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Our Tektronics voltage probe P5100A has a label of "1000 VRMS CAT II, 2500 Vpk". Could you please tell me exactly what does it mean?

I'm trying to measure an RF generator output voltage which is expected to be about 2 kV in amplitude in maximum at 13.56 MHz. I interpreted this label that P5100A can withstand 1000 VRMS ~ 1.414 kV in AC voltage amplitude and 2.5 kV transient spike, so I thought I couldn't use this probe for my measurement.

However, the local company engineer said that I can use this for the measurement I want to make. He said a current flowing the probe should be very low so I don't need to be overly concerned. Is what he said really true? I tested this probe and at least the probe looks working at the voltage amplitude of 1.7 kV. Can I go further so 2 kV is reached?

And in terms of "CAT II", What does it try to tell me? Wikipedia said CAT II is associated with low voltage installations such as household appliances, portable tools. It is a rather broad concept. So, Should I consider this seriously?

pipe
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Donggyu Jang
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  • 2.5kV isn't usually transient spike, but peak to peak of your waveform. – PlasmaHH Mar 15 '17 at 15:22
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    1000 VRMS is a sine wave with peak values 1000*1.41 = +/-1400V or 2.8kVpp. CAT II have instruments usually for 230VAC measurements, as you said home appliances. Instruments for industrial network 400VAC-600VAC have CAT III logo. Now, I wouldn't use a 2500Vpp probe (880 VRMS) for measuring a signal of 2k VRMS. – Marko Buršič Mar 15 '17 at 19:17
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    Also "current flowing the probe should be very low so I don't need to be overly concerned" is a story for kids. A current will be pretty high when insulation breaks down. You'd better find some other dealer if you don't want to get some death injury. – Marko Buršič Mar 15 '17 at 19:25
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    The CAT levels have nothing to do with probe current and V rating is always a small percentage of transient Vpk rating – Tony Stewart EE75 Mar 16 '17 at 19:33
  • I'm curious what application involves multiple kV of RF. If it's a 50 ohm load that's > 50 kW! – pericynthion Mar 17 '17 at 02:35
  • It is RF plasma generation. Of course, the impedance matching is done so the equivalent impedance of the the matching box, the plasma load, and the cable between these becomes 50 Ohm. – Donggyu Jang Mar 17 '17 at 03:14

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This category number refers to the Insulation Standard for location of a power source for human safety. Arc flash energy ( and thus risk to thermal explosions) increases in each zone by increasing category numbers is implied by the unstated current and energy available, if an arc were to occur, not the actual breakdown voltage rating.

So a Cat II probe is only suitable for Cat I & II zones.

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Tony Stewart EE75
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  • Thanks for giving me a good answer for meaning of CAT II. Can I ask you a little more? For example, when 300 V CAT II is written on the label of the intrument, is 300 V a rms-voltage or an amplitude of the AC signal (in V = Acos(wt + phase), A is the amplitude of the signal while A/sqrt(2) is rms-voltage)? And can I think that 300 V is the maximum voltage that the instrument can work with? – Donggyu Jang Mar 17 '17 at 03:13
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    rated V would mean rms unless [pk] was otherwise stated – Tony Stewart EE75 Mar 17 '17 at 03:22
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I found the manual of the product and looked at it carefully. There is a figure showing meanings of VRMS and Vpk. VRMS is the RMS voltage in AC signal and Vpk is the difference between the voltage peak and zero. Vpk is applicable when there is a transient spike (possibily superimposed with a normal AC signal). Both VRMS and Vpk are maximum allowed voltages in P5100A voltage probe.

CAT II looks some sort of conceptual guideline of when and where the product can be used. In home applicance, CAT II alone will be enough information for an engineer. For some of scientific purpose, VRMS and Vpk are information that can be relied on.

With your help, I think I got clear understanding of the label. Thanks you:)

Donggyu Jang
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