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

Question

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.

Answer 1

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.

Answer 2

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.