Instrumentation Safety /Malpractice

Thread Starter


Joined Oct 26, 2011
In the bygone days, you could hardly find a scope with an intact ground pin--they were often floated on line voltage equipment (up to 460V) using a low capacitance isolation transformer--yes there was known and accepted risk--yes, even I, myself, got a few nasty jolts from violating the basic and well known "one hand rule" as it takes two hands to complete the circuit--unless of course, you are standing on a wet floor--much akin to driving too fast and/or smashing your car without wearing seat belts.

The "one hand rule", of course, did not protect against those nasty little set screws found on every vintage Tektronix oscilloscope control knob where harmonics generated by high power equipment found their way into the ether via stray human body capacitance.

On occasion, the ground pin presents a significant danger when the unexpected completion of a high power fault circuit can actually blow up a scope probe in your hand--potentially as dangerous as electrocution.

One of the guys I worked with told me that in a previous workplace, he had to get approval from the engineering manager before he was allowed to 'float' an oscilloscope. Awkward, but not a bad policy.

Putting things in perspective: While spending a career around this sort of stuff, I never knew of anyone personally that was electrocuted by instrumentation malpractice--on the other hand, I did have numerous acquaintances that were killed in auto accidents--however, a good more people drive cars than float instrumentation.

Yes, there are better tools and methods available to us today--this kind of stuff should be taught in all engineering schools.
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Joined Jul 7, 2009
I too have floated my scopes to make e.g. an occasional high-side measurement across a shunt. However, this task is for knowledgeable people who understand the risks and how to prevent accidents. Unfortunately, a simple argument can be made that an accident will happen if enough people do it over a sufficient period of time. I've never been "hit", but I know it's only a matter of time -- it'll happen during a momentary lapse of attention or when something slips. Accidental, yes, but I still could be accidentally dead -- and that's why I don't do it anymore. :p

Personally, I would hold the company liable who let such accidents occur because there are tools available (properly-designed differential amplifiers) and procedures that can allow such measurements to be made perfectly safety. You can get such an amplifier today for around $300 that will measure up to around 1 kV with a 25 MHz bandwidth (and higher in both parameters for more money). And they have been available for a long time -- I remember using one on a vacuum tube Tek scope in the 60's. Thus, there is simply no excuse for making these measurements the old way. If you think that's expensive, compare it to the cost of a trip to the emergency room or a funeral.

Oh, and someone else will obviously pipe up about the poor-man's differential amplifier -- using two probes on two channels and subtracting the signals. That's a perfectly safe method too when properly applied. So there's really no excuse to have people doing unsafe things.

Thread Starter


Joined Oct 26, 2011
I have in my bag of tricks a Bell current transducer assembly complete with ±power supplies and load resistor--it will handle over 100A without electrical contact and is extremely fast--very handy at times.

Also have a few current transformers.

Tektronix current probes are too expensive for me--upkeep is high as well.


Joined Dec 26, 2010
I can remember an oscilloscope housed in a thick perspex box, with some controls extended outside the case on insulated handles, powered by an inverter from a large lead-acid battery. Recording was by means of a camera held outside of the box.

It was used for measurements of fairly low voltages riding on a high voltage line at up to 12kV DC. Some few years before I saw it, when it was first introduced, it was considered pretty clever stuff. The "modern" "transistorised" scope would run for some hours on the big battery, although the latter had a pretty hard life with deep discharges because it was tempting to use it a bit too long. I think that a low voltage cut-out was eventually added to save the battery from our enthusiasm.

The advent of various sensing devices, optically coupled or otherwise, eventually put paid to this contraption, although of course nowadays you could also get far more compact self-contained oscilloscopes that could have gone inside that insulated box. There was still a chore of changing or charging batteries, but at that point they became small NiCads which were much easier to handle.
One downside of early simple analogue versions of these arrangements was that the calibration was not so stable and could suffer as the battery ran down, but later FM and digital versions overcame these snags.