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Time Delay RCDs Explained — When and Why to Use Them

RCDs & RCBOs

Time Delay RCDs Explained: When and Why to Use Them

A time delay (S-type) RCD is deliberately slow to trip, so a downstream 30mA RCD operates first. That keeps a layered protection system selective: one circuit fault doesn’t disconnect the whole installation.

Quick answer

Use an S-type RCD upstream of instantaneous 30mA devices (TT systems, large boards, multi-occupancy buildings, EV distribution boards) so only the device closest to the fault trips.

What is an S-type (time delay) RCD?

In an installation with several layers of RCD protection, the devices have to be coordinated so a fault trips only the one nearest to it. This is selectivity, or discrimination. Time delay RCDs are how you achieve it.

An S-type RCD (S for Selective) has a built-in delay, typically 60 to 200 milliseconds, before it opens its contacts. That pause lets downstream instantaneous RCDs trip first and clear the fault before the upstream device operates.

Without the delay, both devices would try to trip at once. Whichever is marginally faster wins, and you have no reliable selectivity: a minor downstream fault could take out the whole upstream supply.

RCD discrimination A fault on one circuit trips only its own 30mA device because the upstream S-type RCD waits.Upstream S-type RCD (100 / 300mA)Deliberate time delay30mA RCBO: circuit 1Stays on30mA RCBO: circuit 2Fault: trips first30mA RCBO: circuit 3Stays onThe 30mA device clears the fault before the S-type reacts, so the other circuits keep their supply.
A fault on circuit 2 trips only its own 30mA device; the upstream S-type waits.

Where time delay RCDs are used

Application Upstream device Downstream device Why time delay is needed
TT earthing systems 100mA or 300mA S-type 30mA instantaneous RCBOs Upstream must not trip when one 30mA circuit clears a fault
Large distribution boards 300mA S-type at incomer 30mA RCBOs on each circuit Keeps supply to unaffected circuits during a fault
Multi-occupancy buildings 100mA or 300mA S-type per floor or flat 30mA RCBOs per circuit A fault in one flat must not affect other occupants
EV charging distribution boards 300mA S-type at supply incomer 30mA or Type B RCDs per charger One charger fault must not disconnect all chargers

S-type vs G-type

Designation Delay Application
S-type (Selective) 60–200 ms Upstream of instantaneous 30mA devices. The standard for discrimination.
G-type (short time delay) 10–50 ms Where a shorter delay is acceptable. Less common in UK domestic work.

For UK domestic and commercial work, S-type is the standard. It is tested to BS EN 61008 and is designed not to trip too quickly at higher fault currents, giving downstream instantaneous devices time to clear first.

WCED time delay RCD range

The WCED WARTD series provides A-Type time delay RCDs in 2-pole and 4-pole versions, 63A to 100A, at 100mA sensitivity. They are specified upstream in TT systems and larger distribution boards where full discrimination across several downstream 30mA circuits is needed.

Shop WCED time delay RCDs

A-Type 100mA time delay RCDs, 2-pole and 4-pole, 63A to 100A. In stock with next working day dispatch.

View the time delay RCD

Frequently asked questions

Does a time delay RCD still provide protection?

Yes. It still trips on a fault; it just waits a short, defined time first. If the downstream device fails to clear the fault, the time delay device operates. The delay is short enough (typically under 200ms) that danger from a downstream fault stays minimal.

Can I use a time delay RCD in a domestic consumer unit?

Time delay devices normally sit upstream of a domestic consumer unit, for example as the incomer RCD in a TT system feeding a sub-board, or as a floor-level RCD in a multi-occupancy building. Inside a standard domestic consumer unit, instantaneous 30mA RCBOs on each circuit give the right protection without upstream time delay devices.

What is the difference between a time delay RCD and a high-sensitivity RCD?

They are independent characteristics. Sensitivity (30mA, 100mA, 300mA) sets the leakage current at which the device trips. Time delay (S-type, G-type) sets how quickly it trips once that threshold is reached. A time delay RCD is usually rated at 100mA or 300mA and used upstream, so the downstream 30mA device always operates first.

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