Check your protection – why that 30mA RCD is so important

In Belgium, most domestic electrical installations have an earthing model called TT (Terra-Terra). The electricity supply company (Sibelga or Fluvius) does not supply the earth connection to the property, instead each property has its own earth terminal, a metal rod or loop, embedded in the ground.

A 30mA RCD (Residual Current Device) is the most critical safety component on a 230V TT electrical system because it provides life-saving shock protection where standard circuit breakers fail to work.

Here is a simple explanation of why it is so important:

1. TT Systems Have High Earth Resistance

In a TT system, your property’s earth wire connects directly to a metal rod driven into the ground, rather than back to the utility company’s main earth network. Because soil and rock are relatively poor conductors compared to copper wire, this “earth path” has a high resistance.

2. Standard Circuit Breakers Won’t Trip during an Earth Fault

If a live wire accidentally touches the metal casing of an appliance (like a toaster or washing machine):

• In other systems (such as TN used in most countries in Europe): A large fault current rushes back through low-resistance wires, instantly tripping a standard circuit breaker or blowing a fuse.
• In a TT system: The high resistance of the ground limits the fault current. It might only be a few hundred milliamperes or a couple of amperes. This is not enough current to trip a standard 16A or 20A circuit breaker, leaving the appliance casing permanently energized at dangerous 230V.

3. The 30mA RCD Instantly Cuts the Power

An RCD does not wait for a massive surge of current. Instead, it constantly balances the current going out (Phase 1) against the current coming back (Phase 2 or neutral).

• The 30mA threshold: If even a tiny amount of electricity leaks away to earth (e.g., through the ground rod or through a human body), the RCD notices the imbalance. If that leak reaches 30mA (0.03 amps), it trips instantly (in under 40 milliseconds).
• The safety limit: 30mA is universally recognized as the maximum current a human body can tolerate before ventricular fibrillation (fatal heart stoppage) or muscle paralysis (preventing you from letting go) occurs.

Summary Comparison

System Feature	Without an RCD	With a 30mA RCD
Fault on a TT system	Casing stays live at 230V. Circuit breaker does not trip.	RCD detects the leak and cuts power in milliseconds.
Human shock risk	Extreme danger of fatal electrocution if touched.	Safe protection against fatal electric shock.

A 300mA RCD is not enough because it allows up to 10 times more current to pass through a human body than what is required to cause fatal heart failure.

While a 300mA RCD can detect electrical leaks to prevent electrical fires, it is completely incapable of protecting human life against direct contact.

1. The Human Body Lethal Threshold

Electrical currents affect the human heart and muscles at incredibly low levels:

• Below 30mA: A person can usually still let go of the live object. While painful, the shock is unlikely to cause permanent damage.
• At 50mA to 100mA: Muscular paralysis occurs (you cannot let go), and ventricular fibrillation (where the heart flutters wildly and stops pumping blood) is highly likely within seconds.
• At 300mA: Fatal cardiac arrest is almost instantaneous.

2. What a 300mA RCD is actually used for

A 300mA RCD is designed for equipment and fire protection, not personal safety.

• It is installed at the main incoming supply of a TT system to detect larger faults that could generate heat and start a fire.
• Because a TT system relies on the ground rod, a major insulation breakdown could leak current into the earth. A 300mA RCD catches these larger leaks to protect the building, but it will completely ignore a 50mA current flowing through a human being.

The Protective Difference

RCD Rating	Triggers At	Primary Purpose	Human Safety Level
30mA	0.03 Amps	Life Protection (Direct Contact)	Safe: Trips before heart failure occurs.
300mA	0.30 Amps	Fire Protection (Indirect Contact)	Lethal: Will allow enough current to kill a person without tripping.