30mA vs 300mA RCD: Which Sensitivity Level Do You Need

You know that jolt you get when you accidentally touch a live wire on a 9V battery? Annoying, but harmless. Now imagine that same feeling multiplied by twenty, coursing through your chest. That's the difference between an inconvenience and a funeral. And honestly? It's why picking the right RCD sensitivity isn't just code compliance—it's the difference between walking away and not.

I've spent over a decade knee-deep in distribution boards, fault-finding on circuits that other people swore were fine, and watching 30mA RCDs trip at the worst possible moments. I've also seen 300mA RCDs sit there quietly while a cable smoldered behind a wall. The debate between these two sensitivity levels is not new, but most people get it wrong because they think it's a simple 'one size fits all' question. It isn't.

So let's cut through the noise. You need to understand the physics of what your body can handle, the practical realities of nuisance tripping, and the specific application where each device shines. By the end of this, you'll know exactly which residual current device belongs in your panel. No guesswork.

The Core Difference: Protecting People vs Protecting Property

Look, the fundamental job of an RCD is to detect current leaking to earth and disconnect the circuit before that leakage becomes dangerous. But dangerous to what? That's the question.

A 30mA RCD is designed to save your life. The human heart can tolerate approximately 30 milliamps of current flowing through it for about 40 milliseconds before ventricular fibrillation sets in. After that, you're relying on someone else knowing CPR and an ambulance arriving fast. The 30mA trip threshold is the accepted international standard for personal protection because it sits just below that lethal threshold. It's fast, it's sensitive, and it's non-negotiable on circuits that supply sockets or portable equipment.

A 300mA RCD is a completely different animal. This device is not trying to stop your heart—it's trying to stop your building from burning down. A sustained earth leakage current of 300 milliamps generates enough heat to ignite insulation, wood, dust, and pretty much anything else flammable in its path. The 300mA residual current device is a fire prevention device first and foremost. It allows for the natural, unavoidable leakage that occurs in long cable runs and equipment with filters, while still catching the kind of fault that would cook a cable slowly over hours or days.

Why You Can't Use a 300mA RCD for Personal Protection

Here's where things get real. If you put a 300mA RCD on a socket circuit, you might as well not have one at all for personal safety. The human body can't handle 300mA. Period. At that current level, your muscles clamp down instantly—you can't let go of whatever you're touching. Your heart goes into fibrillation within a single cycle. The 300mA device won't even notice you're dying until it's far too late.

I've tested this (not on people, obviously, but on calibrated test rigs). A 30mA RCD typically operates between 15mA and 30mA of leakage. That's the sweet spot. A healthy adult might feel a tingle at 10mA, can let go at 15mA, and starts having trouble at 30mA. The 30mA sensitivity catches you before you cross that line.

For a 300mA device, the typical operating range is between 150mA and 300mA. That's ten times higher. You'd be unconscious before the device even started thinking about tripping. Seriously. That's not an exaggeration.

Why a 30mA RCD Might Drive You Crazy on Large Installations

Now, the flip side. A 30mA RCD is incredibly sensitive. Too sensitive for some applications. Modern electrical equipment is leaky by nature. Every switched-mode power supply, every inverter, every LED driver, every surge protector leaks a tiny amount of current to earth. Individually, it's nothing. But when you add up twenty computers, thirty LED downlights, and a couple of phone chargers on the same circuit? Suddenly you're sitting at 15mA of background leakage on a device that trips at 30mA.

One spike. One slightly damp morning. One aging power supply. And the 30mA RCD trips. Now the entire office is in darkness because someone plugged in a dodgy phone charger.

This is the classic nuisance tripping scenario, and it's why you see 300mA RCDs used as the main switch in commercial and industrial distribution boards. They provide fire protection for the whole installation while allowing all that background leakage to exist without causing chaos. Downstream, you install 30mA RCDs on specific socket circuits where people are actually plugging things in.

Time Curves and Selectivity: Getting the Coordination Right

It's not just about the sensitivity in milliamps. You also need to think about when the device trips. This is where time-delay characteristics come into play, and honestly, it's where most electricians screw up.

A standard 30mA RCD is instantaneous. It trips within 40 milliseconds at its rated sensitivity. That's fast enough to protect a human. But if you put that same fast device upstream of another 30mA RCD, you get what I call the 'race condition.' Any downstream fault trips both devices, and now half your building is dark because a single socket had a problem.

