Author Topic: test equipment grounding ???  (Read 2915 times)

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Offline KM4FER

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test equipment grounding ???
« on: February 05, 2015, 07:20:37 pm »
Back in the day, maybe 40-50 years ago, oscilloscopes and power supplies had three binding posts; I'll call them Positive, Negative and Ground with a little metal jumper that you could put between the Ground terminal and Negative if you wanted to reference to earth ground.  If you wanted to measure between two nodes that were both offset from Ground you just flipped the jumper so Negative and Ground were not connected and put Positive lead on one node and Negative lead on another and you measured the difference between the two. 

This was a handy feature as long as you paid attention to what you were doing.  Of course if you forgot to remove the jumper and connected the Negative lead to a node with voltage on it life got interesting very quickly.

In newer equipment Negative seems to be always tied to Ground.  Is this purely a safety issue or is there some other reasons manufacturers went that route?

earl...
 

Offline belasajgo

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Re: test equipment grounding ???
« Reply #1 on: February 05, 2015, 09:44:44 pm »
On high quality lab power supplies you will find three binding posts, only the cheap power supplies don't have the "ground" binding post.
 

Offline Zero999

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Re: test equipment grounding ???
« Reply #2 on: February 05, 2015, 09:57:35 pm »
In my experience, most cheap bench power supplies are floating. It's ATX and laptop PSUs which tend to be earthed.
 

Offline babysitter

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Re: test equipment grounding ???
« Reply #3 on: February 06, 2015, 06:48:52 am »
I second the "power supplies still have it".

Oscilloscope manufacturers learnt quite early that by grounding a scope they can sell accesoires like differential probes and isolation transformers and legitimate that by increasing operator safety, as the use of scopes spread from "experts only" to "people that barely know what they do".  O0
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Offline ivaylo

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Re: test equipment grounding ???
« Reply #4 on: February 06, 2015, 08:31:09 am »
Yeah, I have an Agilent MSO6104 with the BAT option (can run on batteries for a few hours). Yet the manual says it must be grounded (it has a special ground socket on the back) even when running on batteries. Why is that?
 

Offline Richard Crowley

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Re: test equipment grounding ???
« Reply #5 on: February 06, 2015, 09:06:25 am »
Yeah, I have an Agilent MSO6104 with the BAT option (can run on batteries for a few hours). Yet the manual says it must be grounded (it has a special ground socket on the back) even when running on batteries. Why is that?
As babysitter said, it was probably prompted by the legal counsel as a protection against frivolous claims by idiot users. We could start a huge discussion here (and it probably already exists elsewhere) about the ridiculous "safety measures" and warnings published by manufacturers in response to frivolous legal action.  Should it be necessary to warn people to not use their electric toaster in the bathtub?
 

Offline Dave

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Re: test equipment grounding ???
« Reply #6 on: February 06, 2015, 10:49:36 am »
Yeah, I have an Agilent MSO6104 with the BAT option (can run on batteries for a few hours). Yet the manual says it must be grounded (it has a special ground socket on the back) even when running on batteries. Why is that?
I'm going to guess because the inputs have a shared ground connection, connecting one probe's ground wire to, let's say, a live circuit, the user could get electrocuted simply by touching the shield of other channels' BNCs.

If you look at handheld oscilloscopes, they always have those special BNC connectors with plastic outer shells (the actual ground contact is made on the inside). On top of that, the inputs are all isolated.
« Last Edit: February 06, 2015, 10:51:27 am by Dave »
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Offline Zero999

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Re: test equipment grounding ???
« Reply #7 on: February 06, 2015, 07:50:47 pm »
Yeah, I have an Agilent MSO6104 with the BAT option (can run on batteries for a few hours). Yet the manual says it must be grounded (it has a special ground socket on the back) even when running on batteries. Why is that?
As babysitter said, it was probably prompted by the legal counsel as a protection against frivolous claims by idiot users. We could start a huge discussion here (and it probably already exists elsewhere) about the ridiculous "safety measures" and warnings published by manufacturers in response to frivolous legal action.  Should it be necessary to warn people to not use their electric toaster in the bathtub?
Yes, it's pretty standard for oscilloscopes. My Owon 'scope has a similar warning and an earthing stud on the back.

I don't pay any attention to it. As long as the device under test is run off a bench PSU or an isolation transformer it's perfectly safe, unless you do something silly like connect multiple oscilloscopes to different voltages so one unit is at a lethal potential to the other.
 


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