Why am I grounding my HV probe to mains to read a secondary (different) voltage in that circuit?

In this scenario I'm measuring output of a 4 wire 5kV transformer: line voltage on primary, two HV wires on secondary. For any normal multimeter reading, if I want to measure any single component on a circuit, I'm putting my leads on that individual component. For instance, if I'm measuring a 5V piezoelectric sensor on a car, I'm not putting my meter probes on battery negative and 5V positive; I'll get wacky readings. So why does my HV probe require me to ground on 110, and touch the lead to the 5kV secondary? Shouldn't my probe (ground AND hot points of contact ) be connected only to the secondary side?

Edit: Probe ground to secondary: incorrect reading. Probe ground to primary: correct reading of 5kV

To clarify my question: regular voltage measurements, as I'm familiar with, involve reading voltage AT the individual component in a circuit, because those components on that circuit many times operate under voltages different than the input voltage that's powering the device. Voltage readings on the device means I'm attaching both Com and V to the device.

Yet the proper way to measure the output voltage of this transformer was to take that reading from two completely different parts of a circuit, one a 5kV, and one a 110. If voltage reading comes from the difference in potential, wouldn't this yield 4,000 - 110, which would be 3,890V?

Resistance measurements (for OP to complete).

    Terminals            Resistance
------------------------------
Black to white       Ω
Black to HT1         Ω
Black to HT2*        Ω
Black to frame**     Ω
HT1 to HT2*          Ω
HT1 to frame         Ω

* To be found. Not visible in photos.
** Frame or wire attached to frame by ring terminal.

• Probably for safety. If you have an HV probe. you need the ground side to go to a safe ground such that the probe output is always a safe voltage. Show a schematic of what you want to measure. It sounds like you want to measure the output of a floating HV secondary. If so, ground one side of the secondary as well. Jul 31, 2021 at 4:41
• @JaredCravens to the safety ground of the mains, surely! Edit into your question a link to the manual, or add a picture of the part of the manual that tells you to clip the probe ground to the mains. AND POST A SCHEMATIC OF WHAT YOU'RE TRYING TO MEASURE. There's a schematic button on the editor toolbar Jul 31, 2021 at 5:02
• Jared, you can see your setup. We can't. There were a number of errors in your original post which caused confusion (which you haven't acknowledged in your comment above) and there are still references to a piezoelectric sensor in a car which has nothing to do with the question. We're looking for context to help you. Measuring voltage across an isolation transformer doesn't make sense so your readers would like to know why you are doing it. "I'll get wacky readings" is not a good technical description. What readings do you get? Why not include a photo of the setup? Over to you. Jul 31, 2021 at 10:42
• @JaredCravens Thanks for the picture. What is the 'incorrect reading'? What makes you think it's incorrect? I notice the secondary ground is also the core. Is the core grounded to mains safety ground? If not, why not? Jul 31, 2021 at 13:00
• @JaredCravens Transformer core grounded to mains safety ground is usually required for mains operated things. This is usually achieved by the transformer being mounted in an earthed chassis with conducting fasteners. The secondary can be floating, or grounded on one side, depending on how you want to use the secondary. I've just read Transistor's answer - you MUST be able to get a resistance reading (aka continuity)(with power off of course) between the two points you're trying to measure HV on, otherwise you'll read rubbish. I'd expect something in the 100s to kohms range Jul 31, 2021 at 14:26

The photos you've added reveal that one of your "secondary" connections is the transformer laminations. You need to check for continuity between the HT wire and the frame. Take a resistance measurment and see if you can see a secondary connection to the frame.

There is a possibility that the transformer is an auto-transformer.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Autotransformer configuration. Note that in this configuration there is no internal connection to the transformer frame.

You should be able to find which wire is common using resistance measurement techniques.

If the configuration is as shown in Figure 1 then it should be clear that measuring between HT and frame will just give you a reading of capacitive coupling. Correct meter common reference should be to the common terminal on the transformer.

I don't see any fuse in your test setup. This is bad practice and may bite you some day.

