Clarification about floating measurement with an insulated oscilloscope

[giacomo]

In another topic about the tektronix ths series of handheld oscilloscope an user of the forum has underlined that from the manual the maximun allowed floating voltage without insulated probe would be 30Vrms, that sound low, so i wondered if the reasons are more regard humans safety or if we must also pay consideration for the scope safety or for not screw our measurments.

I thought that the question was also applicable to insulated oscilloscopes in general.

[madires]

Most likely to encourage you to use insulated probes when dealing with lethal voltages. 30Vrms won't hurt you, but 300Vrms will.

[capt bullshot]

The 30Vrms spec. matters only for your personal safety.
If it's ok for you (and your gut feeling of safety), you can use an unsinsulated probe for the max. rating of the scope common mode voltage. Just keep in mind, it's dangerous for you, not the scope.

[bdunham7]

There are regulations that prohibit exposed conductive parts from having more than 30V_rms or 42V_pk to ground.  An uninsulated probe connector will be at the same potential as the ground lead, thus the specification on the scope.  The scope doesn't care whether the probe is insulated or not, but you should.

[lor3n7o]

Hello everybody.
The curious thing is that this THS700 oscilloscope comes with P6117 probes (of which I am attaching the datasheet).
However, the P6117 has a fully insulated bnc connector covered in plastic material.
But according to the manual, even with this probe it is possible to carry out floating measurements for only 30V.
I have never noticed this limitation on floating voltage with standard probes, so I have sometimes made the measurements between high and low voltage with p6117 probes and nothing has happened, but now I have doubts.

At this point I am very curious to know what is particularly about the hi voltage floating tek P5102 probe.

[TimFox]

The P5102 probe is designed for floating measurements, where the Low/Common node is at high voltage.  This is covered in the Tektronix manual, which you can find through Google.
Specifically, there is a finger guard to keep you from the Low contact, and there is an insulated alligator clip included, along with a strong statement not to use any other clip.  The compensation screw is to be adjusted only with an insulated screwdriver, but that is probably not an HV consideration.
Basically, if you use a P6117, you are more likely to touch the Low node, which therefore must be less than 30 V rms from ground to meet regulations.

[tautech]

Yep, there's scope probes and isolated scope probes where a normal max rating for the reference lead might be CATIII 600V or CATII 1000V.
These 10x probes for the SHS1000 series have such ratings for both the probe tip and reference lead and need to if you're to play with dangerous voltages.



A similar diagram to the Tek image in the OP is here:
https://int.siglent.com/products-overview/shs1000/#navs

[David Hess]

Besides its insulation from common/ground, the P5102 also has a 5 megohm input resistance instead of the common 10 megohm input resistance of a x10 probe high impedance probe indicating that it has a shunt resistance across its output.  The significance of this is that when AC coupling is used, the probe still divides the DC input so it is safe to use AC coupling with high DC input voltages unlike most other high voltage probes.

[lor3n7o]

thanks for your answer, but I didn't understand what you mean by the measurements made in AC. I presume the P5102 probe works for both AC and DC measurements.
Could you have a way to explain better?
Thank you.

[TimFox]

A simple x10 probe could have a 9 megohm resistance in the probe, coupled through the AC-coupling input capacitor in the CRO to the 1 megohm input resistance (plus high-frequency compensation capacitors).  When connected to an AC signal superimposed on a high DC voltage, the entire DC voltage appears across the input BNC and input capacitor.  In DC-coupled mode, the CRO input resistance will see 1/10 the input voltage (AC and DC).

[lor3n7o]

Taking for sure the following parameters retrieved from the probe and oscilloscope datasheet.
- Oscilloscope input resistance: 1M
- Oscilloscope input capacity: 25pF
-Resistance of the probe when connected to the oscilloscope 5M.
-Capacity of the probe when connected to the oscilloscope 11.2pF.
- 10x probe factor.
I thought of two potential configurations that could be inside the probe if it had components on the reference lead.
I am attaching the datasheet of the probe and a photo of the two configurations that could be in the P5102 probe.
Both are based on the principle of separating the reference lead from the oscilloscope's BNC.
What do you think?

