What Is a Time-Delay RCD? S-Type Uses and Trip Times

What Is a Time-Delay or S-Type RCD?

A time-delay RCD incorporates an intentional delay before it operates. It is commonly identified by an S marking, meaning selective.

The delay allows a downstream general-type RCD or RCBO to operate first when the residual-current fault occurs on one of its circuits.

This can help:

• Maintain the supply to unaffected circuits.
• Prevent an entire board or sub-installation being disconnected unnecessarily.
• Coordinate upstream and downstream residual-current protection.
• Improve continuity where several protected circuits are supplied from one upstream device.

The S marking describes the device’s time-delay characteristic. It does not, on its own, identify:

• Its rated residual operating current, such as 30 mA, 100 mA or 300 mA.
• Its residual-current waveform type, such as Type AC, A, F or B.
• Its current rating, such as 63 A, 80 A or 100 A.
• Whether the device is an RCCB or RCBO.

A product may therefore be described as a 100 mA Type A S-type RCCB, with each part of the description defining a different characteristic.

How Does an S-Type RCD Provide Selectivity?

RCD selectivity means arranging devices so that the protective device nearest the residual-current fault operates without unnecessarily disconnecting the upstream supply.

Consider a distribution circuit protected by an upstream S-type RCD that supplies several final circuits, each protected by a general 30 mA RCBO.

If a fault occurs on one final circuit:

1. The downstream 30 mA RCBO detects the residual current.
2. The downstream device operates without an intentional delay.
3. The upstream S-type RCD waits for its defined delay period.
4. If the downstream device clears the fault, the upstream RCD remains closed.
5. If the fault remains, the upstream RCD can operate as required by the design.

The downstream device must also have a lower rated residual operating current than the upstream selective device where current selectivity is required.

What Is the Difference Between 30 mA, 100 mA and 300 mA RCDs?

The mA figure is the device’s rated residual operating current, written as IΔn. It does not describe the time delay.

The correct sensitivity depends on:

• The purpose of the RCD.
• The installation’s earthing arrangement.
• The required disconnection time.
• Expected protective-conductor and leakage currents.
• Downstream residual-current devices.
• The equipment and circuit being protected.

A 100 mA or 300 mA device can be either general or time delayed. Check for the S marking and manufacturer’s data rather than assuming the delay from the mA rating.

👉 Compare 100 mA time-delay RCDs for selective upstream applications.

Can You Get a 30 mA Time-Delay RCD?

Yes, 30 mA time-delay or S-type devices exist. However, the combination of a 30 mA sensitivity and an S-type delay does not make the device suitable for 30 mA additional protection.

Additional protection requires a sufficiently fast operating response. An S-type device is intentionally delayed and therefore cannot fulfil that function.

A 30 mA S-type RCD may be specified for a particular equipment or coordination application, but it must not be confused with the general non-delayed 30 mA device used for additional protection.

This distinction explains why the following descriptions are not interchangeable:

• 30 mA general RCD: No intentional delay; may provide additional protection where all requirements are met.
• 30 mA S-type RCD: Intentionally delayed; not suitable for additional protection.
• 100 mA S-type RCD: Common upstream selective arrangement, subject to the installation design.

What Is the Operating Time of an S-Type RCD?

When tested at its rated residual operating current, an S-type RCD is expected to operate within its defined time-delay band.

The minimum time matters as well as the maximum. If an S-type device operates too quickly, it may not provide the required selectivity with a downstream general device.

The exact testing and acceptance criteria should follow:

• The applicable product standard.
• The current edition of BS 7671.
• The RCD manufacturer’s instructions.
• The test instrument configuration.

Do not treat 130–500 ms as a universal disconnection-time allowance for every circuit. It is the expected operating-time range for the S-type device under the specified test condition.

What Is the Maximum Trip Time for a 30 mA RCD?

A general non-delayed RCD complying with the relevant product standard should operate within the applicable product-standard limit when tested at its rated residual operating current.

For current BS 7671 verification, the common field-test benchmark at IΔn is an operating time not exceeding 300 ms for a general non-delayed device.

