What Is RCD Testing? Process, Frequency, And UK Compliance

If you've ever looked at your consumer unit and noticed the switches labelled "RCD," you might have wondered what they actually do, and more importantly, whether they're working properly. Understanding what is RCD testing comes down to one thing: confirming that the device designed to save your life in an electrical fault actually functions when it matters. An RCD (Residual Current Device) cuts the power in milliseconds when it detects current leaking along an unintended path, such as through a person's body. Testing proves it can still do that job reliably.

For landlords, business owners, and homeowners across London and the South East, RCD testing isn't optional, it's a core part of electrical compliance. Regulations like BS 7671 and the Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020 set clear expectations around when and how these devices must be checked. At Electrical Testing London, our engineers carry out RCD testing as part of every EICR we perform, using calibrated instruments to verify trip times against precise thresholds.

This article breaks down exactly how RCD testing works, what the process involves on site, how often you need it done, and what the UK regulations require. Whether you're a landlord preparing for a compliance inspection or a homeowner who simply wants peace of mind, you'll have a clear picture of where you stand by the end.

Why RCD testing matters for safety

An RCD works by continuously comparing the current flowing out through the live conductor against the current returning through the neutral. When those two values differ by 30 milliamps or more, the device trips and cuts the supply within 40 milliseconds. That speed is what separates a survivable shock from a fatal one. But an RCD can pass a visual inspection and still fail to trip at the correct threshold, which is precisely why regular, instrument-based testing is non-negotiable.

How RCDs prevent electrocution

The human body conducts electricity. If you touch a live conductor, current flows through you to earth, and without an RCD, that current continues until the circuit breaker detects an overload or serious harm occurs. An RCD detects the imbalance between live and neutral current almost instantly and cuts the supply before your heart can go into fibrillation. At 30mA the risk is already serious, and at 100mA or above the outcome is frequently fatal without rapid disconnection.

A standard 30mA RCD trips in under 40 milliseconds, which is fast enough to prevent cardiac arrest in most healthy adults.

Testing confirms the device achieves that disconnection within the required time. An RCD that only trips at 80mA instead of 30mA offers far less protection than its label suggests, and you would have no way of knowing without a proper instrument test.

What happens when an RCD fails silently

RCDs can degrade over time. Mechanical components wear, contacts corrode, and the internal test circuitry can give a false impression of health. The built-in test button on your consumer unit only confirms the mechanical trip works; it does not measure the actual trip current or disconnection time. A full RCD test using a calibrated instrument applies a known fault current and records the precise response against pass thresholds.

This is why understanding what is RCD testing goes beyond pressing a button once a year. Silent failure is a genuine risk in older installations, particularly in properties built before the 17th Edition of BS 7671 introduced mandatory RCD protection across more circuits. If an RCD fails on a circuit serving a bathroom, kitchen, or outdoor socket, the consequences of a fault become significantly more serious before any protection activates.

What gets tested during an RCD test

A qualified engineer runs several distinct checks during an RCD test, and each one targets a different aspect of how the device responds to a fault. Simply understanding what is RCD testing at this level clarifies why pressing the built-in test button is not a substitute: that button only trips the mechanism mechanically, it does not verify the trip time or the current threshold against any measurable standard.

Disconnection time

The most critical measurement is how quickly the RCD disconnects the supply when a fault current flows. The engineer applies the rated trip current through a calibrated instrument and records the time taken for the device to trip. For a standard 30mA Type AC RCD, the pass criteria are:

Failing the disconnection time test means the device offers reduced protection even if it appears fully functional from the outside.

Trip current threshold

The engineer checks the actual current level at which the RCD trips using a ramp test, where the instrument gradually increases the fault current from zero until the device operates. This records the precise trip point and confirms the RCD is not operating outside its rated tolerance.

A 30mA RCD must trip before the current reaches 30mA, not above it. Any device that only operates at 40mA or 50mA is outside tolerance and needs replacing immediately. Your installation records should capture both the trip time and measured trip current for every device tested to maintain a proper compliance trail.

How to test an RCD safely

RCD testing at a basic level requires two separate approaches, and understanding both helps you see what is RCD testing in full. The first is a quick monthly press of the built-in test button, which every occupier can carry out themselves. The second is an instrument-based test carried out by a qualified engineer, which is the only method that produces verified, compliant results for your records.