[David Hess]

The probe is simpler than that and does not have a balanced input.  Ground goes through without any series resistance.

The P6055 is an example of a x10 probe with 1 megohm input resistance with the same configuration.  The P6009 is an example of a x100 probe with 10 megohm input resistance with the same configuration.

[lor3n7o]

The probe is simpler than that and does not have a balanced input.  Ground goes through without any series resistance.

The P6055 is an example of a x10 probe with 1 megohm input resistance with the same configuration.  The P6009 is an example of a x100 probe with 10 megohm input resistance with the same configuration.

the p5102 is 10x and 5Mohm. So do you think it would have 9Mohm series resistor to make the 10x partition and in addition an additional 10Mohm resistor at the input to lower the total input resistance to 5Mohm ??
thanks for the replies.

[jonpaul]

Bonjour a tous: It seems a lot of related topics in this thread:

Probe use in differential and single ended
Safety ratings
Probe design
Floating portable scopes

It is wise to do a bit of research in this area:

Probes: Tektronix Circuit Concepts Oscilloscope Probe Circuits
https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

Specific models: https://w140.com/tekwiki/wiki/
eg https://w140.com/tekwiki/wiki/P5102

The 30 VAC or 42.2 VDC rating of a scope or probe  is a UL class of SELV = Safety Extra Low Voltage, in theory the max V for touching safety.
P5102 etc are designed to prevent contact and used in dangerous power applications.
The rating in the spec of 30V reflects the UL ratings and protects again liability in case of lawsuit.

I have used scopes and probes since 1960s including on high power and high voltage devices , to 55 kV.

I have seen scopes floated to 20 kV with appropriate precautions (LLNL)

Most TEK 10X probes have a 300-400 V peak rating.

 HV probes P6013, P6015, P6015A have x100, X1000 and ratings to 20 kV, 40 kV peak.

I have never need a differential HV measurement, but used some diff active  HV probes, such as Yokogawa 701921, 700V

Finally ALL of these "safety" probes have so much bulk, insulation and complication that usefulness is low.

Depending on your expérience and the danger of the devices being probed, you must judge if the risk is worth the benefit. Amateur Radio, Broadcast and power engineers know how to work and measure safely. Novices may not.

Kind Regards,

Jon

PS: LEGAL NOTICE  I AM NOT GIVING ANY ADVISE, I HAVE NO LIBAIBLITY IF YOU ARE INJURED OR WORSE!

[David Hess]

The probe is simpler than that and does not have a balanced input.  Ground goes through without any series resistance.

The P6055 is an example of a x10 probe with 1 megohm input resistance with the same configuration.  The P6009 is an example of a x100 probe with 10 megohm input resistance with the same configuration.

the p5102 is 10x and 5Mohm. So do you think it would have 9Mohm series resistor to make the 10x partition and in addition an additional 10Mohm resistor at the input to lower the total input resistance to 5Mohm ??

No, like the examples I give, it has a much lower series resistor of about 4.5 megohms.

[lor3n7o]

The probe is simpler than that and does not have a balanced input.  Ground goes through without any series resistance.

The P6055 is an example of a x10 probe with 1 megohm input resistance with the same configuration.  The P6009 is an example of a x100 probe with 10 megohm input resistance with the same configuration.

the p5102 is 10x and 5Mohm. So do you think it would have 9Mohm series resistor to make the 10x partition and in addition an additional 10Mohm resistor at the input to lower the total input resistance to 5Mohm ??

No, like the examples I give, it has a much lower series resistor of about 4.5 megohms.

According to your idea, your configuration would be like this ?

[David Hess]

According to your idea, your configuration would be like this ?

That is about right.  I am not sure about the exact values and am too lazy to calculate them at the moment.