100mA S-Type (time-delay) RCDs are used as upstream protection devices where discrimination is essential. They sit ahead of 30mA devices and allow downstream RCBOs or RCDs to trip first in a fault, preventing full-board outages and avoiding nuisance power cuts on critical circuits.
Quick Answer: A 100mA S-Type RCD is used as the main incomer or upstream device when you need time-delay protection for selectivity. It trips more slowly than 30mA RCDs, ensuring downstream devices operate first. They’re commonly used in submains, TT systems, outbuildings, small three-phase boards and circuits powering heat pumps or EV chargers where discrimination is required.
You can browse the full range here: 100 mA Time-Delay RCDs.
This guide explains when S-Type protection is required, how time-delay devices maintain selectivity, and which circuits typically rely on 100mA RCDs.
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What Does a 100mA S-Type RCD Do?
A time-delay (S-Type) RCD is designed to trip after downstream RCD/RCBO protection. Instead of reacting instantly, it introduces a small delay so that 30mA devices protecting final circuits operate first.
The “100mA” rating refers to the device’s residual operating current. In simple terms, it is designed to trip on a higher level of earth leakage than a 30mA device, which is why it is used for upstream fault protection and selectivity rather than additional protection on final circuits.
This ensures:
- Discrimination - only the faulted circuit trips
- No full-board outages
- Stable protection on TT systems
- Safer fault isolation
These devices prevent nuisance trips that would otherwise shut down multiple boards, outbuildings or heat-pump/EV systems.
When Do You Need a 100mA Time-Delay RCD?
Installers use 100mA S-Type RCDs in upstream positions where downstream devices must trip first.
Common scenarios include:
- Submains feeding garages, outbuildings or annexes
- Small three-phase boards
- Heat pump supply circuits (indoor/outdoor split systems)
- EV charger supply circuits where discrimination is required
- Distribution circuits feeding secondary consumer units
- Main RCD incomers on TT systems
- Commercial circuits with multiple layers of RCD protection
Why 100mA S-Type RCDs Are Used on TT Systems
TT earthing arrangements rely heavily on RCD protection due to higher earth-loop impedance. In many practical TT setups, a 30mA device is not ideal as the sole incomer because it offers no selectivity against downstream protection and may be more prone to unwanted tripping.
- It will trip too quickly
- It cannot discriminate against downstream RCBOs
- It may nuisance-trip on load switching
This is why TT systems typically use:
- A 100mA S-Type RCD as the main incomer (fire protection + discrimination)
- 30mA RCBOs or RCDs downstream (additional protection for final circuits)
Regulation note: On TT systems, upstream RCD protection is commonly used for fault protection where earth fault loop impedance is too high for overcurrent devices alone to achieve automatic disconnection. A 100mA S-Type RCD is often chosen where selectivity with downstream 30mA devices is also required.
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100mA S-Type RCD Trip Times Explained
A 100mA S-Type RCD is intentionally slower than a standard non-delayed RCD. That built-in delay gives downstream 30mA devices time to disconnect first, which is what creates selectivity.
In practice, S-Type devices are expected to introduce a deliberate delay rather than trip instantaneously. Exact operating times depend on the test current and the device standard, so always confirm against the manufacturer datasheet and current testing guidance.
When Installers Choose an S-Type RCD
Installers normally select a 100mA S-Type RCD when:
- A submain feeds another distribution board
- A house supplies a garage, annexe or garden building
- A heat pump or EV charger requires upstream discrimination
- A TT system needs stable upfront RCD protection
- A three-phase system has multiple RCD layers
- You want to stop nuisance trips affecting entire premises
Upstream discrimination is the key - preventing unwanted whole-site outages.
Recommended 100mA Time-Delay RCDs
These models are popular for TT installations, submains and distribution circuits:
- 4-Pole 80A Type A 100mA Time-Delay RCD
- 2-Pole 80A Type A 100mA Time-Delay RCD
- 4-Pole 63A Type A 100mA Time-Delay RCD
- 4-Pole Type A 100mA Time-Delay RCD (Variant)
- 4-Pole 100A Type A 100mA Time-Delay RCD
- 2-Pole 100A Type A 100mA Time-Delay RCD
Browse all time-delay options: 100 mA Time-Delay RCDs
FAQs
Installers often ask these when specifying S-Type protection:
Can I use a 100mA S-Type instead of a 30mA?
No - 100mA provides upstream protection only. Final circuits still require 30mA additional protection.
Do S-Type RCDs replace the need for RCBOs?
No - downstream RCBOs provide localised disconnection. S-Type devices handle upstream discrimination only.
Are 100mA RCDs only for TT systems?
They’re common on TT, but also widely used on submains, outbuildings and three-phase boards.
Can I use 100mA S-Type with EV chargers?
Yes - especially when the charger requires discrimination from upstream RCD protection.
Does a TT system need a 100mA RCD?
Often, yes. Many TT installations use a 100mA S-Type RCD as the upstream device so downstream 30mA protection can still provide additional protection and circuit-level discrimination. The exact arrangement depends on the installation design.
👉 Need reliable upstream protection or TT system stability? Browse all 100 mA Time-Delay RCDs