Time Delay RCDs Explained — When and Why to Use Them

A time delay (S-type) RCD is deliberately slow to trip — allowing a downstream 30mA RCD to operate first. This maintains selectivity across a layered protection system so a single circuit fault doesn't disconnect an entire installation.

In any well-designed electrical installation with multiple layers of RCD protection, the devices must be coordinated so that a fault on one circuit trips only the device closest to that fault — not every device upstream. This principle is called selectivity or discrimination, and time delay RCDs are the mechanism that makes it work in residual current protection systems.

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

An S-type RCD (the S stands for Selective) incorporates a built-in time delay before it trips. When a residual current fault is detected, instead of tripping immediately, the device waits a defined period — typically 60 to 200 milliseconds — before opening its contacts. This pause allows any downstream instantaneous (non-delayed) RCDs to trip first, clearing the fault before the upstream device operates.

Without a time delay at the upstream device, both devices would attempt to trip simultaneously. Whichever happens to operate marginally faster would clear the circuit, but you would have no reliable selectivity — you could lose the entire upstream supply on a minor fault downstream.

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	Preserves supply to unaffected circuits during a fault
Multi-occupancy buildings	100mA or 300mA S-type per floor/flat	30mA RCBOs per circuit	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 time delay

Designation	Delay	Application
S-type (Selective)	60–200 ms delay	Upstream of instantaneous 30mA devices — standard for discrimination
G-type (Short time delay)	10–50 ms delay	Where shorter delay is acceptable — less common in UK domestic

For UK domestic and commercial applications, S-type is the standard. It is tested to BS EN 61008 and must not trip in less than 60ms at 5× the rated sensitivity — giving downstream instantaneous devices ample 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 configurations, 63A to 100A, at 100mA sensitivity. These are specified for upstream use in TT systems and larger distribution boards where full discrimination across multiple downstream 30mA circuits is required.

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 time delay RCDs →

Frequently Asked Questions

Does a time delay RCD still provide protection?

Yes. A time delay RCD will still trip on a fault — it just waits a defined period before doing so. If the downstream device fails to clear the fault first, the time delay device will eventually operate. The delay is short enough (typically under 200ms) that sustained danger from a downstream fault is minimal, while still being long enough for a 30mA device to operate.

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

Time delay devices are typically used upstream of a domestic consumer unit rather than within it — for example, as an incomer RCD in a TT system feeding a sub-board, or as a floor-level RCD in a multi-occupancy building. Within a standard domestic consumer unit, instantaneous 30mA RCBOs on each circuit provide the correct level of protection without requiring upstream time delay devices.

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

These are independent characteristics. Sensitivity (30mA, 100mA, 300mA) determines the leakage current at which the device trips. Time delay (S-type, G-type) determines how quickly it trips after detecting that threshold. A time delay RCD is typically rated at 100mA or 300mA and is used upstream — its higher sensitivity threshold combined with its delay ensures the downstream 30mA instantaneous device always operates first.

Further reading:
RCD trip sensitivity: 30mA vs 100mA vs 300mA explained
4-pole vs 2-pole RCDs for 3-phase
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