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.
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. A standard 30mA device is not suitable as the sole incomer because:
- 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: A 100mA S-Type RCD provides fault protection and selectivity on TT systems, preventing the main incomer from tripping before final circuits disconnect safely.
Explore all time-delay options here: 100 mA Time-Delay RCDs
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.
👉 Need reliable upstream protection or TT system stability? Browse all 100 mA Time-Delay RCDs