The solution is selectivity. A 300mA RCD with an 'S' (selective) or time-delay characteristic is designed to sit upstream. It deliberately waits—typically between 130ms and 500ms—before tripping. This gives the downstream 30mA RCD time to clear the fault first. If the downstream device works, the upstream 300mA RCD never trips. If the downstream device fails or the fault is bigger, the 300mA time-delay RCD catches it.

The S-Type Device Explained

The 'S-Type' RCD is a specific variant you'll encounter frequently. It's designed for selectivity. It has a deliberate delay and a higher trip threshold, usually 300mA. You'll find these as incoming main switches in apartment blocks, office floors, and industrial units.

- The S-Type 300mA RCD ignores short-duration leakage spikes (like motor startup surges or capacitor charging currents). - It allows downstream 30mA RCDs to operate first. - It ensures that if a major fault develops, the entire installation doesn't burn while waiting for a small device to react.

Here's a quick list of where you should and shouldn't use each:

- 30mA Instantaneous RCD: Socket circuits. Portable equipment. Bathrooms. Construction sites. Anywhere a person might touch a live conductor. - 30mA S-Type RCD: Rarely used. Only for specific medical applications or circuits requiring high immunity to nuisance tripping while maintaining personal protection. - 300mA Instantaneous RCD: Almost never used. It's too slow for people and too fast for selectivity. - 300mA S-Type RCD: Main switch in commercial/industrial boards. Fire protection for entire installations. Upstream of multiple 30mA RCDs.

The Type Matters: AC, A, F, and B

You can't just pick a sensitivity and call it a day. The waveform of the fault current also matters. And this is where a lot of 'experts' get caught out.

Older RCDs, classified as Type AC, were designed to detect sinusoidal AC leakage only. That worked fine in the 1970s when everything was a resistive heater or a simple motor. Today? We have electronics everywhere. Those electronics create pulsating DC leakage and mixed-frequency waveforms. A Type AC 30mA RCD might not trip at all on a pulsating DC fault from a rectifier circuit.

You need the right type for the load:

- Type A RCD: Detects AC and pulsating DC. This is now the minimum standard for most residential and commercial circuits in many countries. - Type F RCD: Handles mixed frequencies from variable speed drives and inverters. - Type B RCD: The full monty. Detects AC, pulsating DC, smooth DC, and high-frequency AC. Needed for EV chargers, solar inverters, and industrial drives.

So when you're choosing between 30mA and 300mA, you also need to specify the type. A 30mA Type A RCD is the standard for modern socket circuits. A 300mA Type B RCD is what you need on the incoming supply for a solar installation.

Practical Decision Guide: Which One Goes Where?

I've seen enough panels to give you a simple framework. These are the rules I follow after thousands of installations and fault-finding calls.

Always use 30mA for personal protection. That means: - All socket outlets up to 32A - Lighting circuits in bathrooms or areas with high earth potential - Cables buried in walls at less than 50mm depth - Circuits supplying outdoor equipment - Construction site distribution

Use 300mA for fire protection and selectivity. That means: - Main incoming switch for whole installations - Sub-main distribution to multiple final circuits - Circuits supplying fixed equipment with known high leakage (like servers or industrial machines) - Agricultural and commercial buildings where background leakage is unavoidable

Use both in a tiered system. That's the professional answer. A 300mA S-Type RCD at the origin, feeding multiple sub-circuits, each protected by a 30mA RCD. The big device stops the fire. The little devices stop the dying. It's not complicated, but it requires planning.

Let me give you a real-world list that I literally hand to my apprentices:

1. Socket circuits in a house: 30mA Type A, instantaneous. 2. Main switch in a house (if required by code): 100mA or 300mA S-Type, depending on local regs. 3. Office floor distribution: 300mA S-Type Type A as main, 30mA Type A for each socket sub-circuit. 4. EV charger: 30mA Type B (or 30mA Type A with DC detection in some cases). 5. Industrial welding circuit: 300mA Type B S-Type, with 30mA Type B for any associated sockets. 6. Temporary power for construction: 30mA Type AC (or Type A for modern sites) on every circuit.