Re: Allowable voltages on Scope

I suggest powering the oscilloscope from an isolation transformer; with
appropriate safety caveats of course. Private email to OP outlining same.

Bob Swinney
"Wild Bill" <wb_wildbill@xxxxxxxxx> wrote in message
news:1143572287.893025.66400@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Using earth-grounded sensitive instruments to investigate
utility-connected power sources can involve special equipment for many
measuring procedures.

Isolating accessories are often required for operator safety.
Opto-isolated interfaces are available for insertion between the probes
and the ocilloscope, to insure that the scope's chassis remains at
earth ground potential.

The great feature of oscilloscopes is that they're very sensitive, and
can display very low signal levels, even without direct connections.
Placing a probe tip near a transformer or inductor will often provide
adequate signals for the scope to display.
The added length of a screw-on hook tip can act as a pickup, sort-of
like a small antenna.

Typically, an isolation transformer is utilized to troubleshoot/test
line operated equipment, but in the case of an RPC (or a motor VFD), a
fairly huge isolation transformer would needed, to be able to sustain
the load current demands.
Many high voltage industrial electrical installations can't be measured
without special equipment.

Using an inductive method would be a safe approach of isolation when
accurate voltage measurements aren't required. Scope probe hook tips
can provide a small inductive and/or a capacitance-coupled pick-up
method. Placing the closed hook tip near a magnetic field will often be
sensitive enough to produce useable waveforms. Laying the plastic
housing of the hook tip directly on an insulated cable will have the
same effect.
A small plastic cap (something like those automotive vacuum tap caps)
could be placed on the end of the hook tip probe to insure that it's
fully insulated.

A snoop loop makes a more sensitive inductive pickup. The loop doesn't
need to be wrapped around a magnetic field, just close enough that some
inductive coupling takes place.
Such a loop can be fabricated on the terminal side of a panel-mount
type of BNC connector. Many scope probes include a probe tip-to-BNC
adapter (to enable the probe tip to be mated to a BNC connector), which
will allow the snoop loop to be easily fitted to a scope probe.
I've read articles about sensitive probes that were built using heads
from floppy disk drives, but haven't tried the idea.

A simpler technique would be taping a length of insulated wire along
the side of the individual power leads. Connecting the scope probe hook
to the conductor in the isolated pickup wire should be sufficient for
phase angle displays. (connect to only one end of the pickup wire).

If you're wanting to check the phase relationship of the
manufactured/generated 3rd phase of an RPC, you wouldn't need to
connect directly to any of the electrically hot terminals.

I think you'll want to check the RPCs phase relationships with a
running motor connected to it, but maybe not.

Many RCMers and HSMs seem to be getting more involved in machine
electrical and electronic circuits. Hopefully, all or most, will know
enough about the equipment they use to avoid damage and injury.

Although it's not used often, my Sencore 3100 Auto Tracker has full
auto-ranging capabilities to 100MHz and two isolated 2kV inputs for the
x10 probes. Additionally, the LCD display provides AC, DC, P-P,
frequency, time, and even ohms (separate test leads for ohms).
Almost any measurement is as simple as connect the probe, then read any
parameter.

WB
..............

Sierevello@xxxxxxx wrote:
I have a Kikusui COS6100M Scope and want to look at the voltages coming
out of my RPC to see how out of phase each waveform is from the correct
120 degree phase angle. Someone told me not to apply the 240v to the
scope that it will damage it. They advised to get a large resistor and
drop the voltage across that and then use the 5v and look at the
waveforms. That would be a large resistor and I would not know where to
find one that large that would handle the watts.

The manual says that the scope has allowable inputs of:

Channel 1,2,3: +- 250volts at 20KHz (DC+AC Peak)
EXT Trigger Channel 4,5: +-50volts (DC+AC Peak) at 1KHz and lower.
Probe Inputs (10:1): +- 600volts (DC+AC Peak) at 1 Khz and lower
Z Axis Input +- 25volts (DC+AC Peak) at 1KHz or lower.

Also, what about grounding the scope? I heard someone mention about the
chassis of the scope becoming charged and a possible shock hazard. What
causes this to happen and what is the remedy?

Any help greatly appreciated!!!

Thanks, Steve



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