[SI-LIST] Re: Ground Connection for Current Probe
- From: "Dandy, Jonathan S" <Jonathan.S.Dandy@xxxxxxxxxxxxx>
- To: "SI-LIST (si-list@xxxxxxxxxxxxx)" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 23 Jan 2013 16:57:11 +0000
The ground connection on current probes is intended to be used in high dV/dt
environments. These probes typically have a grounded shield between the
wire-under-test and the transformer - any capacitively-coupled current will
then flow in the ground instead of the transformer windings. Due to finite
ground impedance through the cable, scope, and DUT ground connection, some of
this will show up in the signal. If that is an issue, the ground connection on
the probe head is available to provide a lower-impedance connection to the DUT
ground. Of course, as Curt mentioned, there are times when it is not advisable
to connect the DUT ground to a scope ground. For an inrush-current
measurement, the local ground connection may not make much difference, but if
you are trying to measure current around a fluorescent ballast or other large
in-band dV/dt source, adding the local ground can noticeably improve your SNR.
Jonathan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Curt McNamara
Sent: Wednesday, January 23, 2013 8:06 AM
To: ah.vinod@xxxxxxxxx; SI-LIST
Subject: [SI-LIST] Re: Ground Connection for Current Probe
Best if you can take the measurement without the ground, as there is always a
risk of shock or damage when you connect grounded test gear to something else.
All grounds are at different potentials, and when you connect them together
even a small difference can produce a large current (due to the small
resistance). For example, a 1V difference with a 100 milli-ohm resistance would
be a 10A transient.
I would take the measurement without the ground first. If those results are not
satisfactory then you could move to connecting the grounds together.
You can minimize the risk of ground differential by making sure both equipments
are connected to the same AC feed. For example if they both were plugged into
an outlet strip (though it sounds like your currents may be too high for that).
Another approach is to use an isolation transformer for the equipment under
test, and then the ground from your test gear becomes the ground for the
system. It may be possible to use a sensitive voltmeter to determine the ground
potential difference before connecting the equipment together.
The underlying issue here is that AC connected equipment has a wide variation
in how "ground" is connected to the mains.
On the AC side (US at 120V), neutral is commonly tied to the ground at the
junction box, while the safety ground should be a wire that leads back to that
same junction box (however it might be connected to the conduit). The neutral
can carry large return currents, so it can be well above ground potential. Two
outlets in the same fixture might be connected to different circuit breakers in
the junction box. That means that different AC feeds can have very different
ground potentials. When these are connected together, the large currents
mentioned above can flow.
On the DC side, the ground may be connected to AC/safety ground through the
power supply, through the chassis, or in some cases not tied at all.
It is a good practice to establish a safety plan for all work involving the
mains, and have another engineer approve it before starting the work.
Curt
Curt McNamara, M. Eng. P.E. // principal electrical engineer | electrical
engineering
Logic PD
T // 612.436-5178
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of vinod ah
Sent: Wednesday, January 23, 2013 6:24 AM
To: SI-LIST
Subject: [SI-LIST] Ground Connection for Current Probe
Hi All,
I am using a current probe of 50MHz bandwidth with 15A max AC current. The
probe is used for measuring in-rush current due to hot-plugging of a device
(the device is bus powered and hence inrush current) to a host which supplies
power to the device. The current probe has ground connection tip at probe tip
end (the other end of probe is a BNC connector which gets connected to
oscilloscope where it makes solid ground connection with respect to
oscilloscope ground).
Since the current probe works on Hall effect/sensing magnetic friend around a
current loop, Is it necessary to connect the ground tip to board ground before
i make the current measurement. Since there are 2 domains involved i.e. Fields
(magnetic fields) and induced current through the probe and the current is of
slightly higher frequency (around 1MHz), not sure on the grounding aspect.
Regards
Vinod A H
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