RCD / GFCI Trip Calculator

An RCD (Residual Current Device) or GFCI (Ground Fault Circuit Interrupter) is a safety switch that cuts power the moment it detects electricity leaking outside the intended circuit path — protecting people from electric shock and fires. Enter your system voltage, fault loop impedance, RCD/GFCI rating, maximum trip time, normal load current, and background leakage current into the RCD/GFCI Trip Calculator to see the calculated fault current. Secondary outputs include trip margin, effective sensitivity, and code compliance status. Also try the use the Ground Fault Calculator.

Results

Fault Current

Trip Margin

Effective Sensitivity

Code Compliance

What is the difference between RCD and GFCI?

RCD (Residual Current Device) and GFCI (Ground Fault Circuit Interrupter) are essentially the same device with regional naming differences. Both detect current imbalances and trip when fault current exceeds their rating. See also our Cable Derating Calculator.

How do I measure ground fault loop impedance?

Loop impedance is measured using a specialized tester that applies a test current and measures voltage drop. The measurement includes transformer impedance, line conductors, and equipment grounding conductor resistance.

What causes nuisance tripping in GFCI breakers?

Common causes include high background leakage current from equipment, moisture, damaged insulation, or neutral-to-ground faults downstream of the GFCI. Normal leakage should be well below the device rating.

What is a good loop impedance value?

Loop impedance should be low enough to ensure adequate fault current for protective device operation. Typically less than 1.6Ω for residential 120V circuits and proportionally lower for higher voltages. You might also find our calculate Ground Rod Resistance useful.

Can I trip an RCD remotely for testing?

Yes, RCDs can be tripped remotely using a test circuit that creates an artificial imbalance between live and neutral conductors, similar to the built-in test button function.

Why is my fault loop impedance too high?

High loop impedance typically results from loose connections, undersized conductors, long circuit runs, or poor grounding connections. These conditions reduce fault current and may prevent proper protective device operation.

What are the standard RCD sensitivity ratings?

Common ratings are 5mA (medical), 10mA (special locations), 30mA (general protection), 100mA (fire protection), and 300mA (industrial applications). Selection depends on application and safety requirements.

How fast should an RCD trip?

Standard RCDs should trip within 300ms at rated current, and within 40ms at 5 times rated current. Faster trip times provide better protection against electric shock and fire hazards.