Sensitive Earth Fault
The core-balance current transformer (or CBCT) is normally of the ring type, through the centre of which is passed cable that forms the primary winding. An earth fault relay, connected to the secondary winding, is energised only when there is residual current in the primary system.
The advantage in using this method of earth fault protection lies in the fact that only one CT core is used in place of three phase CT’s whose secondary windings are residually connected. In this way the CT magnetising current at relay operation is reduced by approximately three-to-one, an important consideration in sensitive earth fault relays where a low effective setting is required. The number of secondary turns does not need to be related to the cable rated current because no secondary current would flow under normal balanced conditions.
This allows the number of secondary turns to be chosen such as to optimise the effective primary pick-up current.
Core-balance transformers are normally mounted over a cable at a point close up to the cable gland of switchgear or other apparatus. Physically split cores (‘slip-over’ types) are normally available for applications in which the cables are already made up, as on existing switchgear.
SOURCE: Network Protection & Automation Guide -Current and Voltage Transformers
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
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Hi
Please let me know what is different between class 3FS10 and 1FS10 in core balance and when we use 3FS10 or 1FS10.
CBCT size will depend on the cable cross section/ bus bar size.
Please clarify me difference between 3P3L & 3P5L PT
hi
Using Core-Balance Current Transformer For Earth Fault Protection.
Using Core-Balance Current Transformer For earth leakage protection.
can i connect earth leakage relay with nornal protection ct which is use for basically over current & earth fault protection??
Hi,
Why do we connect CT & CBCT in distribution panel?
can we do the HI pot test on cable if CB CT connected in cable . please advsie
Do CBCT have any direction for primary current ( Like Normal CT ) ???
Hello,
I want to design and calculate CBCT,which Formula i shoulad use?
Hi,
I have doubt about core balance CTs installation. Due to size of cables we have to use 2 CTs per cubicle = 2 single core cables per phase. This CTs are very close to each other (touching in some cases). Is it eventually problem (risk) that they will interfere each other? Problem with EMC?
Thanks for advice.
Hi, can you tell me how to select a core balance current transformer? they have specified it to be PS Class? what calculations do I follow and where do i find them?
Observation : 3 separate armoured 1core cables for 3 phase passes through common cbct. During motor loading higher than rated current spurious earth fault is coming. Reason and remedy are reqd.
Can we use core balance CT in 11kv or 22kv?will it withstand those voltage
In 4 core cable (3p+N) do we pass N core through core balance?
In case of 4Core or 3.5 Core cable, if Neutral is passed through CBCT, then in case of any phase unbalance, the Earth fault CT connected to it will not trip since the differential current shall flow through the neutral. Thus the relay will not trip. But, if only the three phase cores of the cable is passed through the CBCT, then occurance of any phase unbalance as well as during earth fault the Relay connected to that CBCT will trip.
Whatever you said above, is TRUE only in 3 ph 4 wire system, for e.g. 415V (LV) distribution system. If it is 3 ph 3 wire system , like in 11kV or 6.6kV distribution systems, even if there is unbalanced loading, that is difference in R,Y, B phase currents, algebric sum (vectorial) current Ir+Iy+Ib =0 & hence there will/should be no current in CBCT secondary & relay will not operate. Only if there is Earth fault in one or two phases, there will be current in CBCT secondary & relay will operate.
Hi
Can anybody tell me what is the meanings of CBCT’s parameters below?
Zero phase current ratio Pri./sec. 200mA/100mV (75~125mV at 2kohm); This zero phase sequence current transformer has the rated current of 80A. I used it for signaling a earth fault relay of a motor control unit.
Thanks.
In 4 core cable (3p+N) do we most pss N core through core balance?
Any advise about how to select/size the convenient CBCT? and who are the most common manufactures for this product.
Thanks