Does a TT System Need a 100mA S-Type RCD?

Does a TT System Need a 100 mA RCD?

Not necessarily.

A TT system normally needs an RCD where the earth-fault current is insufficient to operate a fuse or circuit breaker within the required disconnection time.

The RCD does not have to be rated at 100 mA in every TT installation. Depending on the design, fault protection may be provided through:

• Individual 30 mA RCBOs
• A general non-delayed upstream RCD
• A 100 mA S-type upstream RCD with downstream 30 mA protection
• A 300 mA selective device in an appropriate distribution arrangement
• Another coordinated residual-current arrangement

A 100 mA S-type device is frequently selected where:

• An upstream RCD is required for a TT distribution circuit.
• Downstream final circuits have 30 mA RCBO protection.
• Selectivity between the upstream and downstream devices is required.
• The S-type operating time satisfies the permitted disconnection time.
• The earth-electrode resistance is compatible with the selected IΔn.

Why Do TT Systems Commonly Rely on RCD Protection?

In a TT earthing system, the installation has its own earth electrode rather than relying entirely on an earth terminal provided through the supply network.

The earth-fault return path includes:

• The installation earth electrode
• The soil
• The supply transformer’s earthing arrangement
• Protective conductors and the fault path

This path commonly has a higher impedance than the metallic earth loop present in a TN system.

As a result, an earth fault may not produce enough current to operate an MCB or fuse within the maximum permitted disconnection time.

An RCD can detect the residual-current imbalance and disconnect the circuit without relying on a very high earth-fault current.

For a TT installation using an RCD for fault protection, the design must satisfy:

RA × IΔn ≤ 50 V

Where:

• RA is the resistance of the earth electrode and associated protective conductors.
• IΔn is the rated residual operating current of the RCD.
• 50 V is the conventional touch-voltage limit under normal conditions.

This calculation is only one part of the assessment. The RCD must also operate within the required disconnection time.

What Does “100 mA S-Type RCD” Mean?

Each part of the description defines a separate characteristic.

A 100 mA RCD is not automatically time delayed. The device must carry the relevant S-type marking and be specified as a selective RCD.

Likewise, S-type does not automatically mean Type A. The residual-current waveform type must be selected separately.

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What Is a Typical TT RCD Arrangement?

One possible arrangement is:

• A 100 mA S-type RCD protecting the upstream distribution circuit.
• Individual 30 mA general-type RCBOs protecting downstream final circuits.

During a residual-current fault on a final circuit:

1. The downstream 30 mA RCBO detects the fault.
2. The downstream device operates without an intentional delay.
3. The upstream S-type RCD remains closed during its delay period.
4. If the downstream device clears the fault, unaffected circuits remain energised.
5. If the fault persists upstream or the downstream device does not clear it, the upstream RCD can operate.

This arrangement can provide:

• Fault protection for the TT distribution circuit
• 30 mA additional protection on applicable final circuits
• Improved selectivity
• Reduced loss of supply to unaffected circuits
• Better control of cumulative leakage across the installation

It is not the only compliant TT arrangement.

An all-RCBO consumer unit may also provide fault and additional protection, provided all parts of the incoming and internal wiring that require RCD protection are suitably covered.

What Are the Trip Times for a 100 mA S-Type RCD?

At its rated residual operating current, an S-type RCD should operate within its specified time-delay band.

The lower limit is important because the device must remain delayed long enough to allow a downstream general RCD to operate.

The upper limit is equally important because the S-type device must still disconnect within the time permitted for the circuit it protects.

TT final circuits versus distribution circuits

TT systems can have different maximum disconnection times depending on the circuit.

• Applicable final circuits: Commonly require disconnection within 0.2 seconds.
• Distribution circuits: May be permitted a longer time, commonly up to 1 second, subject to the applicable requirements.

Because an S-type RCD may take up to 500 ms at IΔn, it should not simply be assumed suitable for a TT final circuit requiring 0.2-second disconnection.

Its common role is upstream protection of a distribution circuit where the permitted time and actual operating characteristics support the design.

Full testing guide: Time-Delay RCD Trip Times and Testing

What Earth-Electrode Resistance Is Required for a 100 mA RCD?

