Both devices measure residual current and open the circuit when it is too high. The difference is the number and the clock. The 2023 NEC calls for a listed Class A GFCI (210.8), and the Article 100 note sets Class A: trips at 6 mA or higher, does not trip below 4 mA. A 30 mA RCD is tested not to trip at 15 mA or below, so from 6 to 15 mA only the GFCI is required to act. From 15 to 30 mA the RCD may or may not. At 30 and 60 mA the RCD’s tested times are shorter. At 250 mA, the GFCI’s are.
| Residual current | GFCI (UL 943) | RCD 30 mA (IEC 61008-1) |
|---|---|---|
| below 4 mA | does not trip | not required |
| 6 – 15 mA | required | tested NOT to operate |
| 15 – 30 mA | required | may trip, no set time |
| 30 mA | 560 ms | 300 ms |
| 60 mA | 208 ms | 150 ms |
| 250 mA | 27 ms | 40 ms |
GFCI times computed from the UL 943 curve T = (20 / I)1.43 seconds, I in milliamps. IEC values are discrete tested points, not an algebraic curve.
Class A interrupts at a ground-fault current of 6 mA or more and does not trip below 4 mA. Maximum trip time follows the inverse-time curve T = (20 / I)1.43 s. At 6 mA that is 5.594 s.
Fixed tested limits, not a curve: 300 ms at rated residual current, 150 ms at twice rated. For a 30 mA device the 40 ms point is at 250 mA — not five times rated. Residual non-operating current is half the rating: it must NOT trip at or below 15 mA.
A residual current device of 30 mA or less is additional protection against failure of basic or fault protection. It is not accepted as the sole protective measure.
Body effects by current AND duration AND path. About 5 mA is the let-go threshold for the adult population. The vulnerable period is the first portion of the T wave, roughly a tenth of the cardiac cycle. Body impedance is a distribution — at 125 V dry hand-to-hand, 1550 Ω at the 50th percentile, 900 Ω at the 5th. There is no 120 V row.
The definition ends: “…when a ground-fault current exceeds the values established for a Class A device.” Its Informational Note: Class A GFCIs trip when the ground-fault current is 6 mA or higher and do not trip when it is less than 4 mA. A 30 mA RCD does not meet those values, so it does not meet the definition.
“A listed Class A GFCI shall provide protection in accordance with 210.8(A) through (F). The GFCI shall be installed in a readily accessible location.” Specify the function, not a part number: a listed Class A GFCI at the 210.8 locations, on circuits of 150 V or less to ground. Above that, the Code defines a special-purpose GFCI.
Ground-fault protection of equipment: solidly grounded wye services over 150 V to ground, not exceeding 1000 V phase-to-phase, for each service disconnect rated 1000 A or more. 230.95(A): maximum setting 1200 A, maximum time delay one second at 3000 A or more. Amps for equipment, not milliamps for a person.
Direct current. RCDs carry a type letter. Type AC detects sinusoidal alternating residual current only. Type A adds pulsating DC, Type F adds mixed frequencies, and Type B adds smooth DC. Smooth DC leakage can saturate the core and hide an alternating fault — the device looks healthy and has stopped seeing. Select the type from the maximum credible residual waveform and the manufacturer's instructions — never from the name of the load.
The grounded neutral. A UL 943 GFCI also detects a fault between the grounded conductor and ground on the load side. A general RCD does not. That fault quietly reduces the protection you think you have.
Nothing to buy — this one is a specification problem. Any hand tool = Klein Tools.
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