Author Topic: NST question about center tap  (Read 323 times)

Offline MRMILSTAR

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NST question about center tap
« on: November 08, 2023, 10:59:43 PM »
Does anyone know if either side of the primary coil of a NST is connected to the core? I know that the center tap of the secondary coil is connected to the core. The reason I am asking is that I measured the resistance between the hot lead of the primary coil input to the ground lug of the NST and measured 582 ohms. I also measured the resistance of the neutral lead of the primary coil input and also measured 582 ohms. I didn't expect this. I expected a very large resistance, practically infinite ideally. Perhaps this is a NST about to fail. I have been operating it without the center tap connected to ground which I will now correct. The transformer in question is made by Jefferson Electric and is a 15 KV, 60 ma model. It is an old-fashioned unit with no GFIC.

I measured another NST and got a resistance of 208K ohms. Still less than I expected but much better. This one is also an old fashioned unit without GFIC.
« Last Edit: November 08, 2023, 11:14:56 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline klugesmith

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Re: NST question about center tap
« Reply #1 on: November 08, 2023, 11:03:49 PM »
We can't tell about _your_ NST.   I will check a few at home tonight if I remember.
Is it possible that you're seeing effects of a SGFP circuit (often casually but mistakenly called a GFI or GFCI)?

Offline MRMILSTAR

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Re: NST question about center tap
« Reply #2 on: November 08, 2023, 11:13:53 PM »
Its an old-fashioned NST with no GFIC.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline klugesmith

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Re: NST question about center tap
« Reply #3 on: November 09, 2023, 03:13:26 AM »
I just checked one here.   Resistance between primary terminals is about 2 ohms. Between primary and grounding conductor is more than 10 megohms.  Secondary to grounding conductor is in the 5k to 10k range.

Offline MRMILSTAR

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Re: NST question about center tap
« Reply #4 on: November 09, 2023, 05:07:51 AM »
I just checked one here.   Resistance between primary terminals is about 2 ohms. Between primary and grounding conductor is more than 10 megohms.  Secondary to grounding conductor is in the 5k to 10k range.

That is about what I would expect. I wonder if my previous operation without the center tap being grounded has weakened the insulation on the primary coil? Maybe the 582 ohms is a sign of imminent failure. It still seems to operate properly and reaches the nameplate voltage with no load. I use it on my pulsed power machine for charging the big capacitor so its never on more than about one minute at a time. From now on I will operate it with the center tap grounded.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

High Voltage Forum

Re: NST question about center tap
« Reply #4 on: November 09, 2023, 05:07:51 AM »

 


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