What You Need To Know About Arc Fault Detection Devices (AFDD's)

Why would I need an AFDD?

Before answering that question it is important to understand the existing protection scheme for a domestic electrical installation:

• Additional protection – This is provided by an RCD not exceeding 30mA
• Fault protection – This is automatic disconnection of the power and the fault, typically provided by circuit breakers, RCD’s, RCBO’s and fuses
• Basic protection – This is the insulation of live parts preventing the user coming into contact with them

Below is a typical 2-way split consumer unit (click here to find out more about different types of consumer unit).

Basic protection is provided by the cover of the consumer unit and the busbar cover, these prevent you from coming into contact with the live parts inside.

In your consumer unit there will be a circuit breaker for each of the circuits in your property, providing fault protection. These will protect your circuits from;

1. Overload e.g. plugging too many things into an extension lead or a socket adaptor
2. Short circuit current e.g. a leak in your bathroom spreads to the lights in the room below, water comes into contact with the line and neutral conductors at the same time
3. Earth fault current e.g. you accidently drill through a cable buried in a wall, the current flows through you from the line conductor to earth

You also have two RCD’s, these are each protecting a bank of circuits, they are there to provide additional protection in the event of a line to earth fault. However, due to the way an RCD works it takes a lot less current to cause it to operate as opposed to a circuit breaker. So in the event of a line to earth fault they will generally operate first.

As you can see you’re fairly well protected. You can plug too many things into a socket and the circuit breaker will trip before your cable burns out (then sets fire to your house), you can flood your bathroom and fill your lights up with water and your circuit breaker will trip preventing you from coming into contact with electricity, you can drill a hole in your wall straight through a cable and the RCD will trip before the circuit breaker even gets a look in, again protecting you from getting an electric shock.