What Is a Residual Current Device (RCD)?
As an Amazon Associate we earn from qualifying purchases.
The person typing “what is a residual current device” wants to know what the device does, why it trips when nothing appears to be wrong, and how it differs from a breaker. A residual current device, or RCD, is a safety switch that watches the current going out on the line conductor and the current coming back on the neutral. In a healthy circuit the two match. When some current leaks out to earth or to another path, the two no longer match, and the RCD opens the circuit. The device is built to protect people from electric shock, which is a different job from the overload protection that a standard breaker provides.
Quick answer
- What it compares: the current in the live (line) conductor against the current in the return (neutral) conductor.
- What trips it: an imbalance, which means current leaking to earth or along another path.
- What it protects: people, from shock, mainly. A standard breaker protects the wiring from overload and short circuit.
- Common rating: a 30 mA sensitivity is the figure the sources describe for personal protection.
How it works
The Wikipedia article on residual current devices describes the principle directly: these devices “are designed to quickly interrupt the protected circuit when it detects that the electric current is unbalanced between the supply and return conductors of the circuit.”[1] The article adds that any difference between the two currents “indicates leakage current, which presents a shock hazard.”[1]
The device does this with a differential current transformer. The Wikipedia article states that RCDs “operate by measuring the current balance between two conductors using a differential current transformer,” and that if the line and neutral currents do not sum to zero, there is leakage to earth or another path.[1] The Queensland government technical note describes the device as a circuit breaker that “senses when the active and neutral currents in a circuit are not equal and, when that inequality reaches a predetermined value for a predetermined time, opens the breaker contacts.”[2]
Both the “predetermined value” and the “predetermined time” matter. A small imbalance that lasts only a moment may not trip the device, and a larger imbalance trips it sooner. The exact limits depend on the device’s rating and standard.
Why it trips when a circuit looks fine
An RCD does not measure voltage or overload. It measures imbalance. That is why it can trip on a circuit that looks normal from the outside. The Wikipedia article explains the cause: leakage current “presents a shock hazard,” and an imbalance means current is going somewhere the device has not been told to expect.[1] The sources we opened do not list the common causes of leakage, so this article does not rank them.
What the 30 mA rating means
The Luceco and ECA presentation lists “RCD’s for personal protection 30mA” and states that “In = 30mA: standard allows for trip from >15mA to 30mA.”[3] In other words, a 30 mA device is designed to trip somewhere between about 15 and 30 mA of leakage, not at a single exact value. The same presentation says high sensitivity RCDs rated 30 mA “are designed to disconnect the supply within 40ms at 5 In,” where In is the rated current, so 5 In means five times that rating.[3]
The Queensland technical note gives the broader definition and the same family of names. It says these devices “have also been called ‘safety switches’ and ‘earth leakage circuit breakers.'”[2] The same device appears under names such as RCCB, RCBO and GFCI depending on the region and the standard, so check the label rather than the name.
RCD versus breaker
A breaker and an RCD do different jobs, and the Luceco presentation makes this point directly: a 30 mA RCD for personal protection “cannot be used in isolation” and “must be used with overcurrent protection device.”[3] The overcurrent device protects the wiring from overheating. The RCD protects a person who touches a live part and sends leakage to earth. A circuit may need both functions, and the presentation says so for personal protection.[3]
Testing an RCD
Most RCDs have a test button. Pressing it creates a deliberate imbalance and should trip the device. The sources we opened describe the device’s function but do not give a step-by-step test procedure for a specific product, so follow the manufacturer’s instructions for your unit. A test button confirms the mechanism moves; it does not measure the sensitivity of the device or the condition of the wiring behind it.
For a broader look at checking the outlet itself, read our guide to GFCI outlets and open grounds. For a quick presence check at a receptacle, see how to test a wall outlet.
Safety
An RCD reduces the risk of shock. It does not make live parts safe to touch, and it does not replace isolation and proper testing. The Queensland technical note is written for departmental infrastructure and sets out where RCDs are used under its standard, so it is a useful reference for the device, not a site procedure. Work on a live installation should be left to a qualified electrician, and the rules that apply where you live govern the installation.
How we chose
This is an explainer, so it does not pick products. We used three sources that cover different angles: the Wikipedia article gives the general principle and the differential current transformer, the Queensland technical note gives a formal definition and the alternative names, and the Luceco and ECA presentation gives the rating and trip-band details. We did not use retailer pages or forum posts for any technical claim.
What we could not verify
- The sources we opened are not US sources. They cover Australian, UK and general practice, and the US code and listing rules may differ. Check the rules that apply to your installation.
- We did not find a single consistent response-time figure across the sources. The Luceco presentation gives 40 ms for a 30 mA high-sensitivity device at 5 times the rated current. Other figures depend on the device and the fault.
- The source material does not say how often an RCD should be tested in service, so this article does not give an interval.
FAQ
Is an RCD the same as a GFCI?
Both detect leakage by comparing currents. The Wikipedia article lists RCD, RCCB and GFCI as names for the same class of device, depending on the region.[1]
Why did my RCD trip with nothing plugged in?
The RCD trips on an imbalance, so leakage somewhere on the protected circuit can trip it even with no load switched on. The sources describe the principle but do not list the causes, so an electrician should check the wiring.[1]
Does an RCD protect against overloads?
No. The Luceco presentation says a personal-protection RCD “cannot be used in isolation” and must be used with an overcurrent protection device.[3]
What does 30 mA mean?
It is the rated sensitivity. The presentation says the standard allows the device to trip from more than 15 mA up to 30 mA, not at exactly 30 mA.[3]
Can I use a voltage tester to check an RCD?
A voltage tester checks for the presence of voltage. It does not measure an RCD’s sensitivity. For the tool itself, see how voltage testers work.
Is the test button enough?
The test button checks that the mechanism moves. It does not confirm the device’s sensitivity or the condition of the wiring. Follow the manufacturer’s test procedure.
Bottom line
A residual current device trips when the current in the line and neutral conductors no longer match, which signals leakage that could cause shock. It protects people, it does not replace an overcurrent device, and its rating is a trip band, not a single exact number.
Sources
- [1] Residual-current device, Wikipedia. https://en.wikipedia.org/wiki/Residual-current_device (opened 2026-10-29)
- [2] Technical Note TN145, Use of Residual Current Devices in Departmental Infrastructure, Queensland Department of Transport and Main Roads, July 2015. https://www.tmr.qld.gov.au/-/media/busind/techstdpubs/Technical-notes/ITS-Electrical-technology/TN145.pdf?la=en (opened 2026-10-29)
- [3] Luceco RCD presentation, hosted by the Electrical Contractors’ Association (ECA). https://www.eca.co.uk/getattachment/4b6097b2-a46f-439b-a0b1-bddef034ea0b/Luceco-RCD-Presentation.pdf (opened 2026-10-29)
