RCD and RCBO Types Explained: Symbols and Device Markings
Understand the type symbols and device labels, including why a B-curve RCBO is different from a Type B RCD.
By EICR Certificates London · Updated 1 November 2026
RCD types AC, A, F and B describe residual-current detection characteristics; an RCBO also has separate overcurrent markings. These labels answer different questions about protection, so reading one letter or number in isolation can be misleading.
For landlords, agents, homeowners and businesses, understanding the markings helps when discussing an Electrical Installation Condition Report or an upgrade quotation. EICR Certificates London explains the distinctions below. Identifying a label does not establish whether a device suits the connected equipment, works correctly or is compatible with the consumer unit.
RCD Types AC, A, F and B at a Glance
AC, A, F and B classify the residual-current waveforms a device is designed to detect. The descriptions below are a text reference, not a substitute for the manufacturer’s identification of a particular model.
| Type | Symbol description | Detection characteristics | What the label does not establish |
|---|---|---|---|
| AC | A sinusoidal wave: a smooth, repeating alternating-current shape. | Sinusoidal alternating residual current. | Suitability for equipment producing DC components, or the condition of an installed device. |
| A | Alternating-wave and pulsating-current shapes shown together. | Alternating residual current plus pulsating DC residual current. | Detection of smooth DC, or suitability for every electronic appliance. |
| F | Type F identification with the manufacturer’s associated waveform markings. | Type A characteristics plus specified composite residual currents containing mixed frequencies. | Universal compatibility with speed-controlled equipment or unrestricted detection at any frequency. |
| B | Combined waveform markings identifying broader detection, including smooth direct current. | Broader residual-current characteristics, including smooth DC. | An overcurrent curve, a recommended circuit rating, or compatibility with a particular board. |
These classifications can describe the residual-current function of either an RCCB or an RCBO. An RCCB has no integral overcurrent protection; an RCBO combines that protection with residual-current detection. See RCD, RCBO, MCB and RCCB Explained for the differences between device functions.
The IET’s “Which RCD Type?” explains the classification background. AC, A, F and B are not a universal ranking from basic to best: the required characteristics depend on the application. None of these labels establishes successful testing, the correct residual-current rating or compatibility with other protective devices.
Why Type A and Type AC Are Different
Type A adds pulsating-DC detection to the alternating-current detection associated with Type AC. Electronic components can change the shape of earth leakage from appliances, so an electrician must consider more than the fact that equipment uses an AC supply. A device’s response depends on the residual current it encounters.
Present-day selection under BS 7671:2018+A4:2026 and assessment of existing protection are related but separate tasks. An EICR considers the installed arrangement, connected equipment and inspection findings. A type label is useful evidence, but it cannot establish that replacement is needed or settle an observation code. Type A detection must also not be confused with smooth-DC detection.
- Alternating current: repeatedly reverses direction; a sinusoidal waveform has a smooth, regular shape.
- Pulsating DC: flows in one direction while its level rises and falls in pulses.
- Smooth DC: flows in one direction at a substantially steady level.
How to Read RCD and RCBO Markings
The markings identify several independent characteristics, so one letter or number cannot describe the whole device. Read only information that is safely visible; do not remove covers or expose terminals to find a label.
| Marking | Meaning | Does not prove |
|---|---|---|
| Rated current, A | The device’s rated load current. | The circuit’s safe capacity or, on an RCCB, integral overload protection. |
| Rated residual operating current | Operating sensitivity: 0.03 A equals 30 mA. | A measured trip time or satisfactory additional protection. |
| Waveform symbol | The residual-current classification. | Successful operation in the installation. |
| Product reference | Identification for finding the matching manufacturer documentation. | Interchangeability with another manufacturer’s device. |
| Product standard | The applicable product requirements, such as BS EN 61008-1 for RCCBs or BS EN 61009-1 for RCBOs. | Verification of the complete circuit. |
The test button is a functional control, not a type identifier or an inspection result. Its presence does not replace RCD trip-time testing. The Schedule of Test Results records measurements; the faceplate states device characteristics.
B32 Does Not Mean a Type B RCD
On an RCBO, B32 describes the overcurrent curve and rated current; it does not identify Type B residual-current protection.
| Overcurrent terminology | Residual-current terminology |
|---|---|
| B, C and D describe operating curves. | AC, A, F and B describe detection characteristics. |
RCD types AC, A, F and B classify residual-current detection, while an RCBO’s B32 marking identifies its overcurrent curve and current rating, not a Type B RCD.