• No continuity between HT wire and frame. If there was, this would be a short, correct? The bottom line here is that the component (neon gas tube) is fed it's 5kV by these two wires. Voltage is two wires to move electrons. So why can I not measure it by sticking with both of my secondary wires? The secondary common, regardless of how it's wired, is still half of the 5,000V worth of moving electrons, just as the hot HT wire. Jul 31, 2021 at 13:41
• If there is no continuity between HT and frame then you can't expect to measure a voltage between them. "... this would be a short, correct?" No, it would be the coil resistance. Forget about "electrons". Just think of current. In the neon gas both electrons and positive ions flow in opposite directions. I can't make any sense of your last sentence. You need to find out where the other end of the HT winding is terminated. A better photo would help. Try to show the secondary coil terminations. Jul 31, 2021 at 13:51
• Also, check for continuity between the primary and the HT lead. Jul 31, 2021 at 13:55
• I was wrong, I have 10.6kΩ between both secondaries. I can't understand any event in which measuring voltage at the two wires that feed a load will not yield a measurement of the voltage going to the load. Maybe my theory of operation/fundamentals are just wrong, and you're assuming I know something which I've failed to understand. A voltage reading as I've always understood is made by reading from the power and ground that directly feed a load, but somehow I'm supposed to get that reading from two completely different voltages, A 5kV hot and a 110V negative that doesn't feed my load. Jul 31, 2021 at 14:25
• "I have 10.6kΩ between both secondaries." There is only one secondary. Do you mean between the HT wire from the coil and the wire on the transformer chassis? (You need to be precise.) If so 10.6 kΩ seems high. Note that you haven't reported on checking for a connection between primary and HT lead. Jul 31, 2021 at 14:54

Summary: The test configuration seems invalid, leading to unexpected measurements.

The topic of the transformer frame (core) not being grounded was raised in earlier comments, and that is likely to be the cause of your incorrect measurements.

Details: What you have there is a neon sign transformer (NST). The label seems to show that it's a "Franceformer 5020 SE", rated output 5000 V and 20 mA. (I would be interested what the caution label says on the top.)

I could not find that as a current model on the Franceformer (now part of SFEG) website. The closest I could find (visually and specifications) now listed on their website, is the 3020 SEG-U (datasheet).

I'll start with a warning and disclaimer: The voltage at both the mains input and HV output of the transformer (5 kV in your case) can cause death, injury, fire or other damage. Anything you do is totally your responsibility.

I have some experience with NSTs from long ago. To cut straight to the point, you ask in a comment on another answer:

Which picture are you [...] saying is the correct way to read secondary voltage? Left or Right? Because Right seems wrong, but that's giving me a correct measurement, and that's how I was taught to use a HV probe.

The response is: Neither of them is completely correct, and that is why your results are confusing - your test configuration is invalid.

Your NST has only one HV output wire (this type of NST is variously called unbalanced / single-ended / single-end grounded / one-end referenced / one-end grounded). That HV output wire is where your photos show the HV test probe measurement "point", so that is correct.

However on this type of NST, the output voltage is referenced to ground (as you are trying to measure in your left photo) but you have not grounded the transformer. The other end of the HV secondary is internally connected to the transformer frame (core). That is where your other output wire is already connected, as we see in your photos, and the frame should be grounded - usually via the mains earth wire.

See below (from Questions and Answers on Neon Transformers):

You seem to be measuring the correct voltage in your right-hand photo because the "common" (a.k.a. ground) clip for your HV measurement probe is connected to the transformer mains neutral input, which should be within a few volts of ground. So by luck, your measurement seems to work. But that configuration, without the transformer being grounded, is still invalid and unsafe.

You need to:

• Make the mains connections safe. Your photos seem to show uninsulated twisted wire connections, from the transformer's mains input wires to another cable with a mains plug. These connections could becomes loose, unintentionally touch each other or other objects etc. with dangerous consequences.

• I was taught that the mains input to those neon sign transformers is AC polarity-sensitive i.e. the transformer black input wire must go to the mains hot (live), and the transformer white input wire must go to the mains neutral. I don't know the reason for this, but the fact that these transformers use different colors for the mains input wires (white and black), rather than two wires of the same color, is consistent with polarity being important.

• The transformer's body itself must be connected to mains ground. Failure to do so can cause the transformer body to reach a dangerous voltage by capacitive coupling, as well as incorrect output voltage.

• The image below (from signindustry.com - Essential Tools of the Trade), shows the correct test configuration, similar to your right-hand photo - except your transformer has only 1 HV output wire, not the 2 HV outputs shown in that image (that's a different type of NST). So the other end of the neon sign would be grounded via the wire connected to your transformer's frame, if using your type of transformer:

Then you should be able to measure 5 kV between the HV output wire and the grounded transformer frame.

• Good answer, Sam. Do you reckon the OP's secondary is referenced to the frame or not? I can only explain the readings if it's an auto-transformer isolated from ground and grounding the frame would then give a neutral reference through the mains-neutral link at the supply. I was unable to find a datasheet for it either. Aug 1, 2021 at 7:45