15
\$\begingroup\$

I just got myself some new toys for my lab at home like 30V DC power supply, KKmoon signal generator and a Rigol DS1052E. I want to float everything, because I don't want to spend any money on stuff like differential probes or isolation transformers. I am doing 100% DC stuff and want to do some probing in my circuits. My questions are:

  1. What is the right way to float my home lab?
  2. Do I float my three devices separately? For example, somehow removing the ground from the socket coming from the device?
  3. Can I plug everything into a mains power strip and remove the ground of the mains power strip before plugging it into the wall?

Please let me know. Thanks.

\$\endgroup\$
  • 36
    \$\begingroup\$ DON'T float your equipment. \$\endgroup\$ – JRE May 29 at 19:41
  • 8
    \$\begingroup\$ Why do you want to float everything in the first place? \$\endgroup\$ – Unknown123 May 29 at 19:54
  • 16
    \$\begingroup\$ One example of why you shouldn't float your equipment, is that if your scope is floating and you measure a high voltage, then your entire scope floats up to that. That includes things like all the probe ground clips that don't look like they're connected to anything, the probe ports on your scope that don't have anything plugged in, as well as the chassis right behind the buttons and knobs on the scope that you are poking around. \$\endgroup\$ – DKNguyen May 29 at 20:17
  • 10
    \$\begingroup\$ Spending money on safety is always the right answer. If you can't afford to do something safely, I'd re-evaluate whether it's worth doing. \$\endgroup\$ – Daniel May 29 at 20:35
  • 16
    \$\begingroup\$ It seems like a shame to buy all this nice new equipment and then die before you get a chance to enjoy it. :-/ \$\endgroup\$ – jeffB May 30 at 13:25
56
\$\begingroup\$

You can't float everything without an isolation transformer - the neutral will be connected to ground at the entry into the building. For safety any class I equipment does need to remain grounded, in my lab I have an isolation transformer only for the equipment under test, the scope and power supplies still have the case grounds, and are tolerant of the inputs/outputs floating up to 50V from ground (per their manuals). If I need to measure high side DC stuff, I can connect the high side to the scope ground (since the isolation transformer allows that offset) but a differential probe is still needed for measuring small signals with a large offset - it's far cheaper to buy one than smoke an oscilloscope

\$\endgroup\$
  • 12
    \$\begingroup\$ If I could, I would upvote this a thousand times. OP, if you really, absolutely, positively, unquestionably need to make a floating measurement, float the DUT, do not float your equipment. Stay safe out there. \$\endgroup\$ – Vladimir Cravero May 29 at 20:52
  • 2
    \$\begingroup\$ Grounding is generally the simplest way to operate class I equipment safely, but that doesn't mean it's the only way. An ungrounded scope would pose two hazards: 1. internal faults, which would be rendered harmless if the scope was powered by its own safety isolation transformer; 2. the scope case becoming live as a result of the scope's ground lead being connected to high voltage in the device under test, which would only be possible if the device under test had a source of high voltage. Are there any dangerous scenarios I'm missing? \$\endgroup\$ – supercat May 30 at 20:06
  • \$\begingroup\$ @supercat That's about it. As you say, having an isolation transformer on the scope supply provides the supplemental isolation that allows it to run without the grounding, but can then mean that an internal fault is invisible, until another fault occurs somewhere else. My case is testing equipment (mostly motor controllers) that have high voltage, non-isolated supplies built in that would then charge the case on the scope even without any fault in the EUT. \$\endgroup\$ – Phil G May 30 at 21:06
  • \$\begingroup\$ @PhilG: Testing a mains-referenced device with a floating scope ground can be dangerous, but mains-referenced devices aren't the only things where an earthed scope ground could be problematic. I wonder how hard it would be or how much it would cost for a scope to include a coaxial contactor assembly which would disconnect all the probes (both power and ground leads) if any significant current flowed through any of the ground leads? \$\endgroup\$ – supercat May 31 at 13:26
11
\$\begingroup\$

What is the right way to float my home lab?

