Klein Et40

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The Klein ET40 is a pocket-sized digital AC/DC voltage tester that shows voltage through a set of LEDs rather than a numeric display. If you searched for “klein et40”, you likely want to know how to read its indicator levels, how far you can trust them, and what it does not measure. It covers AC from 12 to 240 volts and DC from 1.5 to roughly 24 volts. This guide explains how to use it on household circuits and what Klein’s own pages say about its ratings, batteries and limits.

Quick answer

The ET40 tests AC from 12 to 240 volts and DC from 1.5 to roughly 24 volts, using LED levels rather than a number.
Klein rates it CAT III 250V, Class 2, double insulation, and lists IP41 for dust and water resistance.
It runs on two AAA batteries, which Klein includes, and it powers off after about two minutes of non-use.
It does not test continuity, and its accuracy is stated as roughly 70 to 100 percent of the indicated voltage.

What the ET40 measures

The tester has separate AC and DC indicator sets. For AC, the LEDs light at 12 volts, 24 volts, 120 volts and 240 volts. For DC, the levels are 1.5 volts, 9 volts, 12 volts and a top level above 17 to 24 volts. Klein’s page explains that the DC polarity indicators show positive and negative, and that both lights come on when the input is AC. That lets you tell the two types apart without a menu.

The press material Klein published for its residential voltage testers gives the common uses as outlets, light fixtures, doorbells and batteries. Those are the jobs this tester fits. It is designed to tell you whether a circuit is live and roughly what voltage it carries, which is different from measuring an exact value.

How to read the LEDs

Start by touching the two leads to the conductors, or to the terminals of the device you are checking. Watch which LEDs light and how bright they are. A level that lights fully means the input is at or above that level. Because the readings are approximate, a partly lit level tells you the voltage is close to that step, not that it equals it.

Klein states that the ET40 reads roughly 70 to 100 percent of the indicated voltage. The exceptions are 2 volts DC, where the page gives 84 to 100 percent, and the top DC level above 17 volts, where it gives 94 to 100 percent of 17 volts. In practice this means the tester will not give you a precise 120.0 volt figure. It will tell you the outlet is live at the 120-volt level, which is the answer most household checks need.

For AC outlets, a healthy 120-volt receptacle should light the 120-volt level. If only the 12-volt or 24-volt LEDs light, the circuit is either not energized at the outlet or the connection is poor. Check that the leads are firmly seated and try a second outlet on the same circuit before you draw a conclusion.

Safety rating and conditions of use

Klein’s specification lists the safety rating as CAT III 250V, Class 2, double insulation, with pollution degree 2. The CAT rating tells you the tool is designed for installation-level circuits within the stated voltage. It does not make the tool safe for work above that rating, and it does not replace a proper test procedure. Read the CAT rating for what it is: a limit on the environments in which the tester is designed to be used.

The IP rating is IP41, which Klein describes as dust and water resistant. That is a modest rating. Keep the tester dry, and do not rely on it for wet outdoor work. Klein lists a drop protection height of 9.8 feet, or 3 meters, which is useful for a tool that lives in a pocket or on a belt.

Batteries, auto-off and the low battery light

The ET40 runs on two AAA batteries. Klein lists them as included with the tester. It has a low-battery indicator and powers off after about two minutes without use. Auto-off saves the batteries, but it can surprise you if you pause between readings. Press the tester on again if the LEDs do not respond.

Replace the batteries when the low-battery indicator appears, and keep spares in the case or pocket you carry the tester in. A dead tester can give you a false sense that a circuit is dead, so check the battery before you rely on a negative result.

What the ET40 does not do

The ET40 does not test continuity. Klein states that continuity testing is not available on this model. If you need to check whether a wire is open, or whether a fuse passes current, use a multimeter or a continuity tester. The ET40 is a voltage check, not a full electrical diagnosis.

It also does not show a number. The LED levels are the only output. If you need a measured voltage to a decimal place, a digital multimeter is the right tool. For background on what a CAT rating covers and why it matters for a voltage check, see what a CAT rating means on a multimeter.

Practical checks with the ET40

A common use is confirming that an outlet is live before you work on it. Test a known live source first, check the tester reads it, then test the outlet or fixture you need to verify, and then test the known source again. This sequence catches a tester that has failed, which is the most dangerous kind of false negative. It is a standard habit for any voltage tester.

For a light fixture, test both the hot and neutral side of the circuit at the box, if you have access and are qualified to do so. Remember that a DC reading on an AC circuit is not expected, so the polarity lights should show both on for AC. If the DC indicators light on a supposed AC circuit, stop and investigate before continuing.

If you are checking outlets in a home, the same method applies. For a deeper walk-through of testing receptacles with a meter, see how to use a multimeter on an outlet.

How we chose

This guide covers only the Klein ET40. We started with Klein’s own product page, because the manufacturer’s listing is the most direct source for the ranges, the safety rating and the battery details. We then opened Klein’s UK product page to compare the same specifications, and Klein’s residential press release for the use-case wording. We did not include marketplace ratings or review scores, because this article is about how to read the tester and what it can and cannot do.

What we could not verify

We did not find an independent lab test of the ET40 accuracy figure. The 70 to 100 percent accuracy statement comes from Klein.
The sources we opened do not say whether the ET40 is discontinued. The Klein pages show a buy button and no discontinued notice.
We could not confirm the full operating temperature range from the pages we opened.
The press release describes the ET40 for residential use. We did not find data on long-term durability or on performance in wet or dusty conditions beyond the IP41 figure.

FAQ

What voltage range does the klein et40 cover?

Klein lists AC from 12 to 240 volts and DC from 1.5 to roughly 24 volts, shown as LED levels.

Does the ET40 test continuity?

No. Klein’s specifications list continuity testing as not available on this model.

What batteries does it use?

Two AAA batteries. Klein lists them as included.

Why do the LEDs light only partly?

Klein states the tester reads roughly 70 to 100 percent of the indicated voltage, so a partial light means the input is near that level rather than exactly at it.

Is the ET40 rated for outdoor use?

It is IP41, which Klein describes as dust and water resistant. That is a limited rating, so keep it dry and do not rely on it for wet outdoor work.

How do I know the tester is working before I use it?

Use a known live source to confirm the tester responds before and after checking an unknown circuit. Replace the batteries when the low-battery indicator shows.

Bottom line

The Klein ET40 is a compact, battery-powered AC/DC voltage check for outlets, fixtures and batteries. Read the LED levels as approximate, confirm the tester on a known live source, and use a multimeter for any task that needs a number or a continuity check.

Sources

Klein Tools, Digital AC/DC Voltage Tester ET40: https://www.kleintools.com/product/ET40 (opened 2026-10-29)
Klein Tools UK, ET40 Electronic AC/DC Voltage Tester: https://kleintools.co.uk/product/ET40 (opened 2026-10-29)
Klein Tools press release, AC/DC voltage testers for residential wiring: https://www.kleintools.com/content/klein-tools-introduces-easy-use-acdc-voltage-testers-residential-wiring (opened 2026-10-29)