When using modern, high technology measurement devices one can often be tripped up by the simplest things. The most common is the ground loop. Time and again this issue rears its head. So, let’s talk about how to avoid a ground loop.
What is a ground loop?
Most sensors and their cases will be made from metal. Usually the mechanical structures under test are made of metal. These metals are normally ferrous and, almost always, will conduct electricity.
Generally, accelerometers are of the non-isolated type. That is, the case of the accelerometer is electrically connected to the ground of the cable that carries the sensor signal.
This means the electrical potential of a large metal object being tested could be at a different potential to the measurement system. Perhaps the measurement system is running from mains power or a battery. The result is that a current is induced in the cabling between the measurement system and the metal test object. Thus the cables carrying the signals are corrupted by the current flows. The only solution is to correct the issue and stop the current flow. Another ‘easy’ solution will not have the desired effect.
How to avoid a ground loop
Normally, one would ensure that the measurement system and any computers using the measurement system are powered from the same source and so they will be at the same potential. Even this is not as easy as it sounds. Low cost power supply units often omit the common ground path from AC/DC input to DC output.
Good quality measurement systems will always have a ground connection. One could attach an accelerometer to an engine block and then ground the measurement system to the same ground as the block. This would be the vehicle 0V. Then, if there is an issue, the ground cable can make the connection and so any current flowing will not adversely affect the signal path. The 0V should be the same 0V that is powering the measurement system and any laptops.
It is always best to ensure that the piece of metal under test is itself grounded.
Are there any other considerations?
What is the catch?
Sometimes the above will not help, but running the laptop from its internal battery rather than from the common ground will. Technically this should not help or even work. However, experience has shown that although we don’t always understand what is happening, sometimes it makes the difference. That is why we call it the black art of ground loops!
What can be done to help this issue?
It is possible to apply electrical tape between the base of the accelerometers and the metal they are physically connected to. This will isolate the accelerometers and they will still respond in a similar fashion, but it is likely that the tape will change the frequency response of the accelerometer. Using a product like Blu Tack would mean the accelerometers may not respond as expected, as it will act like a spring. Also it will melt easily.
Petro wax is the most commonly used method for affixing accelerometers. Unfortunately, that method does not ensure that there are no electrical connections between the accelerometer and the test piece.
An acceptable solution is to purchase isolated accelerometers or isolation bases for the accelerometers.
What about impact hammers?
When using impact hammers a metal tip is often required to excite at the desired frequency range. These hammers are usually not an isolated design and so when an impact is made the signal can become corrupted. In these cases, it is not possible to apply a material (e.g. tape) to the tip or use a non-metallic tip. This would adversely affect the excitation frequency range. The only solution is to ensure the test piece is properly grounded.
My equipment is connected to the mains power socket, so surely it’s grounded?
Well, not really. It is likely that the earth connector in the wall socket is not actually connected to anything and therefore the potential of the supply is actually floating relative to another supply. In my experience this is a big problem in the Far East. If that is the case there is no choice, but to ground the equipment directly. Usually, with a large pole buried into the ground.
James Wren
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Ground loops are most problematic when high power electric machinery is in connection with the accelerometers. A good example is an electrodynamic shaker what can have a 40 kW amplifier running from 3 phase mains. Shaker table is connected to protective ground for safety reasons. Other protecting ground used for the shaker controller can close a physiically big ground loop with that. Only safe method is to use insulated base accelerometers. Other means of insulating the accelerometer base are never as good as an accelerometer with a built-in insulation, I agree.
Hi Peter,
Thank you for your comments. Your quite correct the isolated base is the best solution.
Ground loops were always a large problem in the Power Generation industry. I remember one power station (now demolished) that had four earth points, one in each corner. There was an earth potential difference between them of several volts, not significant in the thousands of volts of the generator outputs, but a major issue when trying to measure microvolts from accelerometers. We used armoured and screened cables, with the armouring earthed close to the turbine and the signal screen only earthed back at the measurement system. the internal circuits of the accelerometers were insulated from local ground. Having a differential (floating) input on any measurement equipment also helps.
Hi Terry,
Thanks for posting on our blog.
Great story and that matches with our experience perfectly, power stations are very complex electrically speaking. Thanks for sharing your wisdom.
I had a simmilar problem at an animal rendering plant, I was able to balance the centrifuge decanter not connecting the measuring equipment to the utility line rather running it directly from the battery.
Hi Wolbeck,
Thanks for sharing your experiences.
This sort of information is very useful, each person’s experiences are different when it comes to grounding. As we all share we will have more ideas to solve these issues in the future.
Thank you.
I faced a similar problem a few days back . I was measuring engine vibration on an engine test bed , which consists of an eddy current based dyno. The dyno & the engine are mounted on the same metallic bed. Now when i installed my accelerometers on the engine block directly & took some measurements & then analysed that data in frequency domain , i found really high v g values in the frequency range of 4000-5000 hz which is something that i have never encountered before. Also when i installed the accelerometers with some isolation tape that high frequency data was considerably reduced with drastic reduction in the g rms levels. Both test bed & the measurement system were grounded to the same ground . Can somebody please provide some insights & the possible cause/causes of the aforementioned problem.
Hello Vishvajeet,
Thank you for posting on our blog.
As soon as you mentioned the eddy probes the dots started to connect. You see eddy probes often work with a high frequency modulation signal to create a magnetic field, the value measured and the signal returned by the eddy probe is proportional to the variation in that field caused, by a magnetic object passing through the field. I would expect that the modulation signal was making it’s way into your acceleration signal path. The use of the isolation tape stopped or reduced this.
I would recommend double checking the ground used with the eddy probes, it may not be quite as you expect.