Answer / duraivelu j

there are two type of ct. one is metering ( class 0.1,class
0.5,1,&5)another one is Protection (PS, 5P10,5P20)the
metering ct has very less error, so we can measure actual
power by accuratly,the protecton ct has more error but that
has more saturaion level, so during fault condition this ct
will not saturate. bur Metering ct will saturate when fault
condition and that time we can't measure the current vale.

Answer / mhd hassan

5p10 = 5% and 10 time nominal current

Answer / er.k.k.murty

From: Er.K.K.Murty, B.E.(Hons)Electrical,FIE.
Retd.Chief Engineer(Testing & Commnu),M.P.State Electricity
Board.
Jabalpur(INDIA).
Email ID:kkmurty05@yahoo.com

Measuring Current transformer: This is also known as
Metering CT ,it should confirm to IS:27O5-2 or IEC-60044-1
The cross section area of the cores are so designed that
,the core saturates at about 5 times the rated current (with
low saturation current) .This would facilitate safety to the
instruments, while heavy fault current flows in the Primary
circuit.
Before going into the details of characteristic / duty to
be performed by the metering CTs, one should be aware of
some definitions / terminology relevant to the equipment.
7.1 Instrument Security Factor (IFS/FS): The ratio of
instrument limit primary Current to the rated primary current.
Note: In the event of system fault current flowing through
the primary winding of a CT, the safety of the apparatus
supplied by the CT is greatest when the value of the
instrument security factor (SF) is small.
. Rated Instrument Limit Primary Current (IPL): The Value
of the minimum primary current at which the composite error
of the measuring CT is equal to or greater than 10%,while
the secondary burden is equal to the rated burden.
Note: The composite error should be greater than 10%, in
order to protect the apparatus supplied by the CT against
the high current produced in the event of system fault.
8.0 Accuracy classes;

(a)Standard Accuracy classes: 0.1,0.2,0.5 and1.0 class-The
accuracy limits are defined from 5% of current onwards,
however the declared accuracy limits ie 0.1,0.2,0.5 & 1.0%
respectively and 5,10,30,60 minutes respectively would be at
100% to 120% of rated current at 25% to 100% of rated
burden. {Ref:IS:2705(part-2)}
(b)Special Application Accuracy classes: 0.2S and 0.5S are
used for special applications. The accuracy limits are
defined from1% of rated current, however the declared
accuracy limits ie 0.2% & 0.5%and 10&30 minutes respectively
would be at 20% to 120% rated current at 25% to 100% of
rated burden. Precision metering of High value HT consumers
and Tariff meters etc are categorized as special application
metering and need ‘S’ class CTs. {Ref:IS:2705(part-2)or
IEC:60044-1}.
Note: Almost all the Distribution Utilities follow the
convention mentioned here under;
(i) 0.2 and 0.2S are used for the voltage above 33kV level.
(ii) 0.5 and 0.5S are used up to 33kV level.

5P10 & 5P20 Class CTs: They are generally used for Over
current (Instantaneous and IDMT relays) and E/F protection
of Feeders, Transformers etc.
5 signifies the % limit of composite error
P signifies Protection core,
10, 20 signify the quantum of fault current as multiples of
the rated current or ALF(Accuracy Limit Factor) up to which
the CT shall have the defined percentage of composite error
i.e. ± 5%.
Standard Accuracy Limit factors as per IS: 2705 and
IEC:60044-1 are 5, 10, 15, 20 & 30.
Accuracy limit factor is inversely proportional to the rated
burden i.e. if the burden is lower than the rated one, the
limit of accuracy shall be maintained beyond the declared one.
The formulae for actual ALF;
ALFACT= ALFRTD x (Sin+Sn(Rated Burden))/( Sin+Sa(Actual
Burden)).
Example:- Calculate actual Accuracy limit factor if
secondary burden is reduced to 1/3rd, if CT rated as 200/5
A,5P10,15VA and Rct =0.15Ω?
ALF Rated=10(From CT data 5P10),
Sin (Burden due to internal resistance of the CT) = (5A)2 x
0.15=25 x 0.15=3.75VA,
Sn (Rated burden) = 15VA (From CT data),
Sa(Actual Burden) = 15÷3= 5VA,

ALFACT = ALFRTD x (Sin+Sn(Rated Burden))/( Sin+Sa(Actual
Burden))

ALFACT = 10x ((3.75+15))/((3.75+5)) = 10x18.75/8.75 = 21.42
It is clear from the above example that the ALF increases if
the connected actual burden is lower than the rated one. The
CT in this condition shall maintain accuracy up to fault
current of 21.42 times of rated current as against 10 times
the rated current and shall not saturate till then.
PS Class CTs: Though abbreviation PS is not elaborated
anywhere, however it may be called as “Protection Special
Class” CT core. This core is used particularly where current
balance is precisely required to be maintained. In
Differential protection, balance is the prime requirement
between secondary currents of associated CTs of either side
of the equipments. Differential and restricted E/F
Protection of Transformers and Overall Differential
Protection of Generators need such CTS.
The 5P10P or 5P20 class CTs cannot match the Characteristic
as that of PS class. The core of PS class is such that very
high current is needed for saturation of the core. Knee
point voltage of the CT is of valid importance. The
developed voltage across the relay terminals should be lower
than the Knee point Voltage of the CTs.

