CT clamps explained measuring current, direction, and what reversed means
A CT clamp (current transformer) is one of the most misunderstood components in solar installations. Reversed clamps cause battery won't charge symptoms, wrong export readings, and system confusion. This guide explains what they measure, why direction matters, and how to test if yours is installed correctly.
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Book a diagnostic — £75 → How diagnostics workWhat is a CT clamp and how does it work?
A CT clamp is a sensor that measures electrical current. It is not connected directly to the power line — it clamps around the cable and measures magnetically. Here is how it works.
A split-core CT clamp looks like a small hinged metal ring (usually split into two halves). The ring opens, clamps around an AC power cable, and closes. A cable is passed through the ring's centre without any electrical connection — it is purely magnetic coupling.
When AC current flows through a cable, it creates a varying magnetic field around the cable. The CT clamp's coil detects this changing field and produces a proportional voltage signal. This signal is sent to the inverter's input terminal (usually a 3.5mm jack or connector).
Unlike measuring voltage or resistance, CT clamps do not require cutting or splicing the power cable. They clamp around an existing cable and extract measurement data. This makes them safe for retrofit installations and post-commissioning diagnostics.
Where CT clamps are installed in a solar system
Different installations use CT clamps at different locations. Each location serves a specific purpose.
Clamped around the main cable from the electricity meter to the consumer unit. Measures how much power flows in (import) and out (export) of your property. The inverter uses this to:
Clamped around the battery inverter's AC output cable (or DC cables on DC-coupled systems). Measures how much current is flowing in (charge) or out (discharge) of the battery. Used for:
In some larger systems, a CT clamp is installed on the cable from the solar combiner box to the inverter to directly measure panel output. Most systems instead calculate generation from battery charge + load + export, so a dedicated solar CT is not always fitted.
Why CT clamp direction matters: reversed clamps cause chaos
CT clamps are directional. Install one backwards and readings invert — imports become exports, battery won't charge, and the system becomes confused.
A CT clamp is marked with an arrow or P1/P2 labels indicating the intended direction of current measurement. Current flowing in the marked direction produces a positive voltage. Current in the opposite direction produces a negative voltage.
The inverter interprets this: positive = import, negative = export (or vice versa depending on how the system is programmed).
A reversed grid CT clamp inverts all readings:
A reversed battery CT clamp inverts charge/discharge signals:
"Battery won't charge" is often the first symptom that alerts you to a reversed battery CT.
The kettle test: how to verify a CT clamp is installed correctly
Simple, non-invasive test that takes 3 minutes and requires only a kettle and your inverter app.
Open your inverter app on your phone or log into the web portal. Navigate to the "Power" or "Flow" page that shows import/export readings in real-time. Write down or screenshot the current reading (e.g., "240 W import").
Switch on an electric kettle, shower, or oven. The goal is to create a sudden 2,000+ watt load that is not being met by solar.
The import reading should increase significantly (e.g., "240 W" → "2,500 W"). This indicates the system is drawing power from the grid to feed the kettle — the CT clamp correctly measures this as import.
If turning on a 2 kW load causes the export reading to increase, the grid CT clamp is installed backwards. The system is seeing the increased load as export, which is impossible — you cannot export more power than the solar is generating.
Switch off the kettle. The import reading should drop back to near the original value (give it 10 seconds to update). This confirms the CT clamp is responding to real load changes — not a one-time glitch.
Testing battery CT clamps
If your system has battery and the grid CT test passed, check the battery CT clamp separately:
Single-phase vs three-phase: does my system use one or three CT clamps?
UK residential supplies come in single-phase or three-phase. CT clamp configuration depends on this.
A typical UK residential property has a single-phase supply (230V AC, one live wire, one neutral, one earth). Most solar systems use one CT clamp on the main import/export cable. If the system has battery, a second CT clamp measures battery current.
A single CT clamp on a single-phase supply measures 100% of the power flow to/from your property.
Three-phase supplies have three live wires (phases A, B, C), each 230V to neutral. Most three-phase solar systems install one CT clamp per phase (three total) to measure the total power flow across all phases.
If a system has only one CT clamp on a three-phase supply, it measures only 33% of the power flow. Generation/import figures appear 30% low.
In some battery installations, CT clamps are installed on individual circuits (e.g., one on the essential loads circuit, one on the general circuit, one on the battery). This allows the inverter to prioritize battery discharge to essential circuits during a power cut (Emergency Power Supply mode).
If you have a split-load battery system, verify the installer fitted a CT clamp on each monitored circuit — missing clamps cause incomplete load measurement.
Frequently asked questions
Yes, CT clamps can fail. Common causes: water ingress (clamp installed in a damp location), physical damage (clamp knocked or bent), or internal coil burn-out from a lightning surge or over-current event. If a CT clamp fails, the inverter stops reading current data for that measurement point and may show "sensor error" or "CT clamp disconnected" in the fault log. The affected feature (export limiting, battery charging) will stop working. Replacement is straightforward — contact your installer or book a diagnostic.
Flipping a CT clamp requires turning off the system's AC and DC isolators first. If you are qualified to safely isolate the system, it is a 5-minute job: unclamp, rotate 180°, reclamp, reseat the connector. If you are not confident, contact your installer or a qualified solar engineer. Do not attempt this with the system live — the cable may be carrying current and flipping a clamp with power on is unsafe.
No. Smart Export Guarantee (SEG) payments are measured by your utility company's smart meter — not by the CT clamp. The CT clamp is used internally by the inverter for optimization and export limiting. A reversed grid CT means your inverter can't optimize battery charging or enforce DNO export limits correctly, but your SEG payments are calculated from the meter and remain unaffected.
Most likely your system has only one CT clamp measuring one phase, when it should have three (one per phase). A single CT on a three-phase supply captures only about 33% of total power flow — so import, export, and generation figures all appear low. Ask your installer whether CT clamps are fitted on phases B and C. If not, this is a commissioning error: additional clamps can be added and the system reconfigured. Common on three-phase retrofit installs.
A Rogowski coil is a flexible wire loop that wraps multiple times around a cable. More accurate than split-core CT clamps and less prone to saturation at high currents — common on commercial systems above 50 kW. For UK domestic solar (5–15 kW), split-core CT clamps are adequate and standard. If your system uses Rogowski coils, the same direction principles apply — they can also be reversed, causing the same inverted readings.
Related guides and problem pages
How your DNO sets an export limit and why your CT clamp is essential to enforcing it correctly.
Battery chemistry, BMS, and why a reversed CT clamp causes overnight charging to fail.
How the inverter uses CT clamp data to manage self-consumption, battery, and export.
Step-by-step diagnosis to confirm a reversed or mispositioned CT clamp and what to do next.
Diagnosis for batteries that won't charge overnight — CT clamp issues are a leading cause.
What your installer should have verified on the day — including CT clamp orientation testing.
CT clamp issue or battery won’t charge?
If the kettle test reveals a reversed CT clamp, or battery charging isn’t working as expected, a remote diagnostic identifies the cause and resolves it — usually in a single 30-minute session.