S, Pole Counts and Other Labels Describe Separate Features
An S marking concerns selective time-delayed operation, while pole labels describe device construction.
- S designation: a timing characteristic separate from waveform classification. A delayed device is not interchangeable with protection required for additional protection against electric shock.
- Pole and neutral terminology: describes construction and switching arrangements. Do not infer a universal neutral-switching arrangement from “1P+N”; consult the exact specification.
- Breaking capacity: describes capability under specified fault conditions. It is distinct from both load current and residual operating sensitivity.
- Voltage rating and product code: identify electrical ratings and the manufacturer’s model. Neither establishes compatibility with a distribution board simply because the device physically fits.
Bidirectional RCBOs and Line/Load Markings
Bidirectional describes suitability for power flow in either direction, independently of the RCBO’s residual-current type. Line/load wording and directional arrows provide manufacturer connection information. The electrician must check the precise model and its instructions; an unreadable or absent visible label is insufficient evidence for a householder to establish suitability.
Selection also depends on the circuit, connected equipment and board compatibility. Adding RCBOs to an existing consumer unit therefore needs more than a comparison of labels. Arrange professional assessment for RCD and RCBO protection upgrades when interpretation needs to become a decision about work.
Where AFDD Markings Fit
AFDD identifies arc-fault detection, a separate function from the residual-current classification on an RCBO. A combined device can carry markings for several protective functions, but the presence of one label does not establish the others. Read AFDDs Explained for purpose and applicability.
- Function: AFDD describes detection of dangerous electrical arcing.
- Classification: AC, A, F or B describes residual-current response where that function is present.
Check the manufacturer’s declared functions rather than treating AFDD as another waveform type.
Discuss Your RCD or RCBO Protection in London
EICR Certificates London can assess RCD and RCBO protection and quote for any upgrade work needed. Have the EICR observation wording, property details and any safely visible model information available. Qualified, insured electricians. Evening and weekend appointments available. Upgrade work is quoted according to the assessed scope.
To discuss the report or an existing device, contact EICR Certificates London about your protection.
RCD and RCBO Type Questions
A type label alone cannot settle replacement, suitability or EICR coding questions. A qualified and competent inspector considers the actual installation against BS 7671 alongside the relevant equipment information.
Where Is a Type B RCD Used?
Type B RCDs are used where the assessed residual-current characteristics require their broader detection capability, including smooth DC. Certain converter or drive applications in commercial premises may need this capability, depending on the equipment manufacturer’s instructions. An appliance category alone does not determine the specification: the electrician assesses the expected leakage characteristics and how protection will operate within the installation.
Should I Use a Type A or Type B RCBO?
The appropriate RCBO depends on the circuit and connected equipment, so the type cannot be chosen from the letter alone. An electrician considers residual-current characteristics, manufacturer instructions and consumer-unit compatibility, particularly when adding protection to an existing board. Ask: “Does B in this quotation mean the overcurrent curve or the residual-current type?” That clarification prevents two different specifications being mistaken for alternatives.
Can I Replace an RCD Myself?
RCD replacement should be carried out by a competent electrician because it requires safe isolation, correct selection and verification. Reading an accessible label is different from opening a consumer unit or changing a protective device. A replacement fitting the available space does not establish compatibility or satisfactory operation. Inspection and appropriate testing, including verification of RCD operation, are needed to assess the completed work.
Does a Type AC RCD Automatically Make an EICR Unsatisfactory?
A Type AC label alone does not determine the EICR outcome; the inspector must assess the actual installation. Read how EICR observation codes are assessed for the distinction between findings and their classifications. The Observation Schedule records the circumstances identified during inspection. Connected equipment, the protective arrangement and test evidence inform the assessment; the letters AC do not independently establish either a satisfactory or an unsatisfactory result.
Why Does My RCD Keep Tripping?
Repeated tripping needs investigation; the type marking alone does not identify its cause. Possible issues include appliance earth leakage, deteriorating wiring with low insulation resistance, or leakage accumulating across connected equipment. Electrical fault finding establishes the cause; changing the classification is not a universal remedy. See what to do when an RCD keeps tripping.