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oscilloscope probe grounding clip

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IamSmooth

Sat Mar 08 2014, 11:16PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I've read that the clip is sometimes connected to the chassis, which can cause a ground loop (short). Even if not, it can sometimes have a load effect.

When is it appropriate to connect the clip to a circuit? For example, I've connected it to the ground when I know it is isolated and this removes noise. This is easier than using two probes.

Sigurthr

Sun Mar 09 2014, 03:33AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Generally in a professional installation either the scope or the device under test is isolated from mains via and isolation transformer, thus eliminating ground loops or short circuit conditions. Additionally, a different probe (or poor mans; invert one channel and add it to another channel, then use the two channels w/o ground leads to probe) will eliminate ground loops and short circuit conditions.

The ground clip should only be connected to verified earth ground or isolated from earth points.

Sulaiman

Sun Mar 09 2014, 07:17AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I strongly disagree with using a line powered oscilloscope isolated from PE,
it's a nasty lazy habit from TV repair days (hot chassis)
it can result in the entire 'scope chassis being at dangerous/lethal voltage.
(portable 'scopes have an insulating enclosure, but care required)

I'm not keen on isolating the equipment under test either,
especially for high power electronic systems
where the leakage of line filters etc.
can easily give a mild shock ... if there is no fault
a 'leaky' line filter can cause quite a shock !

Unfortunately, with lots of the equipment that we bench service at work
we have to not connect the chassis of equipment under repair/test to PE
as the ground current through the filters is enough
to trip the 30mA rcd/earth-leakage breaker.
Consequently, 'tripping the bench' is not uncommon
... cue moans of three other engineers ;)

So,
NEVER remove the PE connection to your 'scope
Use an isolation transformer for test/repair/examination
of the equipment if required.
Before connecting the 'scope earth lead to any point,
connect just the tip of your 'scope probe to that point
and check that the voltage is not unsafe
(a few volts rms is probably ok, just 'pickup' of mains)
If there is no 'safe' place to connect the 'scope earth lead
then you need to use a purpose made differential probe,
or use the technique suggested by Sigurthur
(you need to 'calibrate this first;
- set both 'scope channels to the same range,
- invert one of the channels,
- connect both probe tips to the same signal point,
- adjust the 'var' (variable gain) of one or the other channels for zero deflection on the 'scope trace.
- use the probe tip of the inverted channel as 'ground'
- be aware of the voltage being measured relative to
a) the input range set for the channels
e.g. to 'scope a signal at 300v requires the input range to accommodate 300v, so a 1v signal will be very small on the display,
this is why true differential probes are useful.
b) capabilities of 'scope probe

it's early and brain not fully awake, I'll come back and edit later!

Sigurthr

Sun Mar 09 2014, 09:47AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Oh yeah, forgot to mention the hot chassis point, haha, thanks Sulaiman! Heh, that's not a good thing to forget...

Reiterated for clarity; if you use an isolation transformer ON THE SCOPE the scope's chassis will be electrically hot. Using an isolation transformer on the D.U.T. is just fine.

I typically go the "poor man's differential probe" route just because I don't have a suitable isolation transformer for the wattage of devices I have to test (typ. a couple kW of solid state tesla coils) and I don't want a lethal hot chassis.

Never EVER clip the ground / PE lead of a scope's plug. I didn't mention it before because it is an absurdly dangerous and stupid thing to do. I've only heard of one fellow on the net doing it and it was years ago. It's the kind of thing I don't like mentioning so that idiots who read my post don't get any bright ideas.

testtest

Sun Mar 09 2014, 01:40PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Dave Jones on EEVBlog had a good video on this.

Mads Barnkob

Sun Mar 09 2014, 02:13PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Sulaiman

Sun Mar 09 2014, 04:19PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

good video, just thought I'd mention one thing,
for scenarios 1 and 2 using battery or isolated power supply
if the 'scope probe earth clip is connected to the +ve supply
a short circuit will not be formed
BUT
the negative rail, which may be connected to internal circuitry such as
front panel connectors, potentiometers, switches etc
and possibly the chassis,
these points now become -ve with respect to earth
so if you touch what you think is ground / safe
you will actually be touching a -ve voltage rail with respect to earth.
If the internal +ve supply is 3.3, 5.0 or 12v then not really a problem
but 4HVers may be using MUCH higher voltages
(with plenty of power/energy available)
then you risk a lethal shock !
(usually just a mighty painful jolt,
but people do get electrocuted at these voltages)
So
think carefully before attaching a 'scope !

may ruin more than just your day !!!!

If in doubt, power down the equipment, make the connection, power up from a safe distance !

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