Using the built-in test button

Your consumer unit has a test button marked "T" on every RCD. Press it while the supply is live, and the device should trip immediately. Reset it by pushing the lever back up. If the device does not trip, stop using that circuit and contact a qualified engineer straight away, as the RCD is not providing any protection at that point.

Pressing the test button once a month keeps the mechanism from seizing through disuse, but it does not measure trip time or current threshold.

Using a calibrated RCD tester

A calibrated instrument applies a precise fault current to the circuit and records the exact time the RCD takes to disconnect. The engineer tests at multiple current levels, including the rated trip current and five times that value, to confirm the device meets BS 7671 disconnection thresholds throughout its full operating range.

You should always ask for a written test record showing the measured trip time and actual trip current for every device checked. That document forms part of your compliance evidence and supports any future EICR inspection on your property.

How often to test RCDs in the UK

The answer to how often depends on the type of test and the property type. Part of understanding what is RCD testing fully is recognising that there are two distinct testing cycles running alongside each other: a regular self-test that any occupier can perform, and a periodic instrument-based inspection carried out by a qualified engineer. Both are necessary, and neither replaces the other.

Monthly button tests

Every occupier should press the built-in test button on each RCD at least once a month. This takes under a minute and keeps the mechanical components moving, which reduces the risk of the device seizing in a closed position after long periods of inactivity. Write down the date and result each time so you can demonstrate the checks were carried out if an issue is ever raised.

If the RCD does not trip when you press the button, treat the affected circuits as unprotected and arrange an engineer visit immediately.

Periodic instrument testing

For domestic properties, BS 7671 and the IET Wiring Regulations recommend a full EICR every ten years for owner-occupied homes and every five years for privately rented properties. Since RCD instrument testing forms part of every EICR, your devices get formally verified on that same cycle. Landlords in England must also comply with the Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020, which legally requires an EICR at least every five years and before any new tenancy begins.

For commercial properties, the recommended inspection interval is shorter, typically every three to five years depending on the type of occupancy, usage intensity, and any hazardous conditions present on site.

UK rules and records for RCD testing

Knowing what is RCD testing is only part of meeting your legal obligations. The regulations that govern electrical safety in England place specific requirements on inspection frequency, remedial timescales, and record-keeping. Meeting those requirements protects both the people using your property and your position if a dispute or enforcement action arises.

Regulations that apply to your property

The Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020 require landlords to obtain a satisfactory EICR at least every five years and before any new tenancy begins. If an RCD defect is identified, you must complete remedial work within 28 days and provide written evidence to your tenant and the local housing authority.

Failing to act on an unsatisfactory EICR can result in a fine of up to £30,000 under the 2020 Regulations.

Owner-occupiers face no equivalent statutory duty, but BS 7671 still recommends periodic inspection, and insurers may request a current EICR before settling a claim related to an electrical fault.

What your test records must include

Your documentation should state the date and location of each test, the RCD's rated trip current, the measured disconnection time, and the actual trip current recorded by the instrument. Keep these alongside the EICR certificate and any remedial completion certificates issued after follow-up work.

Store copies somewhere secure and provide them to tenants within 28 days of a written request. A complete, dated paper trail is your primary evidence during local authority checks, insurance assessments, and property transactions, so treat your test records with the same care you give to any other legal document.

Next steps to keep circuits protected

Now that you understand what is RCD testing and what the regulations require, the practical next step is straightforward: check when your last EICR was carried out and whether your RCDs were formally instrument-tested as part of it. If your report is more than five years old, or if you have never had a qualified engineer verify your RCDs against BS 7671 disconnection thresholds, your installation may carry risks you cannot detect by eye.

Start by pressing the test button on every RCD in your consumer unit today and noting the results. If any device fails to trip, treat those circuits as unprotected and arrange a visit promptly. For landlords and commercial property owners, acting before a tenancy renewal or lease review keeps you compliant and avoids the cost of emergency remedial work under pressure. If you need a qualified engineer to carry out a full RCD inspection, request a quote from Electrical Testing London and we will arrange a convenient visit.