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I'm troubleshooting an oscilloscope where the cathode voltage is ca. -2000V respect to ground and chassis. There are parts of the circuit (eg. the heater filament, or the blanking / un-blanking circuit) which generate low voltages but they are connected to HV supply lines. Eg. the heater secondary is generating 6.3V in AC but one of the wires is connected to the -2000V to keep the voltage difference of the cathode and the filament within spec for the CRT.

How can I safely measure these low voltages (eg the 6.3V across the filament) within the high voltage circuit?

I have a CAT II 1000V multimeter and CAT III 1000V probes. I assume that the isolation they provide is not sufficient to safely measure in such an environment. If I understand correctly the rated voltage is not just for the voltage I want to measure but it's also a rating of isolation towards the "outer world". Meaning that even though the multimeter and the probes are floating at -2000V the isolation they provide is still rated at 1000V making them not suitable for such a measurement. What options do I have?

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    \$\begingroup\$ As I read it, your understanding is correct. Another thing to watch out for is the mains adaptor, if provided, will have an insulation rating too. This matters because the meter common will be at high potential. Fluke, for example, on their scopemeters have the DC jack / socket arranged in such a way that you can't touch the jack sleeve when it's in contact with the socket. \$\endgroup\$ – Transistor Jul 12 '17 at 7:13
  • \$\begingroup\$ @Transistor you are referring to the insulation rating of the power cable of the device under test ? Can you explain how that can play a role provided that the cable itself is kept distant enough from the meter itself? \$\endgroup\$ – cosenmarco Jul 12 '17 at 9:31
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You are right in assuming that you cannot rely on the meter insulation between you (0v) and the -2000v circuit you want to measure.

You can make the measurement safely, if a little more slowly, by isolating the meter and its probes and wires from the ground. Get a suitable insulator, say a glass or plastic bowl or chopping board, and place the meter on it. This is obviously a battery powered portable meter, not a mains powered one. With the target powered off, set up the meter and connect it. Dress the probes and wires so there is an air-gap, which is resistant to the set-up being bumped or knocked, between meter probes and their wires, and any ground conductors. This is why a bowl may be a better solution than a board, you can allow the excess meter probe wire to sit in the bowl with the meter. Power on, read the meter without touching it, and power off. Wait for high voltage capacitors to lose their charge before unclipping the meter.

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  • \$\begingroup\$ Seems a good approach to avoid injuring myself (which is a very good thing) but what about the risk of damaging other parts of the circuit since the probes (or in this case I would have to use clips which can hold contact tight without manual intervention) are not rated for 2000V? \$\endgroup\$ – cosenmarco Jul 12 '17 at 7:58
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    \$\begingroup\$ The voltage rating is always with respect to some other conductor. In this case, you have 6.3v between probes, and 2000v to ground. That's what the glass or plastic insulating layer is for, to allow the meter to float safely with respect to ground. There is no conductor close to the probes, or their wires, that's at a different potential. Editted answer to clarify. \$\endgroup\$ – Neil_UK Jul 12 '17 at 8:25
  • \$\begingroup\$ Seems a sensible approach. Is there any guidelines about how actual distances? Eg. putting some thick cardboard or a plastic cutting board (how many mm thick?) between the probe wires and the chassis (which is actually a conductor which may be close to the probes) if the wires could get in contact with it ? \$\endgroup\$ – cosenmarco Jul 12 '17 at 8:36
  • \$\begingroup\$ At 2000v, almost any thickness of any insulator is adequate (do not extrapolate this apparently relaxed attitude to higher voltages). More than a few mm of air is also OK, though the problem with air is that it isn't solid, it needs to be bump-proofed. \$\endgroup\$ – Neil_UK Jul 12 '17 at 8:43
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You could use an high voltage probe expressly designed for extending the DMM voltage range, as the following from Fluke:

enter image description here

Description excerpt (emphasis mine):

The 80K-40 is a high voltage probe designed to extend the voltage measuring capability of an AC/DC voltmeter to 40,000 volts peak AC or DC Overvoltage Category I. This means the probe can only be used to make measurements on energy limited circuits within equipment. Examples include high voltage within televisions or photo copy machines. DO NOT use this probe to measure high voltages on power distribution systems. The probe provides high accuracy when used with a voltmeter having 10 Megohm input impedance.

Note that cat II, cat III, etc., rating has more to do with the energy capability of a system than with its voltage level. I.e., cat II rating should be more than enough to protect you from accidental faults when doing measurements inside a scope, provided you use a meter+probe set that can withstand the voltages inside.

Of course, I'm assuming you are using a floating, i.e. non mains-powered, instrument.

EDIT (to answer a comment about cat ratings)

See this document from fluke about cat ratings. Excerpts (emphasis mine):

Transient protection

The real issue for multimeter circuit protection is not just the maximum steady state voltage range, but a combination of both steady state and transient overvoltage withstand capability. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionized and conductive. The result is an arc blast, a disastrous event which causes more electrical injuries every year than the better known hazard of electric shock.

And also this one:

It’s not just the voltage level

In Figure 1, a technician working on office equipment in a CAT I location could actually encounter dc voltages much higher than the power line ac voltages measured by the motor electrician in the CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are clearly a lesser threat, because the energy available to an arc is quite limited. This does not mean that there is no electrical hazard present in CAT I or CAT II equipment. The primary hazard is electric shock, not transients and arc blast. Shocks, which will be discussed later, can be every bit as lethal as arc blast.

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  • \$\begingroup\$ So the HV probe would be nice (in fact I'm waiting for one to arrive) to measure HV to ground but I assume that the precision loss when measuring eg. 20V DC difference between 2 HV lines is too big. \$\endgroup\$ – cosenmarco Jul 12 '17 at 8:05
  • \$\begingroup\$ About the CAT ratings: I know they are about transients but I added also the voltage ratings of the probes and the multimeter and that should be the related to the isolation factors under non-transient (or steady) conditions, right ? EDIT: the multimeter manual says not to measure voltages higher than 1000V DC and 750V AC \$\endgroup\$ – cosenmarco Jul 12 '17 at 8:06
  • \$\begingroup\$ @cosenmarco see my edits. \$\endgroup\$ – Lorenzo Donati -- Codidact.com Jul 12 '17 at 8:18
  • \$\begingroup\$ @cosenmarco and, yes, you will lose accuracy subtracting two readings referred to ground, but if your multimeter has an high resolution mode like the Fluke 87V (i.e. can read an extra digit, albeit at lower absolute accuracy) or at least 4 digits, you could get by having a decent measurement (good for telling if the circuit is ok). \$\endgroup\$ – Lorenzo Donati -- Codidact.com Jul 12 '17 at 8:22
  • \$\begingroup\$ yes that matches with my understanding about CATegories. Now... it doesn't solve my original problem (see other answers and comments) about the steady state isolation capabilities of multimeter + probes under normal measurement conditions. \$\endgroup\$ – cosenmarco Jul 12 '17 at 8:23

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