Using the TT design relationship:

RA × IΔn ≤ 50 V

For a 100 mA RCD:

50 V ÷ 0.1 A = 500 Ω

This gives a theoretical maximum RA of 500 Ω under the basic touch-voltage relationship.

However, 500 Ω should not be treated as a desirable installation target.

The electrode resistance should be as low as reasonably practicable because:

• Soil resistance varies with moisture and temperature.
• Drying and freezing can significantly increase resistance.
• Connections and electrodes can deteriorate.
• Higher resistance provides less design margin.
• Measured values can vary seasonally.

BS 7671 guidance notes that an earth-electrode resistance above approximately 200 Ω may not be stable.

Installers should therefore aim for a robust, repeatable electrode value rather than designing immediately below a theoretical maximum.

Example TT values

These values do not override the need for a stable electrode, correct disconnection time and compliance with all other requirements.

Does 100 mA Automatically Provide Selectivity with 30 mA?

No.

A difference in residual-current rating provides current separation, but it does not guarantee time selectivity.

If a general 100 mA device is installed upstream of a general 30 mA device, a sufficiently large fault can cause both devices to operate.

For reliable selectivity, the upstream arrangement normally requires:

• A higher residual operating current than the downstream device.
• An intentional S-type time delay.
• Compatible manufacturer operating characteristics.
• Suitable standing leakage-current margins.
• Correct coordination across every RCD in series.

The common 100 mA S-type upstream and 30 mA general downstream combination supports selectivity, but the manufacturer’s data and circuit design should still be checked.

It is more accurate to call this selectivity rather than assuming perfect discrimination under every possible fault condition.

Does a 100 mA RCD Provide Fire Protection?

A 100 mA RCD can contribute to protection against fire caused by earth leakage or insulation faults in applications where such protection is required.

However, the 100 mA marking alone does not make it a universal fire-protection device.

The designer must establish:

• Whether additional protection against fire is required.
• The maximum permitted residual-current rating.
• Whether an S-type delay is acceptable.
• The residual-current waveform expected from the equipment.
• Whether cumulative leakage could cause unwanted operation.
• Whether another protective measure is also required.

A 100 mA S-type RCD also does not provide 30 mA additional protection against electric shock.

Where Else Are 100 mA S-Type RCDs Used?

Potential applications include:

• TT distribution circuits
• Submains supplying secondary distribution boards
• Garages, workshops and outbuildings
• Commercial boards with downstream RCBOs
• Temporary distribution systems
• Small three-phase distribution arrangements
• Remote buildings with local 30 mA final-circuit protection

A 100 mA S-type device is not automatically required for:

• An EV charger
• A heat pump
• A solar inverter
• A three-phase board
• An outbuilding

It is selected where the actual distribution and earthing design requires upstream RCD fault protection and selectivity.

Is a 100 mA RCD the Same as a 100 A RCD?

No. This is another common source of confusion.

• 100 mA: Rated residual operating current, equal to 0.1 amp.
• 100 A: Rated current the device is designed to carry in normal service.

A device could therefore be described as:

100 A, 100 mA, Type A, S-type, four-pole RCCB

That device has:

• A 100 A current rating
• A 100 mA residual-current rating
• Type A waveform detection
• An S-type delay
• Four switched poles

Every characteristic must suit the circuit.

How Do You Select a 100 mA S-Type RCD?

1. Confirm the earthing arrangement. Establish that the installation or circuit is TT.
2. Identify why the RCD is required. Determine whether it provides fault protection, fire-risk protection or selectivity.
3. Confirm the circuit type. Distinguish a distribution circuit from a final circuit.
4. Check the required disconnection time. Ensure the S-type operating range is suitable.
5. Measure or calculate RA. Verify that RA × IΔn does not exceed 50 V.
6. Assess electrode stability. Aim for a value comfortably below the theoretical maximum.
7. Check downstream protection. Confirm the ratings and operating characteristics of 30 mA devices.
8. Select the residual-current type. Choose Type A, F, B or another suitable type for the connected equipment.
9. Select the current rating. Coordinate the RCCB with overload and short-circuit protection.
10. Check pole configuration. Select two-pole or four-pole switching as required.
11. Assess standing leakage current. Avoid placing excessive downstream leakage behind one upstream RCD.
12. Confirm manufacturer data. Check trip characteristics, assembly compatibility and upstream protection.

