Also, I would make AC and DC current measurements at each of the grounding electrode conductors for the building. AC current measurements will be useful for determining the precautions someone will need to take when making mechanical connections (or disconnections). It is neither unusual nor unexpected to see AC current on the grounding electrode system, but just be sure to record it. (FYI…Personal Protective Equipment (PPE) can be cited for any electrical work in panels, etc., but it is ambiguous for grounding electrode systems because it is not expected to have amperage.)
YET, if the amperage is over 3 amperes on any grounding or bonding conductor, I would make sure someone is taking precautions to wear safety glasses and rubber gloves when tightening anything on the grounding electrode system.
Finally, DC current measurements are useful, yet often overlooked. No one really expects DC amperage on an ‘AC system’, right? Guess again. Dissimilar metals (i.e., copper, steel, etc.) can create a battery effect, which will accelerate corrosion. It can also be an indicator that an Uninterruptible Power Supply’s (UPS’s) rectifier, equipment power supply, or other form of AC/DC power conversion is nearing the end of its life-cycle. And, though your average investigator may not be aware, DC current on any AC conductor (known as DC offset) can trigger a tampering tariff by your electrical utility if it causes watt-meter errors. This tariff, or penalty, can be substantial for large companies so it’s in any end-user’s best interest to monitor it.
(See Figure 4 for example of DC Current measurement being made.)
The GES investigation is where your on-site investigation begins and has many important aspects to it. Your can retrieve a sample checklist, above, in the section for Recommended Tools and Equipment. It will cover all the things you may expect to encounter on a site (some we’ve discussed here), but feel free to personalize it for your own needs. And, though I’d never do an earth ground resistance measurement, you may…so that data is in there, as well. But, if you don’t use this checklist…USE SOMETHING!
In our next installment, we’ll take a closer look at verifying the Levels 1-3 issues of an AC electrical sub-panelboards, particularly regarding the issues where power quality is concerned.
PQ Surveys – Part 4
AC Electrical Panelboard Investigations