Answer / nirmallya

METERING CT CL-1
+-1% ACCURACY
PROTECTION CT
CL 5P10
+-5%ACCURACY SATURATION LEVEL 10 TIMES OF THE RATED CURRENT

Answer / ayub

THERE ARE TWO TYPE OF CT USED
1) METERING
2) PROTECTION

IN THE PROTECTION CTS
THERE SUB CT/S BASED ON THERE FUNTIONING

5P10, 5P20 & PS

FOR 5P10 CT : 5 % error in the ct for protection current
sensing/measurement
p: is used for Protection purpose
10 0r 20 : The ct will withstand 10 or 20 time current of
the rated primary current of the ct e.g
300/1 it mean the ct will with stand 3000A, for 1 minute
during the fault sensing current

Answer / pin baloo

Standard accuracy classes:
At rated frequency and with rated burden connected, the amplitude error and phase error and composite error of a CT shall not exceed the values given in the following:
Accuracy class//Amplitude error at rated primary current(%)//Phase displacement at rated primary current(°)//
Composite error eC at rated accuracy limit primary current(%):

5P // ±1% // ±1° // 5%
10P // ±%3 // - // 10%
Marking: The accuracy class of a CT is written after the rated power. E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20
Accuracy limit current IAL:
Current transformers for protection must retain a reasonable accuracy up to the largest relevant fault current. Rated accuracy limit current is the value of primary current up to which the CT will comply with the requirements for composite error eC.
Accuracy limit factor kALF:
The ratio of the accuracy limit current to the rated primary current.
kALF = IAL/IN. (eq. 2)
The standard accuracy limit factors are 5, 10, 15, 20 and 30.
Marking: Accuracy limit factor is written after the accuracy class. E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20.

Answer / sindhu sl

There are three types of core basically used in Current Transformers
they are
1)Metering
2)Protection
3)PS core or Protection Special
Basically CT are the one which is used to measure the current in the lower value as Ammeters and other measuring meters are rated at a lower value.Thus CTs step down the current and this is used for metering or protection purpose.
1)METERING:
Metering cores are used only for measurement.for a 500/1 A CT the core should essentially saturate after 120% of the rated current then only it will give accurate measurements.So saturation must be very much higher for metering core.
it is identified as 0.1S,0.2S,0.5S.These are the accuracy class explained by Indian Standars. 0.1S is more accurate than 0.2 and 0.5.

2)Protection Core:
Protection cores are used when a high surge current enters or in the case of any abnormal current.So the core should not saturate when the primary experiences high current.
it is designated as 5P10,5P20 etc
here 5 denotes the percentage of error
P-Protection
10-Accuracy Limit Factor(ratio of accuracy limit Primary current/Rated Primary Current)

If ALF is high then the Class of protection is also high.

3)Protection Special or PS Core:
This is also used for special type of protection in case of heavy faults particularly differential faults. It is determined by Knee Point voltage or Short time current

Answer / anonymous

CT of protection(PS 5P10 5P20) type is designed for low accuracy and high saturation bcoz at the time of fault curretn is very high. Gives input to relays.
CT of meter(0.1 0.5 1) type is designed for high accuracy and lower saturation. Gives input to ammeters.....
Eg: CT specs 10VA 5P10
This means it has a ratio error of 5% at 10 times rated current I if the real load consumes 10 VA at I.
CT specs 10VA 5P20
Ratio error 5% at 20 times I
Both are same as it gives same ratio error; selection however differs as 5P20 has higher saturation limit

Answer / tahir vora- reliance jamnager-

5P10 means the maximum permissible limit of error (accuracy limit) is 5% at 10 times rated current (Accuracy Limit Factor). CT will give ratio error of only 5% even if fault current reaches 10 times the nominal value (In), after that CT will start to saturate.

Answer / kkmurty05

MrTahir Vora,Reliance Jamnager has said that CT will give 5% ratio error.
(1).It is not ratio error alone but it is the composite error.
(2)The definition of ALF is no doubt correct for rated burden,however ALF increases if the total connected burden is less than the rated burden.

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