Recommended CT connected energy meter rating for a CT with secondary 5A - Page 1
Forum section: electronics / general

author: tolerl
author_rank: Newbie
timestamp: October 31, 2020, 06:53:58 pm
content: I work in a Maldivian Utility Company. Our engineering team is pretty weak so I don't have anybody in the company to ask even though this is the state electric company.A supplier quoted us 2 CT meters . Rating for one is given as 1A(6A) and other as 5A(20A) which I believe is 1A basic current Ib and 6A Max current and in other case 5A basic current Ib and 20A Max current.Supplier is recommending 5A(20A) saying that 5A Ib meters is the standard approach for the 5A CTs and 1A Ib is for CTs with 1A secondary current.But my understanding is 1A(6A) meters fit better with current range of 5A CTs.I would like to clarify what is the standard rating for 5A CTs and point me to is any literature to get more information about matching meter rating with CT secondary rating.Sorry for the bad English.

author: jonpaul
author_rank: Super Contributor
timestamp: November 01, 2020, 06:59:07 am
content: Hello Tolerl, nice to hear from East Europe!How is it in Moldavia?A few notes:CTs require a specified "burden" in VA for correct ratio.The device you connect the CT to must be appropriately rated for scale factor.Most utility and high power applications use a 5A secondary eg 200A pri: 5 A sec.Low power and instrument application can use the less used 1A secondary.Besides ratio and frequency range (eg 50/60 Hz, 45-1000 Hz) beware the opening diameter and insulation rating!EG completely different CT for 240V line, or 12 kV distribution line or 345 kV transmission!DO NOT use at line voltages over rating! SAFETY ISSUE!Strongly suggest you consult:1/ Specifications for the equipment you are connecting to or measuring with2/ Manufacturers specifications and app notes of current transformers.Check the big firms like ABB, Schnieder, GE, etc.Just the ramblings of an old retired EE!Bon dimanche,Jon

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 01, 2020, 08:50:03 am
content: Besides what Jonpaul mentioned, CTs are different if used for metering vs protection. The burden is important as precision can be affected a lot by it.I suggest you read this documenthttps://download.schneider-electric.com/files?p_enDocType=Technical+leaflet&p_File_Name=PLSED310169EN_%28web%29.pdf&p_Doc_Ref=PLSED310169EN_WebAlso this one has an interesting Appendix at the end with technical details:https://stevenengineering.com/tech_support/PDFs/45_INSTRUMENT-TRANSFORMERS.pdf

author: nightfire
author_rank: Frequent Contributor
timestamp: November 01, 2020, 12:15:27 pm
content: There are basically two questions that need to be answered:- which dimension the current is that is to be measured, and how precise you want that to be- Type of CT (put-through or separatable type)Depending on the type of CT, they have a precision class. They also give you a burden voltage that is not to be exceeded- so you will have to also take the meter with its burden AND the wire with its resistance into account.The different kinds of CT have their own (dis)advantages: The type you put the wire through is more precise (or mostly more precise within the same price range) than the clamp-on types, but needs downtime to install and repair.In my last job I administered the platform for energy monitoring of a datacenter, worked with Schneider Meters (some of them were bought by Schneider as they swallowed Power Measurement and DSquare). For the very sensitive ION 7600 meters I spent some effforts in choosing the best CT fpr that Job and finally settled for an 1A secondary CT type. Together with short cables (around 8 meters, i think) to the cabinet where the meters were housed we had an acceptable burden so the meters were not impaired in their precision.We have used some shielded cable (CY type in germany) to connect them.Why 1A? The circuits were 50mm² and fused with 125A NH00 fuses. But the real current that would go over them would usually be in the range of 20A-30A because of redundancy setups etc.And so some (I think they were 100/1 CTs with precision 0.5s) would give us a better accuracy than similar priced 200/5 CTs.BUT: before saying that 1A is better, one has to know the load, min and max. current, and the dimension of the room where they will be built in- if you need real precision, you will want to dive deeper in this matter. If you "only" need some rough idea whats going on, the precision class of those CT (and therefore their pricing) will mostly far more important than other stuff.One sidenote: CT should, if possible, not connected directly to the meter- if the primary circuit is live, the secondary circuit has to be shortened!I used the predecessor ofhttps://www.wago.com/de/reihenklemmen/klemmenblock/p/2007-8873for connecting them- those terminals have also 4mm jacks that can be used to connect other equipment like a portable power quality analyzer or a simple DMM to verify the readings of a CT or voltage tap.

author: AlbertL
author_rank: Regular Contributor
timestamp: November 15, 2020, 05:55:56 pm
content: There's some good info here:https://www.gegridsolutions.com/iti/specs/resources.htm, especially in the "Electric Utilities Reference Guide..." and "Instrument Transformer Technical Information..." documents.