How to Use a Loop Impedance Tester

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Safety first: a loop impedance test puts a current through a live circuit, so it belongs to a qualified person working to their local wiring rules. Confirm the tester’s own input range and the mains voltage before you connect it. Do not test a circuit that your tester’s manual says is unsuitable, such as an IT-type supply, until you have set the mode correctly. Panel and service work belongs to a licensed electrician.

This guide shows how to use a loop impedance tester: it explains what the test measures, how to choose L-PE or L-N, when to use high current or non-trip mode, and why an IT setting can block the L-PE reading.

Quick answer

Connect the tester to the outlet or test point as its manual directs. Select the loop test and the path you need, L-PE (line to earth) or L-N (line to neutral). Choose high current mode where the circuit has an RCD and you want a faster result, or non-trip mode where the manual says the test must not trip the device. Read the impedance in ohms, then compare it with the limits your local wiring rules set for that circuit.

What a loop impedance test measures

A loop impedance test measures the impedance of the path a fault current would take: from the line conductor, through the load side of the installation, and back to the source through earth or the neutral. Fluke’s Fluke 1654B product page describes the two paths as “Loop (L-PE): Phase to earth” and “Line (L-N): Phase to neutral.” Those are the two readings most installation testers offer, and they answer different questions. L-PE checks the earth fault path that protects people from touching a faulty enclosure. L-N checks the path that carries the current when a live conductor shorts to the neutral.

The impedance value matters because it sets how much current a fault can produce. A lower loop impedance means a larger prospective fault current, which is what allows a protective device to trip quickly. A higher reading can mean a long cable run, a loose or corroded connection, or a poor earth. The tester gives you the number; the rules for the circuit decide whether the number is acceptable.

Choose the loop path: L-PE or L-N

Select the path that matches the protection you are checking. The 1654B page lists L-PE and L-N as separate measurements, and Fluke’s support article on the same family describes switching between L-N and L-PE readings. If your meter shows only one loop value, check the manual for which path it measures before you record the result. A label on the screen is not enough; confirm it in the manual.

Record which path you measured with each result. A note that says “L-PE, high current, 230 V” is far more useful later than a bare number in ohms.

High current mode and non-trip mode

Fluke’s 1654B page describes two loop methods. High current mode applies a short, high test current. According to the page, it gives “faster measurements compared to loop tests with non-trip mode for RCD’s.” Non-trip mode uses a much lower test current so that residual current devices are less likely to operate during the test.

The 1654B specifications list the test current as “20 A sinusoidal for 10 ms” at 400 V and “12 A sinusoidal for 10 ms” at 230 V. Accuracy is given separately for each mode: ±(3% + 6 digits) for non-trip mode and ±(2% + 4 digits) for high current mode. Input range is 100 to 500 VAC at 50/60 Hz. The same page notes a thermal limit, with automatic shutdown after about 50 consecutive tests at 10-second intervals as a typical figure.

Choose the mode by the circuit, not by habit. On a circuit protected by an RCD, the mode the manual recommends for RCD-protected circuits is the one to use. If you are unsure whether a high current pulse could trip a device on the circuit, use the mode the manual labels as non-trip, and confirm the device arrangement before testing.

IT systems and the L-PE reading

Fluke’s support article KA-01014 describes a common reason an L-PE reading is missing. It explains that the tester may be in IT mode, which is a test setting for IT systems. In an IT system, the article says, “the PE is not connected to the power supply,” so an L-PE loop impedance measurement cannot be performed in that mode. If L-N readings work and L-PE readings do not, check the IT setting before you assume a wiring fault.

The article gives different steps for older and newer models. On older models it describes a power-on key sequence that displays “IT OFF.” On newer models it describes checking the IT mode status with the backlight key and changing it with the Up and Down keys. After switching the mode off, the article says you should be able to measure both L-N and L-PE loop impedance, with and without RCD. The exact key names differ by model, so follow the manual for your tester.

Step-by-step: a basic loop test

1. Read the manual and the circuit label. Confirm the tester’s voltage range covers the supply, and note the protective device on the circuit.
2. Set the supply type. Check the IT setting. If the installation is not an IT system, the article indicates that IT mode must be off for an L-PE reading.
3. Select the loop test and path. Choose L-PE or L-N as the job requires.
4. Choose high current or non-trip mode. Use the mode the manual recommends for the circuit’s RCD arrangement.
5. Connect the leads. Follow the lead and socket diagram in the manual. Keep the connection firm.
6. Start the test and wait for the result. Do not repeat tests quickly. Allow time between runs, and respect any thermal limit the manual gives.
7. Record the reading with its settings. Note the path, mode, supply voltage, and the circuit or outlet tested.

When the reading looks wrong

A reading that jumps between runs, or an error on the screen, has several common causes. A poor connection at the test point is the first thing to check. Next, confirm the mode: an IT setting that is on will stop an L-PE reading, as KA-01014 describes. Then check the supply voltage. The 1654B specification gives an input range of 100 to 500 VAC, so a supply outside that range will not produce a valid result.

Repeated tests can also raise the reading if the tester heats up. The 1654B page lists automatic shutdown after roughly 50 consecutive tests at 10-second intervals as a typical limit, so a shutdown after many back-to-back runs is expected behavior, not a fault. Let the tester cool and retest.

What the sources do not settle

The three sources used here cover the Fluke 1654B product page in two regional versions and one Fluke support article. They describe the test modes, the L-PE and L-N paths, the IT setting, and the 1654B specifications. They do not give a general procedure for every Fluke model, and they do not set the limit values your local rules require for a given circuit.

How we chose these sources

We opened three readable pages from Fluke: the KA-01014 support article, and the Fluke 1654B product page in its UK and Australian regional versions. Those are the sources behind every specification and mode name above. We did not use retailer listings, forums, or third-party manuals.

Frequently asked questions

Is L-PE the same as L-N?

No. L-PE measures the path from line to earth, and L-N measures the path from line to neutral. Check the manual for the path your tester labels.

Why does my L-PE reading fail while L-N works?

KA-01014 says the tester may be in IT mode, where the PE is not connected to the power supply, so an L-PE measurement cannot be performed. Check the IT setting first.

Which mode should I use on an RCD circuit?

Use the mode the manual recommends for RCD-protected circuits. The 1654B page says high current mode gives faster measurements than non-trip mode for RCDs, but confirm the choice against your circuit and manual.

Related reading

For a wider view of test equipment, see our earth ground tester picks and our multimeter comparison.

Sources

• Fluke Support, KA-01014 (opened 2026-10-28)
• Fluke 1654B product page, UK (opened 2026-10-28)
• Fluke 1654B product page, Australia (opened 2026-10-28)

What we could not verify

• The loop impedance limits for any specific circuit or wiring rule. Those depend on the local standard and the protective device, and no source we opened lists them.
• Whether the 1653B, the 1652 series, or other Fluke models share the same modes, input ranges, and test currents as the 1654B. The 1654B page is marked discontinued in the regional listings we opened, and we did not confirm specifications for other models.
• The exact key sequence for any single model beyond what KA-01014 describes. Use your model’s manual.
• The test current behavior on non-Fluke testers. Other brands use different methods and may not share these modes.

Bottom line

A loop impedance test is simple to run and easy to misread. Pick the path, pick the mode the circuit calls for, check the IT setting when L-PE fails, and record every result with its settings. The impedance number only means something when you compare it with the limits for that circuit, and those limits come from your local rules rather than from the tester.