However, electricians should distinguish between:

• RCD product operating time: The device’s performance under a specified residual-current test.
• Circuit disconnection time: The maximum time permitted for automatic disconnection of supply under the circuit’s fault-protection requirements.
• Additional-protection requirement: The rapid operating characteristic required where a 30 mA RCD provides additional protection.

These values answer different technical questions and should not be treated as interchangeable.

Where Are Time-Delay RCDs Used?

An S-type device is considered where the design includes RCDs in series and selective operation is required.

Potential applications include:

• TT installations: An upstream RCD may provide fault protection for a distribution circuit while downstream 30 mA devices protect final circuits.
• Sub-distribution boards: Where several downstream RCBOs are supplied through an upstream residual-current device.
• Commercial and industrial distribution: Where loss of the complete upstream supply would unnecessarily interrupt unaffected circuits.
• Temporary or mobile distribution: Where RCD coordination is required between supply and downstream assemblies.
• EV or renewable sub-boards: Only where the actual upstream arrangement requires RCD selectivity; not simply because the load is an EV charger, PV inverter or heat pump.

Time-delay RCDs should not be added automatically to every installation. An upstream RCD may be unnecessary where fault protection is achieved through the overcurrent device and the earthing arrangement permits it.

Similarly, an EV charger does not inherently need a 100 mA S-type RCD upstream. The need depends on the full circuit, earthing, equipment and distribution design.

Can a Time-Delay RCD Provide Fault Protection?

An RCD may be used for automatic disconnection of supply where the installation design relies on residual-current protection to achieve the required fault protection.

This commonly arises on TT systems, where the earth-fault loop impedance may be too high for an overcurrent protective device to operate within the required time.

Where an S-type RCD is used for fault protection, its intentional delay must still allow the circuit to meet the applicable maximum disconnection time.

The designer must confirm:

• The earth-electrode and protective-conductor resistance.
• The selected rated residual operating current.
• The maximum permitted disconnection time.
• The S-type device’s operating characteristics.
• Coordination with downstream devices.

A blanket rule such as “use 100 mA for fault protection” is not sufficient. The actual values must be verified against the earthing and circuit design.

How Do You Choose a Time-Delay RCD?

Selecting an upstream S-type device requires more than choosing 100 mA from a product list.

1. Confirm why an upstream RCD is required. Establish whether it provides fault protection, fire-risk protection, equipment protection or selective coordination.
2. Identify the earthing arrangement. Determine whether the installation is TN, TT or uses another arrangement.
3. Select the sensitivity. Calculate the required IΔn rather than automatically choosing 100 mA or 300 mA.
4. Specify the time characteristic. Choose S-type where intentional time delay is needed.
5. Select the residual-current type. Choose Type A, F, B or another suitable type according to the connected loads.
6. Check the rated current. Ensure the RCCB is protected against overload and short circuit in accordance with its design.
7. Confirm the pole arrangement. Select two-pole or four-pole switching as required by the supply and circuit.
8. Coordinate downstream devices. Check residual-current rating, timing and manufacturer selectivity data.
9. Verify disconnection times. Ensure the delayed operation still satisfies the circuit’s fault-protection requirements.
10. Check standing leakage current. Avoid unwanted operation caused by the cumulative protective-conductor current of downstream equipment.

What Is an S-Type RCBO?

An RCBO combines residual-current protection with overcurrent protection. An S-type RCBO adds an intentional time delay to its residual-current operating characteristic.

The device must therefore be selected by:

• Rated current
• Overcurrent curve
• Breaking capacity
• Residual-current rating
• Residual-current type
• Time-delay characteristic
• Number of poles and switching arrangement

S-type RCBOs are less commonly encountered than selective RCCBs, but the same principle applies: the S marking relates to residual-current timing, not the MCB curve or current rating.

How Do You Test a Time-Delay RCD?

Testing should follow the current BS 7671 verification requirements, the test instrument instructions and the RCD manufacturer’s data.

For an S-type device tested at IΔn:

• The tester must be set to the correct RCD type and time-delay mode.
• The expected result is within the device’s selective operating-time band.
• An operation below the minimum delay may indicate that selectivity cannot be relied upon.
• An operation above the maximum permitted time may indicate a device or test issue requiring investigation.

