I have continuity between Usb shell and the rail

I have the crossfire pro with THC, limit switches, and a hypertherm powermax 45xp. laptop is isolated on its own table running on battery. I am getting continuity from the rail to the USB shell when I have the THC cable plugged into the electronics control box. it is not a constant beep but it is a pulse beep and in Ohms it reads 3.320 k Ohms. when the THC is not plugged into the electronics control box and I test continuity from the rail to the USB shell I have to work at getting one single beep not constant on my multimeter but it reads open loop and never registers on the multimeter. I have never fired the plasma torch.

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Actually that is not unusual. One would think it is totally isolated but once everything is connected there is enough paths for the connectivity, although slight, to get through. This was discussed in another thread and I believe my table showed similar results as what you got. Even Langmuir support said there will be some connectivity in the neighborhood of 5k ohms to 10k ohms. I can’t remember. But it won’t be a problem. I have the HP 45 XP as well and had no connectivity what so every when the computer and plasma cutter were not attached. When attached, I had closer to what you got.

Welcome to the forum.

Edit: I found my original post on my ohms, you are going to laugh…maybe:
Resistance from the shoulder of the USB receptacle to any bolt, slat or water table does measure some resistance when the table is all turned on, plugged in. I am getting 3.5 to 3.6k ohms. Interesting.


how long have you been running it like that? And you never had any issues?

I have had my table up and running for since Dec. 2021. No issues. Exactly. You’re fine.

It was just a few months ago that I checked when another user stated he had some connectivity. I checked mine, without the plasma cutter or computer connected and there is absolutely no connectivity so I initially reported that “my table works fine and has zero connectivity.” I later checked it with all the connections and low and behold there was some.

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you must have not read the grounding test section in the THC setup before the initial first cut I guess. that is what I did which lead me to this hesitancy.

You are totally not getting what I am saying. I DID read and performed the test and it was “no connectivity.” The connectivity test is done before connecting the the plasma cutter and computer. It is done after you assemble table and attach the enclosure.

Here is the link I am referring to when I went back and checked to find that there is, indeed, some expected connectivity.


sorry so It looks like you went through the exact situation that I did then.

this is what I read in the instructions and did.

Grounding Considerations

Due to the fact that THC signal voltages can be sensitive, it is important to make sure that USB ground is not connected to the machine frame. This can result in lower than expected voltage readings which can negatively affect performance. Before cutting, we recommend using a multi-meter or continuity tester to check for continuity between the USB plug shell and the gantry tube as shown below. Before performing this test, be sure that:

Machine Power cord is plugged into the electronics enclosure.
USB Cable is plugged into the electronics enclosure and your powered-on computer.
Plasma Cutter is connected to THC VIM and electronics enclosure.
Work clamp is connected to the machine frame.
If you are using a laptop stand, make sure your laptop is sitting on it while performing this test. (Its possible the laptop stand fasteners can conduct through your laptop)

If you detect continuity in this test, please message us at Langmuir Systems Support for steps to resolve.

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Yes. I actually was telling this other user that there should NOT be any connectivity and in this thread that I referenced, there was lots of discussion and very little input from any one else on whether they had any connectivity.

So who knows what other people have. Once your machine is connected and running, there is little reason to check it.


this is true.

Langmuir does not consider that connectivity a true connection. Here is a communication from them to him:

So if some users connect their machines and it does not work correctly, they have you go through their decision tree and if the issue identified is the connection being too much, they suggest a USB isolation device. You will get some error from fire control or it will freeze. It is not going to damage your machine.

Have you sent a message to Langmuir?


I have reinstalled the THC setup now and hopefully I have the same results as you for years to come. thanks for your clarity.

In an attempt to clarify (sorry for the delay), there should be “at least” 10,000 ohms resistance between the enclosure and the table. Less resistance might not necessarily register as “continuity” on the average meter, but could indicate a problem (presumably with the board). I had an issue with the USB shell contacting my enclosure, and my meter did not register “continuity”, but I only had about 2,000 ohms resistance, and as the thread indicates, a whole host of issues. I’m not certain what effect this actually had on my machine’s performance, but I’m of the opinion that it certainly didn’t help.

Hope that adds clarification.



so did you get your board replaced?

so in other words 30.xx M (ohms symbol) is good to go. sorry i’m on a steep learning curve here wishing i’d paid more attention in high school physics among other things. i’m capable given the icu stuff i do, but just not well versed in most electrical things other than healthcare. gathering that the capital M is mega and that means i’m way beyond satisfactory given your 10k figure i think this extech multimeter i have is actually somewhat decent given the amount of sensitivity bc i was getting a faint tone from the unit … depending on whether i swapped contact points of the leads. but this isn’t an ekg so…

also once again … being able to find answers on the forum to my deep diving on every little thing as i double and triple check (as i tend to do) is incredibly helpful. so thanks for all you folks do.

As far as acceptable value, you should just confirm with @langmuir-reilly

Did you check for contact between the USB port and the controller case? That’s where many of us had an issue. I just opened the case, loosened the screws holding the motherboard in place, then slid a piece of thin rubber in near the top of the board to force it over a bit.


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Unplug the Langmuir control box for the wall and see if it goes away. Suppose it does; ensure the bolts inside are not touching the rail. Make sure the isolator is in place. You may want a two-prong adapter on the control box to isolate it from the service ground. As stated, ensure the USB shell is not touching the outside housing. This has been a problem before.


mine is close. it looks like it might be touching. i had zero issues with my first 2 test coupons other than a slight amount of tweaking for mild bevel probably needed to get things perfect and thats with thc on.

yeah mine might be touching, or close, i’m gonna look really closely at that next time in the shop… i’m going to do the 2 prong adapter too and more of a setup like you showed me initially when i signed up for the forum as far as emi proofing. thus far i cut 2 test coupons with zero issues other than slight bevel which is probably just settings tweak needed. laptop was unplugged.

i’m kind of amazed by how well it worked tbh, i was expecting potentially more issues and have actually been a little scared to just make these first cuts. that and the software learning barrier has taken a significant amount of time