×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Paralleling ct's of two circuits into one meter

Paralleling ct's of two circuits into one meter

Paralleling ct's of two circuits into one meter

(OP)
My question is can I meter two three phase circuits with 200:5 cts. with one form 9S meter? Will these cts be additive.

RE: Paralleling ct's of two circuits into one meter

Yes. If the phase relation is 0 degrees. To add CTs means that they shall be parallelled, not seies connected.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Paralleling ct's of two circuits into one meter

What Gunnar said, but they add as phasors (vectors).  If you are looking for highly accurate metering, this probably will not work very well.   

David Castor
www.cvoes.com

RE: Paralleling ct's of two circuits into one meter

Actually, adding as phasors means that the kW metering should be quite accurate even if the circuits are at differing power factors.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling ct's of two circuits into one meter

I presume when you design/modify a CT circuit that you should do some check regarding CT saturation. If you have CT's A and B feeding the meter, then current in CT B contributes to voltage seen at CT A secondary.  Since both CT circuits are similar I presume for simplicity you could assume that each individual CT circuit sees double the impedance of the meter and any shared wiring.

=====================================
(2B)+(2B)'  ?

RE: Paralleling ct's of two circuits into one meter

and of course you add to that the impedance of any other non-shared wiring/devices.

=====================================
(2B)+(2B)'  ?

RE: Paralleling ct's of two circuits into one meter

Saturation is not an issue with properly applied metering class CTs.
I believe that the spec on metering class CTs is to maintain acceptable accuracy at rated burden and 200% current.
Saturation is an issue with protection class CTs. Acceptance testing includes checking saturation voltages to verify acceptable operation at anticipated fault current levels.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Paralleling ct's of two circuits into one meter

I think ePete is considering CT burden rather than saturation. Burden certainly does need to be considered, although most modern electronic meters have such a low burden that it is unlikely to be a problem, maybe more so with long leads and an electro-mechanical meter.

Metering class CTs are designed to saturate to protect the meter from high secondary currents in the event of a through-fault.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Paralleling ct's of two circuits into one meter

Yes, accuracy is certainly the relevant issue, not "saturation".   My apologies for sloppy terminology.

The more direct calc (instead of doubling impedance) would of course be:
Va = Ia * Za_unshared + (Ia + Ib) * Za_shared

Since Scotty mentioned protecting the meter, it brings to mind a question what kind of thermal limit the meter input has? (and could it be challenged?)

 

=====================================
(2B)+(2B)'  ?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources