The thing about RCDs is that we get feedback that they are working. We all (even non-electricians) are likely familiar with at least one RCD that has tripped (possibly repeatedly) which after investigation was found to be due to a faulty appliance or a dodgy cable or whatever.

Perhaps we'll reach a similar situation with AFDDs eventually as they become more commonly installed. We'll then all have seen one of them trip and found it's due to damaged flex or loose terminal or whatever. That would eventually inspire confidence. On the other hand, if we never see them trip, we might conclude that either the events they are intended to protect against are exceedingly rare so they're a waste of money, or that we see plenty of arcs but no trips, so they don't do what they're supposed to do, and again they're a waste.

There's a similar feedback problem with SPDs. If they are working and successfully protecting equipment, we'll probably never know.

Definitely wallywombat 

Definitely wallywombat 

With an R.C.D. we know that mainly it trips off because of earth leakage. There are other reasons but they are less common. But, with an A.F.D.D. with a combined M.C.B. unit, the tripping may just be due to overcurrent, so we may not know which part of the device has caused the tripping off.

Many manufacturers advocate the installation of A.F.D.Ds

BEAMA Guide To Arc Fault Detection Devices (AFDDs)

What's this all about? I was never any good at arithmetic.

Q.3 When required for additional fire protection, RCDs shall be a rated residual operating current not exceeding 300 mA. Why is the minimum AFDD tripping current of 2.5 A so much greater than 300 mA? 300 mA (0.3 A) equates to: 230 V x 0.3 A = 69 W which is related to leakage current and not arc current. RCDs do not detect the specific waveform / signature associated with a stable electric arc. The AFDD tripping time at 2.5 A relates to approximately 100 W and can be explained as follows: The break (tripping) time in BS EN 62606 for AFDDs, is derived from the energy to ignite a cable by degrading the insulation with contact arcing and glowing. The minimum energy value of 100 J with an arc voltage of 40 V was established for the tripping characteristic for series arcing. The total break time tB is therefore derived as follows: 100 W for 1 s equates to 100 J, so the AFDD can promptly interrupt the current and limit the duration of combustion of the cable, thus significantly reducing the risk of the fire spreading. AFDDs detect the specific waveform / signature associated with a stable electric arc

Z.

Zoom, it's no good trying to decrypt this stuff. What is defined as 'a stable electric arc'?

A more pertinent question to ask might be: ' What is the minimum energy value and arc voltage for a series arc in a ring final circuit?' Which, as we know, is what most domestic installations have in one form or other.

On the subject of SPDs, I think there are 2 classes. The first, is where an installation has been constructed, the purpose of which is to provide a mission-critical service, so, for an example, we might have something like a server farm which must be always on, 24/7 and have some baseline resilience against outages. The supply will most likely be 3 phase with back up UPS in abundance. Here, a SPD may be seen as another layer of 'defence in depth' against the possibility of interruption of supply.

This particular species of SPD most likely works and will give additional protection against lightning strikes into the bargain. It will also have an eyewateringly high price tag.

Then, we have another class of device for a totally different market, the domestic one, where joe blogs's 4K smart telly might get zapped by an interruption caused by a steadily deteriorating underground supply cable because the DNOs have been paying out to shareholders instead reinvesting into the distribution network infrastructure.

So, we end up with a very cheap jellybean set of components - electrical lego if you like, chucked into a cheap plastic case and mandated via some very spurious dataset which must have been crossed with that of a tropical country, and chucked into domestic consumer units with gay abandon and much financial gain.

I fear that like AFDDs, and the BS7671 changes which mandate them. the domestic side of these things promise the earth, yet deliver a mere handful of soil when called upon to function as described 'on the side of the tin'.

Zoomup said:

Q.3 When required for additional fire protection, RCDs shall be a rated residual operating current not exceeding 300 mA. Why is the minimum AFDD tripping current of 2.5 A so much greater than 300 mA?

The RCD would only trip on a live conductor to PE (or Earth) fault.

The 2.5 A, I understand, relates to the load(s) downstream of a broken conductor (rated 2.5 A or more) causing a series arc. The AFDD may still well operate for a parallel arc ... to PE, but would operate for parallel arcs L-N (or L-L)  if we had three-phase devices or other systems for which the AFDD were rated) which an RCD would not operate for.

Yes, I understand the issue with the fact in houses we often use T&E ... but not always ... and other cable types and wiring systems are used in other premises where AFDDs are required in Amendment 2 - and, of course, other wiring systems are used in appliances as has already been discussed