CAT II and CAT III Isolation question

[Computeruser]

Background:

I have a good Digital Storage Oscilloscope that is grounded to my North American 120 Volts RMS household electrical system.

I cannot hook up the reference lead of the Scope to a hot household electrical terminal. It will blow up the lead, could damage the Scope and possibly kill me. I have known this for years (decades even). So I cannot use a ground oscilloscope to measure voltage, waveform or transients across a resistor which in the hot household circuit.

I have good Fluke 87V digital meter with isolated black (nominally ground) and red (nominally hot) terminals. Across the terminals it reads MAX CAT III 1000 Volts and MAX CAT IV 600 Volts. Peak volts, I think.

So I can (very, very carefully) hook up black and red leads across a resistor in a hot household circuit. Hot = 120 Volts above ground potential. The meter will float above ground, has guards and rubber holstering to prevent me from putting fingers in there.

Present Issue:

Moving forward, I am getting a B&K Precision 2516 100 MHz Battery Operated, Handheld Digital Storage Oscilloscope with 2 isolated channels and a built in Digital Multimeter. The isolated and floating channels are rated for CAT III 600 Volts (and CAT II 1000 Volts). Peak volts implied.  The Probes for this work are completely insulated and rated for CAT III 600 Volts and CAT II 1000 Volts

So then, my thinking from the Fluke Digital Meter is that I can hook up the reference lead of the probe (nominally the ground lead in my traditional scopes) to a resistor in a hot household circuit and the probe to other side of the resistor to measure voltage, waveform and transients. I have never done such a thing before because of the obvious hazard of electrically grounded instruments.

The Digital Meter in the handheld B&K instrument has isolation ratings of CAT III 300 Volts (lower threshold than the scope channels and probes) and CAT II of 600 Volts (lower than scope channels).

CAT 1 is low voltage electronic circuitry, CAT II is household outlets, CAT III is household and industrial distribution panels and motors, and CAT IV is building connections to the outside world. I have no need for CAT IV but would like to look into CAT III circuits

Is my thinking above correct about hooking up isolated, floating terminals on a Digital Meter or Digital Oscilloscope correct? I obviously do not wish to damage a precision instrument.

Thank you for your time.

[turbo!]

CAT ratings aren't for determining if your instruments will get fried. There's no difference between measuring +600v DC inside some electronic equipment vs traction power for a trolley when there is no issue.

If there's a surge, it can initiate a conductive path. In a power supply, it would probably short out the DC rail and fry the meter or the power supply. On 600v traction power it would burst into a big ball of fire unless the meter is able to prevent the formation of plasma ball or have a means of cutting off the power to it without drawing a further arc.

[Computeruser]

CAT ratings aren't for determining if your instruments will get fried.

I did not mean to suggest they were for that.

There's no difference between measuring +600v DC inside some electronic equipment vs traction power for a trolley when there is no issue.

That is not my understanding. My Fluke meter is good for 1000V inside in a CAT III environment and only good for 600V in an inside/outside CAT IV environment.

Perhaps I misunderstand what you are trying to say.

[David Hess]

Since both inputs of the B&K Precision 2516 are fully isolated from the oscilloscope and each other, it will be safe to use them to make off-line measurements whether the oscilloscope is isolated or not.

[electr_peter]

CAT ratings aren't for determining if your instruments will get fried.

I did not mean to suggest they were for that.

There's no difference between measuring +600v DC inside some electronic equipment vs traction power for a trolley when there is no issue.

That is not my understanding. My Fluke meter is good for 1000V inside in a CAT III environment and only good for 600V in an inside/outside CAT IV environment.

Perhaps I misunderstand what you are trying to say.

Computeruser, please read more abour CAT ratings in http://www.ni.com/white-paper/5019/en/
CAT ratings define what kind of voltage transients with specific load impedance meter/scope can survive. That is, CAT IV 600V rated meter can in principle be used to measure for example 1350V (unless protection devices turn on at this point). On the other extreme CAT IV 600V rated meter could only be useful for some small measurement range (say to 50V), but would still be safe with high voltage transients.

Back to question about isolated measurements. Short answer - yes, you could use isolated scope channel (but check that it is isolated, some scopes can switch between isolation and no isolation) with special isolated probe if you are only measuring CAT II type equipment and you are careful. Some handhels scopes do not have isolated channels - these are a bit dangerous if you don't know what you are doing.
Few things to note. Both scope and probes probably should be set to 10x mode to be rated for higher voltages.

[turbo!]

Thread starter, read the manual. It gives you the answer to you asked about how it can be used in plain English.


On CAT category, you're not understanding transient voltage. On 600v DC traction power, interaction of inductance and sudden load change can cause a kick back voltage. What I said about nothing being different when all is going normal remains the same.

A theater arc lamp runs at something like 35v and 150A.  The lamp is started by imposing a pulse 30kV or so pulse while applying normal voltage across the terminals and the ballast limits the current to 150A.

CAT category is a matter of the ability of the meter to absorb the ignition pulse, the impedance of possible arc and the ability of source to sustain the said arc and the ability of the meter to break the arc by vaporizing away its PCB traces, fuses, and such before the arcs actually get across the input terminals.  The higher the arc current, the harder it is to put it out. You will still destroy meter by trying to measure the high voltage side of a microwave. The impedance of the puny little transformer limits the magnitude and duration of fault.

Energize a 10A load through DC 100V and try to break it with an ordinary switch. You can't. It will arc inside and melt with the current only limited by the load. If you short the load out with the arc going, the switch will explode, depending on how much current the source can provide and how far and quickly apart you can draw the two apart.  You can start an arc by bringing the two together and drawing it out... or with a pulse to jump across the two.


A traction 600v power is fed from 3 phase mains and it doesn't depend on capacitors to hold up like single phase rectification so the fault current is probably such an extent that it will vaporize your meter if you give it a surge to strike an arc.