A non-contact voltage tester, also known as a voltage detector, is an electrical tester that aids in the detection of voltage. When troubleshooting or working on a failed asset, voltage presence is useful information to have. A non-contact voltage detector is the first tool you'll reach for. Non-contact voltage testers operate by sensing a very small current that is capacitively coupled from the live circuit to the tester and back to ground. Non-contact voltage testers illuminate when they detect this current without requiring direct contact.