The inspector should also verify that:

• The correct RCD type has been selected on the test instrument.
• The circuit is free from conditions that could distort the result.
• The measured operating time remains compatible with the required circuit disconnection time.
• The downstream and upstream devices provide the intended selectivity.

Testing the trip time alone does not prove full selectivity. Manufacturer coordination data and the devices’ current characteristics must also be considered.

Browse Time-Delay RCDs and Downstream Protection

• 100 mA Time-Delay RCDs
• All RCDs
• Type A RCDs
• Type B RCDs
• RCBOs
• Type A RCBOs
• Three-Phase Boards and Protection

Check the complete specification before ordering. The sensitivity, current rating, poles, residual-current type and S-type delay must all suit the installation.

Time-Delay RCD FAQs

What is a Type S RCD?

A Type S RCD is a selective residual-current device with an intentional operating delay. It is generally installed upstream where coordination with faster downstream RCDs is required.

What is the rated time delay of a 100 mA RCD?

A 100 mA rating does not automatically mean the device is time delayed. Where it is an S-type device, its expected operating time at IΔn is generally between 130 ms and 500 ms.

What is the rated time delay of a 30 mA RCD?

A general 30 mA RCD has no intentional S-type delay. A 30 mA S-type device may exist, but it is deliberately delayed and cannot provide additional protection.

What is the maximum trip time for a 30 mA RCD at 1 × IΔn?

For a general non-delayed RCD, the common product-standard field-test maximum at its rated residual operating current is 300 ms. The applicable device standard and manufacturer’s data should still be checked.

What is the operating time of an S-type RCD?

When tested at its rated residual operating current, an S-type RCD should generally operate between 130 ms and 500 ms.

Can an S-type RCD provide additional protection?

No. Its intentional delay means it does not meet the operating-time requirement for additional protection, even where its sensitivity is 30 mA.

Does a 100 mA RCD provide selectivity with a 30 mA RCD?

Not automatically. The upstream device normally needs an appropriate time-delay characteristic, and the devices must be coordinated by residual-current rating and operating time.

Is every 100 mA RCD time delayed?

No. A 100 mA RCD may be general or S-type. Check the device marking and datasheet.

Is every 300 mA RCD time delayed?

No. The 300 mA figure describes sensitivity, not operating delay. Check whether the device carries the S marking.

Do EV chargers need a time-delay RCD?

Not inherently. An upstream S-type device may be used where the EV circuit forms part of an arrangement requiring RCD selectivity, but it is not a universal EV charging requirement.

Can a time-delay RCD be used for a TT installation?

Yes, where its rated residual current and operating time satisfy the fault-protection and disconnection-time requirements. It may be used upstream of downstream 30 mA devices to improve selectivity.

What is the Type S RCD symbol?

A selective RCD is normally identified by an S marking within or alongside a rectangular symbol. The device will carry separate markings for its residual-current type and electrical ratings.

What is the difference between an S-type RCD and Type A RCD?

S-type describes an intentional time delay. Type A describes the residual-current waveforms the device detects. A device can be both Type A and S-type.

What is a time-delay RCBO?

It combines overcurrent protection, residual-current protection and an intentional delay in the residual-current operation. Its complete characteristics must be coordinated with the circuit and downstream devices.

Why did both upstream and downstream RCDs trip?

Possible causes include using two non-delayed devices in series, insufficient current selectivity, incompatible device characteristics or a fault large enough to operate both. Manufacturer coordination data and test results should be reviewed.

When Should You Use an S-Type RCD?

Use an S-type RCD where an upstream residual-current device is required and selective operation with downstream devices forms part of the installation design.

Do not select one solely because:

• The installation contains an EV charger or solar inverter.
• The device is rated at 100 mA or 300 mA.
• The site has experienced nuisance tripping.
• A time-delayed device appears more robust.

Instead, confirm:

• Why the upstream RCD is needed.
• The correct residual-current sensitivity.
• The required Type AC, A, F or B characteristic.
• The S-type time delay.
• The applicable disconnection time.
• Selectivity with downstream devices.
• Rated current, poles and overcurrent protection.

👉 Specifying selective upstream protection? Browse 100 mA time-delay RCDs and coordinate them with the correct downstream RCBOs.