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100mA Time Delay RCDs

100mA time delay RCDs, also known as Type S RCDs or selective RCDs, are used where upstream residual current protection needs to coordinate with downstream 30mA RCDs or RCBOs. The built-in delay helps the downstream device trip first during a fault, reducing the risk of unnecessary disconnection across the wider installation.

These devices are commonly used on sub-mains, outbuilding supplies, secondary consumer units, distribution circuits and installations where RCD selectivity is part of the design.

Quick answer: A 100mA time delay RCD is an upstream RCD with a deliberate tripping delay. It is used to support selectivity, so a downstream 30mA RCD or RCBO can disconnect the fault before the upstream device operates.

A Type S RCD should be selected as part of the overall installation design, taking account of the supply arrangement, downstream protection, circuit purpose and required disconnection characteristics. It is not a direct replacement for 30mA additional protection on final circuits where that protection is required.

  • 100mA sensitivity – commonly used for upstream residual current protection
  • Type S time delay – supports discrimination with downstream RCDs and RCBOs
  • Useful for sub-mains – garages, outbuildings, workshops and secondary boards
  • Selective RCD protection – helps reduce unwanted whole-board disconnection

For related devices, browse our RCDs, RCBOs, consumer units and electrical enclosures.

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What Is a 100mA Time Delay RCD?

A 100mA time delay RCD is a residual current device with a deliberate operating delay. It is typically fitted upstream of 30mA RCDs or RCBOs where the installation needs selective tripping.

You may also see these products described as S Type RCDs, Type S RCDs, selective RCDs or time delayed RCDs. In most trade contexts, the intent is the same: upstream RCD protection designed to coordinate with downstream devices.

Why Use a Type S RCD?

A Type S RCD helps improve discrimination between upstream and downstream protective devices. If a fault occurs on a final circuit, the downstream 30mA RCD or RCBO should operate before the upstream time-delay device.

This helps avoid unnecessary loss of supply to wider parts of the installation and makes fault location simpler.

100mA RCD vs 30mA RCD

A 100mA time delay RCD and a 30mA RCD are normally used for different roles. A 30mA device is commonly used for additional protection on final circuits. A 100mA Type S RCD is more commonly used upstream where selectivity and residual current coordination are required.

Device Typical Role Common Use
30mA RCD / RCBO Additional protection Final circuits, socket circuits and circuits requiring 30mA protection
100mA Type S RCD Selective upstream protection Sub-mains, outbuildings, distribution circuits and secondary boards

Common Uses for 100mA Time Delay RCDs

  • Sub-main feeds supplying garages, workshops, garden rooms or outbuildings
  • Secondary consumer units where downstream RCBOs or RCDs are installed
  • Distribution circuits where selective RCD protection is required
  • Commercial installations where unnecessary upstream tripping needs to be reduced
  • Enclosed RCD setups where a 100mA time delay RCD with enclosure is specified

RCD Selectivity and Rated Time Delay

RCD selectivity means arranging protective devices so the device closest to the fault operates first. A time-delay RCD supports this by allowing downstream protection a short window to disconnect the fault before the upstream RCD trips.

The rated time delay and operating time depend on the specific device and test conditions. Always check the product datasheet and the installation design requirements before specifying a 100mA S Type RCD.

100mA Time Delay RCD FAQs

What is a Type S RCD?

A Type S RCD is a selective time-delay residual current device. It is designed to operate with a delay so downstream RCDs or RCBOs can trip first.

What is a 100mA time delay RCD used for?

It is commonly used upstream on sub-mains, outbuilding supplies, secondary boards and distribution circuits where selective RCD protection is required.

Is a 100mA RCD the same as a 30mA RCD?

No. A 30mA device is commonly used for additional protection on final circuits. A 100mA time delay RCD is normally used upstream for selectivity and residual current coordination.

Can I use a 100mA time delay RCD instead of 30mA protection?

Not where 30mA additional protection is required. A 100mA Type S RCD is normally used upstream, with downstream 30mA RCDs or RCBOs providing final circuit protection where needed.

What is RCD selectivity?

RCD selectivity means coordinating upstream and downstream RCDs so the downstream device closest to the fault trips first.

Do you supply 100mA time delay RCDs with enclosures?

Selected products may be supplied in or used with suitable enclosures. Check each product listing for enclosure type, rating and configuration.

UK-Stocked Type S RCDs for Trade Installs

Power & Data UK supplies 100mA time delay RCDs and Type S selective RCDs for sub-mains, outbuildings and distribution circuits. Orders over £30 qualify for free UK mainland next-day delivery when placed before 2pm.

For related protection, browse our RCDs, RCBOs, consumer units and electrical enclosures.