Earth Leakage and Touch Current: The Hidden Safety Spec of an Isolated Driver

A driver can be fully isolated and safe, and still allow a small current to flow to earth when its metal housing is touched. That current, called touch current or protective conductor current, is a real safety parameter measured against a defined standard. It is small, but it is regulated, and it is part of why a driver's safety file is more than a logo.

The source of the current is the input EMI filter. To suppress high-frequency noise, a driver places capacitors between the live and neutral conductors and earth, known as Y capacitors. These capacitors deliberately provide a small leakage path for radio-frequency energy. At mains frequency, they also pass a small sinusoidal current, typically fractions of a milliampere. On a Class I driver with a grounded metal housing, this flows through the protective earth conductor. On an ungrounded Class II or Class 2 driver, it appears as touch current on any touchable conductive surface.

The measurement method is defined by IEC 60990, which specifies networks that model the impedance of the human body and how to measure touch current and protective conductor current for different fault conditions. The limits themselves come from the product safety standards: the relevant limits for lighting control gear are low, typically in the 0.25 to 0.75 mA range for normal operation depending on the standard and the equipment class. These are not derived from what feels comfortable; they are set to keep the current below the perception and reaction thresholds for the general population.

Why does a buyer care? First, because a driver with excessive touch current can cause a faint tingling when a grounded person touches an ungrounded metal sign or housing, which is alarming and can be mistaken for a wiring fault. Second, in environments with many drivers on one circuit, the sum of touch currents adds up, and a large installation can trip a residual-current device that protects the circuit even though every driver is individually compliant. Third, the touch current interacts with the Y-capacitor sizing, so a driver tuned to meet the strictest EMI limits may have a higher leakage, and vice versa.

The balance between EMI and touch current is a genuine engineering trade-off. Y capacitors large enough to shunt noise well also pass more leakage current. Standards set limits on both, so a compliant driver has sized its Y capacitors to satisfy the emission standard while keeping touch current under the safety limit. This is one of the reasons a certificate is worth reading: the touch current figure in the test report, and the conditions under which it was measured, tell the buyer whether the driver will behave on a sensitive installation.

For an installer, the practical note is that a grounded metal enclosure carries the leakage safely to earth, while an ungrounded installation leaves it on the housing. When a waterproof driver is bolted to a metal sign frame that is itself earthed, the touch current flows harmlessly away. When the same driver sits on an insulated wall with an ungrounded housing, the current appears on the surface. Specifying an earthed mounting and confirming the driver's touch current in the report prevents both the tingle and the mysterious RCD trip.

There is a distinction between touch current and the leakage a residual-current device sees. An RCD detects imbalance between live and neutral, and a driver's leakage flows where it is not balanced by the return conductor, so it is exactly the kind of current an RCD responds to. A single compliant driver is well under a typical 30 mA RCD threshold, but a circuit feeding forty drivers can approach it, and the RCD may nuisance-trip as the drivers age and their Y capacitors drift. The remedy is to divide the lighting onto multiple RCD-protected circuits or to use a higher rated protective device where the standard allows, keeping the total leakage budget in mind at the design stage.