How AFCI test the arc fault, someone has a good explanation

[ramironchis]

Hello,
I've been collecting information about AFCIs and GFCIs but I couldn't find a good explanation how they work, how they sense the fault, I saw a video on youtube where they make a small explanation but for me is not enough, I want to know how the breaker difference between normal fault and abnormal, if someone has information and can share I'll really appreciate it.............

thanks in advance

 

[__dan]

Hello,
I've been collecting information about AFCIs and GFCIs but I couldn't find a good explanation how they work, how they sense the fault, I saw a video on youtube where they make a small explanation but for me is not enough, I want to know how the breaker difference between normal fault and abnormal, if someone has information and can share I'll really appreciate it.............

thanks in advance

You asked how the AFCI senses the fault. I will make an educated guess.

The arc is wideband noise. It is said to have all frequencies in its spectrum, from the lowest to the highest, from DC up to ionizing radiation.

The normal power will only have 60 Hz and a few harmonics and transients on it. The arc fault sensor could work by sensing and triggering when it detects the presence of a particular frequency or a frequency spectrum characteristic of arcing faults. The ability to selectively capture the target frequency bandwidth with a filter and amplify it is relatively simple and routine EE. That would be my guess, the AFCI sniffs the line for a high frequency noise spectrum characteristic of an arcing fault.

If it were me, I would take this guess and call the factory for a straight answer. I am only saying this is the range of where I expect the answer to fall into. If the discussion falls into this range, you and the factory would be able to speak the same lanquage.

 

[GoldDigger]

You asked how the AFCI senses the fault. I will make an educated guess.

The arc is wideband noise. It is said to have all frequencies in its spectrum, from the lowest to the highest, from DC up to ionizing radiation.

The normal power will only have 60 Hz and a few harmonics and transients on it. The arc fault sensor could work by sensing and triggering when it detects the presence of a particular frequency or a frequency spectrum characteristic of arcing faults. The ability to selectively capture the target frequency bandwidth with a filter and amplify it is relatively simple and routine EE. That would be my guess, the AFCI sniffs the line for a high frequency noise spectrum characteristic of an arcing fault.

If it were me, I would take this guess and call the factory for a straight answer. I am only saying this is the range of where I expect the answer to fall into. If the discussion falls into this range, you and the factory would be able to speak the same lanquage.

That is pretty much the mechanism. To keep the sensitivity up and reduce (really??) the number of false trips the processor will look at combinations of different frequencies and amplitudes of noise along with time scale in fractions of a cycle, repeated over some number of cycles (referred to as a signature) before declaring a fault. That way a high amplitude at a single frequency and not varying over the course of a cycle can be ruled out as being a radio transmission rather than an arc, for example.

In addition, the AFCI may also require a certain minimum current flow through the breaker or receptacle device to qualify some or all of the signatures.

Also, to use the OP's terms, the breaker will only declare a "normal" fault based on overcurrent, e.g. thermal magnetic. One characteristic of an arc fault is that it often will not correspond with any kind of sustained overcurrent. The breaker can trip based on a signature where there is no overcurrent at all, and it can trip immediately based on a signature for a current level which would not trip for a long time based on the I/T curves.

The third type of fault that an AFCI device may be configured to detect is a ground fault (current imbalance.)