If you want to make measurements that are isolated from ground, the only way to do this is with an isolation transformer if your scope is not isolated. There are very few reasons to do this, a high voltage setup would be one reason. Some AC measurements would be another. Differential probes are best.

Do I float my three devices separately? For example, somehow removing the ground from the socket coming from the device?

A reason to do this would for breaking a ground loop on the scope, so that is most likely the only piece of equipment that you'd need to do this on. If you are doing this for AC measurements, the scope could also be floated. It is unsafe to do this on power supplies, where will the fault current go? Not to ground.

There are better ways to eliminate ground loops, one being a differential probe (kind of pricey). Another would be minimizing the grounds between devices, and making sure they are not plugged in on different circuits or plugs. (I've had a few times where this was an issue)

Can I plug everything into a mains power strip and remove the ground of the mains power strip before plugging it into the wall?

No, also very unsafe. No path for a fault current.

\$\endgroup\$
8
\$\begingroup\$

There is no right way to float your home lab.

1) No. Right way is not to float equipment that must be grounded.

2) No. Don't float any of them. Equipment that have grounded plugs NEED to be grounded for a reason.

3) No. Because again, equipment with ground pins need to be grounded! Having all lab equipment and the device being examined being connected to single power strip at least makes them being powered from same mains phase and having a single point ground reference.

\$\endgroup\$
7
\$\begingroup\$

If everything you do is DC, all you need is a DMM.

Likely what you meant is that it is low voltage but DC to high frequency.

Earth ground is advantageous two good reasons;

1) safety

  • the line filter noise currents to the metal frame will go thru you if not earth grounded but your body has stray capacitance to earth
  • line transients

2) performance

  • EMI reduction with lower CM noise which will be induced into your high impedance circuits.

You may want a consider a static-dissipative work surface, flooring, shoes, wrist strap, soldering iron.

Proper high-frequency DC supply measurements

When it comes to measuring supply ripple, you need an earth grounded low impedance AC coupled coax direct to the scope with an earth ground for a low impedance. A floating ground would inject a high common mode voltage that could get inject noise current into your DUT.

it is always best to measure AC coupled with an external Cap to scope using coax without a probe terminated with cable impedance using an internal option or using a BNC T with 50R inserted.

You don't need a differential probe if you can make one. Most measuremenats can be made with A-B with two matched 10:1 probes. WHen probe ground inductance causes resonance > 20MHz, simply remove clips and use tip and ring.

\$\endgroup\$
6
\$\begingroup\$

You certainly do not want to float your equipment if you still have earthed stuff around you, or literally under your feet. That is especially true for (grounded) anti static mats and wristbands. That way lies an early grave.

\$\endgroup\$
2
\$\begingroup\$

if you want to float your instruments use battery powered instruments, trying to float mains powered instuments doesn't work well, and is potentially dangerous.

Else power your experiments from batteries or other isolated supplies and then you won't need to float your instruments.

\$\endgroup\$
  • \$\begingroup\$ In addition, any instruments which may conduct current between test leads and any exposed metal (including other test leads) should generally be grounded whenever working in the vicinity of dangerous voltages. \$\endgroup\$ – supercat May 31 at 15:51
1
\$\begingroup\$

It really doesn't sound like there is any reason for you to float your equipment. Why do you want to float it? You should specify the reason it is a requirement.

I think what you're actually worried about is ground loops? That would only be an issue with the scope. Do know if your scope is not already an isolated variety? Because if it is you don't need to do anything!

\$\endgroup\$
0
\$\begingroup\$

First, install GFCI protection

If you don't follow any other advice here, definitely do install 6-8ma GFCI protection. Either a GFCI receptacle or a GFCI device elsewhere that uses its LOAD terminals to protect these outlets.

This is not absolute protection. Not least, if you get between mains hot and mains neutral, the GFCI will perceive you as just another load.

Do ground, just independently

Since you have a really good reason, you can ground yourself independently by driving separate ground rods for the lab. This will amplify the effectiveness of the GFCI devices quite a bit, because dirt can't conduct enough to trip a breaker, but can conduct the 8ma to trip a GFCI.

This too is not complete, but at least these will keep people from laughing at your funeral.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.