# CT connecting to ohmmeter?



## Southeast Power (Jan 18, 2009)

Can you think of any case where you would be interested if there were current flow to ground?


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## splatz (May 23, 2015)

Suncoast Power said:


> Can you think of any case where you would be interested if there were current flow to ground?


I don't think this is a homework question, is it? 

I think the OP's question is why measure the RESISTANCE of the terminals on a CURRENT transformer. I figured the OP is correct, it's a mistake on the print.


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## Lone Crapshooter (Nov 8, 2008)

I have seen old protection and control equipment with built in ohmmeters. 
I think the assembly had 3 ammeters and a ohmmeter. In 20 plus years I have never seen anyone use the ohmmeter. 
The ohmmeter did not have any loose leads it was selected with switch on the panel but exactly what circuit it was checking I cannot say.
The bottom line is yes they are out there. 

LC


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## QMED (Sep 14, 2016)

Suncoast Power said:


> Can you think of any case where you would be interested if there were current flow to ground?


Yes, but I'm pretty sure an ohmmeter will not indicate said current flow.


It's just weird because there are 3 identical xfrmrs each set up the same way. There's an ammeter for each one but they're not shown on the print anywhere and I cannot find a single ohmmeter on the cabinet. I have found one other minor mistake on the drawings but I'm guessing since they're stamped by a PE any obvious errors would be noticed. I can't go back and double check any of this 'cause I'm off work now but this was just something that irked me while I was tracing systems and decided to take a pic so I could research it later.


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## micromind (Aug 11, 2007)

I very strongly suspect that if the secondary of a CT is connected to an Ohmmeter and there is current in the primary of said CT, the Ohmmeter will blow up and very soon thereafter, the CT will be similarly blown up.


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## Big John (May 23, 2010)

There's another ohm symbol near that breaker. What's that supposed to be?

This looks very much like a drafting mistake.


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## QMED (Sep 14, 2016)

Big John said:


> There's another ohm symbol near that breaker. What's that supposed to be?
> 
> This looks very much like a drafting mistake.






Not sure but it appears to be another ohmmeter and it's connected to the....bus bar? IDK honestly, haven't seen any ohmmeters on the switchboard at all. If you can blow that up enough to see but it's the same for the other to 2 xfrms. T-1, T-2 and T-3.

Here's where they are on the switchboard.



So I was examining T-1 at work a few weeks back and found this.



Those are the 2 supply cables for the primary and some cable that somebody hacked off and wrapped with electrical tape. I could still see the the bare copper when I shined my flashlight up inside of the tape lol. Maybe that's coming from the CT up inside the xfmr and was supposed to be going back to the switchboard...?

On the first picture I posted you can see where it says P203 GND, S75. Maybe that means they brought the ground connection from the center point of the secondary and actually terminated in up in the the load center(P203) and that's where the CT is connected. I didn't see any obvious small instrument type cables that might be heading back to the switchboard. Anyways, obviously I'm not super knowledgable on this stuff although if we had proper wiring diagrams in addition to one lines it might be a little easier  Just trying to wrap my head around it.


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## Southeast Power (Jan 18, 2009)

That 300kva is a decent size transformer.
For whatever reason, there is a CT installed on the ground connection, and that is connected to a meter.
I suspect that the symbol they used is a generic one that they picked out of a library indicating a meter connection.


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## hardworkingstiff (Jan 22, 2007)

This is out of my experience level and I cannot read the schematic, but I did notice a push to test button under what looked like the ohm meters (in that picture). Could the ohm meter be there only to test the CT for continuity (power off of course)? Just asking. (does the push to test button isolate the ohm meter from the CT until it's closed?)


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## QMED (Sep 14, 2016)

hardworkingstiff said:


> This is out of my experience level and I cannot read the schematic, but I did notice a push to test button under what looked like the ohm meters (in that picture). Could the ohm meter be there only to test the CT for continuity (power off of course)? Just asking. (does the push to test button isolate the ohm meter from the CT until it's closed?)


Good question, I'm not sure. I would imagine that it disconnects ammeter from the CT secondary circuit. Don't quote me though lol.


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## Big John (May 23, 2010)

Alright, that's a lot clearer, the ohm symbols are meters: 

Each one of those ohm symbols coming off the CT is supposed to be the ammeter for GF detection. You can see that they correspond to your board. 

The ohm symbols that come off each bus are on either side of the tie breaker, so those would be the sources for your voltage and frequency meters. 

This drawing sucks, though:
- I'd be very surprised if they're bringing straight 600V to that switchboard because that's stupid, I'd bet there are PTs not show.
- I'm certain there are fuses in that Voltage/Frequency pickup that aren't shown.
- This shows solid grounds on the transformers with GF monitors: That makes no sense: You would almost never expect to see the type of high-resistance fault that would continuously pass <10 amps. Either it's a silly design or else the drawing is also wrong here, and these are actually high resistance grounded wyes.


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