What does a CT clamp do on a solar system?

A CT clamp (current transformer) measures alternating current flowing through a cable. On a solar system, CT clamps measure: (1) grid import/export — used for self-consumption optimization and export limiting, (2) battery current — used for accurate SoC calculation and charge/discharge balancing, (3) solar generation (in some systems). The inverter uses these measurements to decide whether to charge battery, use solar, import from grid, or export excess generation.

How do I know if my CT clamp is reversed?

A reversed CT clamp shows inverted readings: exports appear as imports in the app, and vice versa. Battery charge commands appear to be discharge, and discharge appears as charge. The simplest test: turn on a kettle on a sunny day and watch the inverter app. If power import increases when you turn on the kettle, the clamp is correct. If import decreases or export increases when you turn on an appliance, the clamp is reversed.

How do I fix a backwards CT clamp?

A reversed CT clamp must be physically flipped 180° around the cable and reseated onto the connector. This requires shutting down the system (AC and DC isolators off), accessing the electrical enclosure, removing the clamp, rotating it 180°, and reconnecting it. It is a 5-minute job but requires system downtime. You cannot fix a reversed CT clamp via software. Contact your installer or a qualified engineer — do not attempt this if you are not competent to isolate the system safely.

Does the CT clamp position affect export readings in the grid?

The CT clamp on the grid connection measures export for the inverter's own management and optimization, but the export payment (SEG, Feed-in Tariff) is measured by your utility meter, not the CT clamp. If the CT clamp is reversed, your SEG payments are NOT affected — they are measured by the smart meter. However, your inverter will not optimize battery charge/discharge correctly if the grid CT is reversed, and export limiting will not work properly if you have a DNO limit.

What is a Rogowski coil and how is it different from a split-core CT clamp?

A Rogowski coil is a flexible wire loop that wraps around a cable multiple times — used on larger commercial systems or where a rigid clamp won't fit. Split-core CT clamps are rigid metal rings that hinge open. Both measure current, but Rogowski coils are more accurate and less prone to saturation at high currents. Most UK domestic solar systems use split-core CT clamps because they are cheaper and adequate for residential power levels (0–20 kW).

Guide · System Setup · Diagnostics

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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The Basics

What 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.

Physical structure: a ring-shaped sensor

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.

How it measures current: electromagnetic induction

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).

Simple analogy: If the cable is a speaker playing music, the CT clamp is a microphone that listens to the magnetic "sound" and reports it to the inverter. The microphone does not touch the speaker — it just listens.

Non-invasive: no wire cutting required

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.

Measurement Points

Where CT clamps are installed in a solar system

Different installations use CT clamps at different locations. Each location serves a specific purpose.

Grid import/export CT clamp (most common)

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:

•Know how much solar/battery is being used on-site vs exported to grid

•Implement export limiting if your DNO has a cap (0W, 3.68 kW, etc.)

•Optimize battery charging/discharging (prioritise self-consumption)

Battery current CT clamp (hybrid systems)

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:

•Calculating battery state of charge (SoC) by integrating current over time

•Monitoring battery health and cycle count

•Triggering charge/discharge limits based on load or time schedule

Solar generation CT clamp (less common)

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.

Direction & Polarity

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.

CT clamps measure in only one direction

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).

What happens when a grid CT clamp is reversed

A reversed grid CT clamp inverts all readings:

Symptom 1: Power import shows as export (and vice versa)

Symptom 2: Battery won't charge from grid — the system thinks it is already exporting when it should import to charge

Symptom 3: Export limiting doesn't work — the inverter thinks you are importing when you are exporting

Symptom 4: SEG payment is still accurate (measured by your meter, not CT clamp) but inverter optimization is broken

What happens when a battery CT clamp is reversed

A reversed battery CT clamp inverts charge/discharge signals:

•Battery charge commands appear as discharge on the display

•Discharge commands appear as charge

•State of Charge (SoC) countdown appears as countdown in reverse

•Battery won't charge: the system sees the current flowing the wrong way and halts charging to protect the battery

"Battery won't charge" is often the first symptom that alerts you to a reversed battery CT.

Testing

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.

Step 1: Prepare on a sunny day (when solar is generating)

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").

Step 2: Turn on a kettle or similar 2–3 kW load

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.

Step 3: Watch the inverter app (allow 5–10 seconds for data to update)

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.

Correct result: Import increases when load turns on.

Step 4: If import decreases or export increases, the CT clamp is reversed

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.

Reversed result: Export increases or import decreases when load turns on. This is wrong.

Step 5: Turn off the kettle and confirm readings return to baseline

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

For battery systems: use a manual charge command

If your system has battery and the grid CT test passed, check the battery CT clamp separately:

1.Note the current battery SoC (e.g., 50%)

2.Issue a manual "charge battery" command from the app

3.The inverter display should show grid import increasing and battery SoC increasing

4.If SoC decreases when you issue a charge command, the battery CT is reversed

Types & Configuration

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.

Single-phase supply (most homes): one CT clamp per measurement point

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 supply (some larger homes, businesses): three CT clamps

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.

Common issue: Three-phase installation with only one CT clamp. Ask your installer whether all three phases have CT clamps fitted.

Split-load configurations: multiple CT clamps for sub-circuits

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

Can a CT clamp break or fail?

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.

Can I flip a CT clamp myself if it is reversed?

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.

Does a reversed CT clamp affect my SEG export payments?

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.

Why does my system show 30% lower generation on a three-phase supply?

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.

What is a Rogowski coil and is it better than a split-core CT clamp?

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.

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