A current transformer (CT) scales the primary current down to a standard secondary value — normally 5 A or 1 A — so that meters, relays and instruments can be connected safely and consistently. The LZZBJ9-10 is a common epoxy-resin-cast, indoor CT for 10 kV switchgear, with a primary winding that suits both metering and protection cores.
Metering Cores vs Protection Cores
One CT often carries more than one core, and the two cores are designed for different jobs:
Metering core: high accuracy at normal load — class 0.2, 0.5 or 1.0 — so that billing is accurate. It is deliberately made to saturate at a modest multiple of rated current, which protects the meter from fault current.
Protection core: accuracy class 5P or 10P with an accuracy-limit factor (ALF). It must stay accurate well into the fault range so that overcurrent and differential relays see a faithful image of the current.
Typical Ratings (LZZBJ9-10)
Parameter
Typical value
Rated voltage
10 kV (12 kV class)
Rated primary current
50–2000 A
Rated secondary current
5 A or 1 A
Accuracy class
0.2 / 0.5 / 5P10 / 10P10
Rated output (burden)
10–40 VA
Short-time thermal current
up to 40 kA / 1 s
Dynamic current
up to 100 kA
Selecting the Ratio and Class
Base the ratio on the expected normal current, so that the CT operates in its most accurate band. A ratio that is far too large pushes the working point into the low-accuracy region.
Match the class to the duty: 0.5 for billing, 0.2 for high-accuracy metering, 5P10 for overcurrent protection.
Check the rated burden against the actual connected load — the total of the relay, meter, and the lead burden. If the actual burden exceeds the rated value, the CT accuracy falls.
Confirm the short-time thermal and dynamic ratings against the switchgear fault level.
Secondary Wiring Practice
Never open-circuit a live CT secondary. With the secondary open, all the primary ampere-turns appear across the core and a dangerous high voltage develops. Short the secondary with a test block before working on the circuit.
Earth one point of the secondary — and one point only — to avoid circulating currents.
Use conductors of adequate cross-section; a long or undersized run adds burden and degrades accuracy.
Keep the CT secondary wiring away from power cables to reduce induced interference in the measuring circuit.
FAQ
Why does the metering core saturate at a low multiple? So that a fault current does not damage the meter or drive it into a range where it could be inaccurate or unsafe. The protection core is the one that must handle fault current.
Can I use a protection CT for metering? Not where revenue accuracy is required. A protection core is not calibrated for accurate measurement at low current. Use a dedicated metering core.
