Previous in Forum: Power Quality Issue with Installed Passive Filter   Next in Forum: Electrical Panel Wires Burnt Out
Close
Close
Close
4 comments
Associate

Join Date: Dec 2016
Posts: 26

Current Transformer - Sizing

02/09/2021 2:24 PM

In general, it is typical to size the current transformer with +20% margin of Load current. Say, if the spec of the CT is 5P20, - It means for 20 times the CT primary, the error in the secondary will not exceed 5 percent.

Since the accuracy is 5% for 20 times the rated CT primary, Why to size with a margin of 20%?

Login to Reply
Pathfinder Tags: Current Transformer Margin Sizing
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 29823
Good Answers: 1665
#1

Re: Current Transformer - Sizing

02/09/2021 2:58 PM

If you were designing a tightrope to walk across a great height, what would your margin of safety be, and why?

https://resources.pcb.cadence.com/blog/2019-fmea-risk-assessment-for-pcb-component-selection

__________________
Break a sweat everyday doing something you enjoy
Login to Reply
2
Guru

Join Date: May 2018
Location: Under the spreading Bunya Trees, South Burnett, Queensland, Australia
Posts: 532
Good Answers: 47
#2

Re: Current Transformer - Sizing

02/10/2021 12:51 AM

It has a lot to do with asynchronous currents causing saturation of the CT during fault conditions affecting the secondary current for protective devices operation.

As we all know supply authorities always like to run the system above design load to squeeze just a bit more out for the peak demands and rely on the daily duty cycle, so 20% is acceptable. Even the curve of protective devices, say a CB, can have up to 25% overload current flowing before they will operate to trip in a predefined time.

__________________
Hare today, goon tomorrow!
Login to Reply Good Answer (Score 2)
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 290
Good Answers: 17
#3

Re: Current Transformer - Sizing

02/12/2021 11:34 PM

5P20 CT is allowed to have 5% error max at 20 times rated current and that is for protection system design engineers.

20% overload is for the operator to make sure that the CT can continuously be loaded to 120% without suffering any thermal damage, if and when required.

When it comes to error in the operating load range, 5P20 CT can have max 1% error (not 5%). This as per IEC.

__________________
Raghunath
Login to Reply
Guru

Join Date: Jul 2016
Posts: 999
Good Answers: 67
#4

Re: Current Transformer - Sizing

02/13/2021 4:55 PM

In addition to the GAs already posted, commercially & easily available instruments are designed with standard 5 ampere full scale. If you connect the 0-2000A meter with the 0-5A movement to your 2000:5 current transformer, any overload you might want to evaluate will be off the scale, telling you little about the nature of the problem, for instance how much over and for how long?

Login to Reply
Login to Reply 4 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

raghun (1); rwilliams (1); SolarEagle (1); Stef (1)

Previous in Forum: Power Quality Issue with Installed Passive Filter   Next in Forum: Electrical Panel Wires Burnt Out

Advertisement