Computing stuff tied to the physical world

Ground noise

In Hardware on Jan 12, 2012 at 00:01

To try and improve noise levels during measurements (and as general ESD precaution), I went “green”:

DSC 2851

That’s a green ESD mat, covering almost the entire workbench. It’s hooked up to the radiator for grounding. Note that the mat only provides a “dissipative” connection to ground, the surface still has several MΩ’s of resistance. It’s just to get rid of static electricity and to offer a clean protective working surface. Got the mat from Farnell, BTW.

Here’s what I see when pushing a scope probe onto the mat:


A clear 50 Hz pattern of a couple of volts (the amplitude increases when the probe is pushed more firmly onto the mat). This is with a standard 10 MΩ high impedance probe. The big puzzle is: where does this come from?

My explanation for now, is that the scope ground is “floating” a bit, due to the different devices hooked up to the house wiring. Note that the mat is tied to the radiator, not to the ground of AC mains. Since there isn’t any current flowing through the heating system (I hope!), I’m inclined to trust it more as being the “real” ground potential.

It’s no doubt completely harmless. Measuring AC current between these two ground levels with my most sensitive multimeter (a Voltcraft VC940), I see a very occasional blip of up to 0.20 µA flowing. Well under a microwatt.

To give an idea how crazy things can get, here’s the mat with nothing connected – turning it into one big antenna:


Who knows, maybe one day we could harvest even this sort of energy, eh?

  1. One of my earliest electronics experiments was pressing a scope probe into my “lab bench” (sheet of unfinished plywood), with similar results as you show here. There was enough humidity that the wood was somewhat conductive, maybe even more than the static mat. You also see something similar just touching the probe with your fingertip, yes? Electrically I think your body is about 100 pF to ground in series with 1 Mohms or so, depending how sweaty your hands are.

  2. I wouldn’t trust that radiator as a proper ground for one second, unless I’ve personally verified that there is copper tubing running all the way to the metering cupboard and then to a proper ground. Too often radiators in modern houses have coaxial tubing (plastic-aluminium-plastic) where the aluminium is only to block (oxidising) gas exchange and not cut into by the rings that connect the tubing to the manifold, let alone a proper ground. My 2 cts worth. Can’t think of any alternative but putting the whole setup in a Faraday cage though.

    • Hmmm, don’t know about your country, but in the UK central heating and water systems have to be earthed, and the resistance tested to earth tested with some sensitive equipment. If parts of the system are isolated because of non conductive pipes they must be bridged.

      Just my 2 pence worth ;-)

  3. Check this: They have RF harvesting chips but they are quite expensive ($50)

  4. If the tubing is properly grounded it forms a huge ground loop together with the AC mains’ ground wires. I’d expect better results from connecting the mat to the mains earth wire. That would also match the general recommendation that grounding should be as close as possible to a star topology.

    BTW, thanks for adding the edit function for comments.

  5. The heating system should NOT be used as a ground unless all of the following are true: 1) It is a continuous loop of copper pipe. 2) It is bonded to the electrical ground i.e. earth ground. 3) there is 0.0 potential between it and earth ground. Without all the above being true, you are not only creating a potential source of voltage, but it it could injure or kill someone. If you had a short to ground in your device to the mat, it would not trip a breaker or fuse AND it would energize the heating system piping. Someone in another room could touch the heater while also touching ground and be shocked. While you do not see much voltage right now the fact that you see an “occasional blip” would indicate to me that in fact the mat is not actually at ground potential. This could be dangerous to you and any equipment connected to electrical ground if whatever is supplying that “blip” became stronger. OTOH, the blip could be a problem found with the veracity of the electrical ground in the recepticle you are comparing it to!

    • While I share your concerns, and will check more carefully w.r.t. heating ground levels, I don’t think the second half of your comment applies. The mat has 10,000 MΩ resistance (10 MΩ on the bottom side), so no matter what you do 230 VAC can only lead to currents of under 1 µA. It’s an ESD mat, intended to dissipate 2 kV and above potentials only.

  6. Just as a case based personal history: I’ve been shocked by radiators in several houses. And while all fuses were pulled, got 240v jolts through touching a radiator tube and wet plaster at the same time. Try and plaster around radiatortubes that way :)

    Lessons for me: Don’t explicitly trust a radiator to be earthed. Don’t trust the wiring of your neighbours…

  7. Ok, so much for the radiator … 1.6 MΩ to AC mains ground. Time to rethink!

    • Blimee… best you get that tied together properly, or work out how to make use of your central heating RF harvesting array ;-)

  8. Bratib, your cautions about using an unknown plumbing system as a ground source are in general correct. However, if it really is an “all copper” system, modern house wiring practise requires bonding to ground, usually from a source at or near the main power entry distribution board.

    Since the intrinsic resistance of the mat is in the MΩ region, any fault current is limited and unlikely to cause a significant shock hazard as you describe – of course verifying the bonding is in place is a wise precaution. Correctly installed RCD protection will activate (if there is enough current flow) in the scenario you describe.

    It is unlikely to see true zero potential between different paths to a ground reference – the finite cable/appliance leakage currents are small, but unbalanced. The small voltage drops in the resistance of the different paths will be seen as an AC potential difference across those paths.

    This is why RCD’s ratings are usually limited to 0.030A (or sometimes 0.010A); higher sensitivities are possible and would seem “safer”, but at cost of frequent false tripping.

    • I agree with you, but that is current installation code and may not have been code when the system was installed. Of course that is assuming that it was installed correctly and was not changed after the install. And not all electrical systems have RCD protection as that is also a recent code requirement. I have seen many heating systems that are not grounded. I have run service calls that were due to the “heater shocking me”. A hydronic heater system that is not grounded and used as a “convenient” ground for an electrical device can be unsafe if the grounded device shorts out. I most cases these were in old houses and the electrician must have assumed they could attach the ground to the cold water pipe, which turned out to be the heating pipe. While the mat is not likely to cause injury, the cable leading to the mat would be at line voltage potential. Due to the size of the conductor a properly bonded copper heating system would be closer to ground potential than a recepticle ground but is unreliable because it was never designed for that use.

  9. Why don’t you modify a power plug so that it only has earth leads, tie the ground of the mat to those leads, and plug it into that wall outlet you alreadhe have overthere?

  10. As above, but include a nicely “lossy” ferrite bead – it will cut down on RF signals induced in your “earth” cabling getting distributed through the mat to your UIT.

  11. By the way, heating systems MAY be conductive enough to do a gross discharge of a person with a static charge, but they are nowhere near good enough for experiments. In fact, those systems could become charged by the water that flows through them alone!

    There’s a fat chance that, even if you have a full copper system, the ground wire of your AC-mains has less noise and a lower resistance.

    Regarding the “more true ground” feeling: It is false. Heaters are connected to AC-mains ground and the AC mains ground goes into the ground inside (or very near) your home

  12. This reminds me about when we tried to outsource testing to Russia. The facility where these proposed testers worked was old and not well equipped. It appeared that both heaters and the earth pin in some of the power chords had a big chance of being used for data traffic(!!!) by other companies in the building of course… so noone trusted either as proper ground. :-O

Comments are closed.