Browse 100 mA Time-Delay Protection

• 100 mA Time-Delay RCDs
• Type A RCDs
• Three-Phase RCDs
• All RCDs
• Downstream RCBOs
• Type A RCBOs
• Consumer Units
• Three-Phase Boards and Accessories

Check the residual-current rating, Type A or B characteristic, current rating, number of poles and S-type marking before ordering.

100 mA S-Type RCD FAQs

Does a TT system need a 100 mA RCD?

Not universally. A TT system normally needs suitable RCD fault protection, but the sensitivity may be 30 mA, 100 mA, 300 mA or another selected value depending on the circuit, electrode resistance, disconnection time and downstream protection.

Why is a 100 mA S-type RCD commonly used on TT systems?

It can provide upstream fault protection for a TT distribution circuit while delaying operation long enough for downstream 30 mA devices to clear final-circuit faults first.

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

A 100 mA RCD only has an intentional delay if it is an S-type device. At IΔn, an S-type RCD should generally operate between 130 ms and 500 ms.

Is every 100 mA RCD time delayed?

No. A 100 mA RCD can be general or S-type. Check the markings and manufacturer’s datasheet.

Can a 100 mA S-type RCD provide additional protection?

No. Additional protection requires an RCD rated at no more than 30 mA with the applicable rapid operating characteristics.

Can a 100 mA S-type protect a TT final circuit?

Only where its operating characteristics satisfy the maximum disconnection time for that circuit. Because S-type operation can extend to 500 ms, it may not meet a TT final-circuit requirement of 0.2 seconds.

Can it protect a TT distribution circuit?

Yes, potentially. Distribution circuits may have a longer permitted disconnection time, but the exact arrangement and device performance must be verified.

What is the maximum earth-electrode resistance for a 100 mA RCD?

The basic RA × IΔn ≤ 50 V calculation gives 500 Ω. This is a theoretical maximum, not a recommended target. The electrode should be as low and stable as reasonably practicable.

Is Ze the correct term for a TT earth electrode?

Ze describes external earth-fault loop impedance at the origin. For the RCD touch-voltage condition, BS 7671 uses RA, which includes the installation earth electrode and associated protective conductors.

Does 100 mA provide selectivity with 30 mA?

Not by rating alone. The upstream device normally also requires an S-type delay, and the complete devices must be coordinated.

Can I use a 100 mA RCD as a main switch?

An RCCB may perform an isolation or switching function where its product characteristics and assembly permit it, but its current rating and short-circuit protection must be correctly coordinated.

Does a 100 mA RCD provide overload protection?

No, not when it is an RCCB. Separate MCB, fuse or other overcurrent protection is required.

Is 100 mA the same as 100 A?

No. 100 mA is the residual-current sensitivity. 100 A is the normal rated current.

Do EV chargers need an upstream 100 mA S-type RCD?

Not automatically. It may be used where the EV circuit is supplied through a TT distribution arrangement requiring upstream selectivity, but it is not a universal EV charging requirement.

Do heat pumps need a 100 mA S-type RCD?

Not automatically. Selection depends on the earthing, distribution design and heat-pump manufacturer’s residual-current requirements.

Can I use a Type AC 100 mA S-type RCD?

Only where the connected equipment and downstream installation are compatible with Type AC characteristics. Modern electronic equipment commonly requires Type A, F or B protection instead.

What is a 100 mA RCBO?

It combines 100 mA residual-current protection with overload and short-circuit protection. It is not automatically S-type unless an intentional delay is specifically declared.

When Should You Use a 100 mA S-Type RCD?

Use one where the installation needs:

• Upstream RCD fault protection
• A 100 mA residual-current rating supported by the TT design
• Time selectivity with downstream 30 mA devices
• An operating time compatible with the protected distribution circuit

Do not specify one solely because:

• The installation is TT.
• The supply feeds an outbuilding.
• The circuit supplies an EV charger or heat pump.
• The device is being used as a main switch.
• 100 mA is assumed to provide universal fire protection.

Confirm:

• RA × IΔn
• Earth-electrode stability
• Required disconnection time
• Distribution or final-circuit status
• Downstream RCD ratings
• S-type operating characteristics
• Residual-current waveform type
• Current rating and poles
• Overcurrent protection

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