RCD sensitivity (sensibilidad del diferencial, 30 mA)

What RCD sensitivity means

On the front of every RCD you will see a value such as “30 mA” or “IΔn 0.03 A”. That is its sensitivity: the difference between outgoing and returning current at which the device cuts. That difference is the current escaping to earth, through a damaged cable, a faulty appliance or, in the worst case, a person.

Usual values

• 30 mA (high sensitivity). Compulsory on the circuits of a home under ITC-BT-25. It cuts before a shock becomes dangerous for a person.
• 300 mA. Used in industrial installations or as general fire protection. It does not protect people.
• Selective or time-delayed RCDs. Fitted upstream with a higher rating or a delay, so they discriminate with the 30 mA devices on each circuit.

For swimming pools, ITC-BT-31 adds its own requirements, such as 12 V safety extra-low voltage in zones 0 and 1.

When it affects you

A 30 mA RCD that trips now and then is usually doing its job: there is an earth leak worth finding. If the whole home has a single RCD, many small leaks from appliances can add up close to the threshold and cause trips with no clear fault. Splitting circuits across several RCDs or fitting a high-immunity one usually solves it.

What not to do

Never swap a 30 mA RCD for a 300 mA one to stop it tripping: you would lose protection for people. If it trips often, have an electrician find the cause. See RCD keeps tripping and swimming pool electrics in Seville, Spain.