Separate Arc fault protection devices not needed with RCD's?

[Sparkswillfly]

I'm looking at replacing my old switchboard with a new RCD/AFCI one. I asked the electrician who was quoting the job about Arc fault interrupters. I previously understood these were a different device from RCD's-RCD is to prevent shocks, AFI to prevent fires.
I also understood there were devices that did both in combination.

The electrician tells me that the tech has advanced so that RCD's alone will be sufficient without incorporating the AFI feature. If a switch contact or wire etc wears out and arcing occurs, the newer RCD's I'm told will still trip and cut off the power to that circuit, preventing a fire. Is that correct? He seemed pretty adamant about it. I do want to minimise the risk of electrical fires, so just wanted to see what others thought.

Thanks

 

[brianmoooore]

RCD will cut off the power when the arcing/fire gets bad enough to cause leakage between an arcing live conductor and a usually close by cpc, but apart from that, he talking nonsense.

 

[Sparkswillfly]

RCD will cut off the power when the arcing/fire gets bad enough to cause leakage between an arcing live conductor and a usually close by cpc, but apart from that, he talking nonsense.

When does it get bad enough to causing the leakage and arcing, will the RCD cut the power soon enough to prevent a fire from getting going?

 

[littlespark]

The devices work in different ways.

An rcd looks for an imbalance between the load of line and neutral on the protected circuit… if the imbalance goes over a threshold, it trips out. Imbalance is caused by leakage to earth.

arc fault can happen where there is no contact with earth. A loose connection where the cable is touching where it’s supposed to be, but is loose and may disconnect momenterily. This causes the arcing, and very high temperatures, which in turn can cause a fire

 

[pc1966]

When does it get bad enough to causing the leakage and arcing, will the RCD cut the power soon enough to prevent a fire from getting going?

In many cases yes. Where the fault is weakening insulation between line and earth somewhere (which is commonly the case for damaged cables due to crusing or rodent attacks) the GFCI (USA name) / RCD (UK/EU name) will see the current going astray and disconnect. This is sometimes called a "parallel arc" as it is between adjacent conductors.

The usual over-current protection does the same eventually, but it can take a lot more current to do so (tens of amps or more, versus 0.03A or so) and that implies a lot more local heating and so a higher fire risk than an RCD in the same circumstances.

The AFCI (USA name) / AFDD (UK/EU name) operates differently as it is looking for the high frequency noise from an electric arc, not for unusual conventional current flow, and so it can also detect a break in a conductor leasing to a "series arc" where the normal current is flowing but now having to jump a small gap to continue. That gap suffers from a lot of heating as a result and so is also a fire risk. Really this is the special protection from an AFDD that is not provided by more traditional means.

Now quite how good or useful this protection is in practice, and how many fires are really a result of arcs that only an AFDD would detect, is open to debate!

Fires can start from other faults like a high resistance connection (local heating but no arc) or bad design/workmanship (too small a cable for the over current protection), failing parts like motor run capacitors inside appliances, or even common ones like lint in tumble dryers catching fire and being classed as "electrical fires" at source. None of the suppliers of AFDD or the standards bodies have given such an analysis to my knowledge, so the cost/benefit is unknown.

TL;DR Combined RCBO/AFDD devices that provide all of the protection modes are safer, but to an unspecified amount, cost more, and may have more nuisance trip events where no fault is found.

 

[Sparkswillfly]

I got a second opinion from another firm. They are willing to provide the combo AFCI/RCD devices but they don't think the arc protection is necessary. This is because the wiring is tough plastic sheathed cable and the visible wires behind the board look good he said. The house is 40 years old, but I'm also going to have the old wall outlets and light switches replaced which by itself should reduce the chances of arcing anyway right?

They say because the combo AFCI/RCD devices are bigger the new board would take up a lot more space than the current one and I might end up having to get a plasterer or builder to patch up part of the wall. Hmm

I also never have any issues with fuses/breakers tripping, no sparks, burn marks, bad smells or buzzing sounds, so with all that in mind, is it worth getting the arc protection or would the standalone RCD's be sufficient?

 

[Mikegh]

The condition of the installation is a variable so it's hard to say


The AFDs come into play less in a better installation

 

[Mikegh]

Loop impedances and insulation resistances would tell you a lot

I'm not sure if they do all that in the UK on a board change

 

[brianmoooore]

The number of dangerous situations prevented by a RCD in an installation will outnumber the amount of dangerous situations prevented by an AFCI by several orders of magnitude, so if the choice was one or the other, the RCD wins out every time.
That doesn't eliminate the rare event when a AFCI does save the situation, of course.

 

[Mikegh]

The number of dangerous situations prevented by a RCD in an installation will outnumber the amount of dangerous situations prevented by an AFCI by several orders of magnitude, so if the choice was one or the other, the RCD wins out every time.
That doesn't eliminate the rare event when a AFCI does save the situation, of course.

The RCD

Electricians best friend after the Voltstick

I've got more handy money from RCD trips than anything else

Can't depend on either of them but wouldn't be without them

 

[Sparkswillfly]

Just thought I'd provide an update, I have since upgraded the switchboard with the full AFDD detection.(buy once, cry once, if you're going to the trouble of upgrading the board, may as well go the whole hog) Only real issue was the lights tripped the RCD straightaway, something about a neutral removed from one circuit, I didn't really understand that part, and it was quickly fixed.

The day after installation, I've gone and identified which outlets etc on which circuits as its a useful thing to know if you ever have to isolate just one circuit. The interesting thing is, some items are now paired differently than on the old board eg in some cases, outlets that were previously on the same circuit are now on separate circuits, and those which were previously on separate circuits now are on the same circuit. The washer and dryer now share a circuit with two bedroom outlets whereas those 2 bedroom ones were previously separate from the laundry outlets. The lights were all previously on one breaker, now they're split between 2 AFDDs

Is it common for the circuit 'allocation' to be changed when getting a new switchboard? Do I need to be concerned and do I need to call the electrician back?

 

[Electrics]

Am i right in thinking these AFDD devices are only going to come in handy at the CU end where a wire isn't properly terminated?

 

[telectrix]

Just thought I'd provide an update, I have since upgraded the switchboard with the full AFDD detection.(buy once, cry once, if you're going to the trouble of upgrading the board, may as well go the whole hog) Only real issue was the lights tripped the RCD straightaway, something about a neutral removed from one circuit, I didn't really understand that part, and it was quickly fixed.

The day after installation, I've gone and identified which outlets etc on which circuits as its a useful thing to know if you ever have to isolate just one circuit. The interesting thing is, some items are now paired differently than on the old board eg in some cases, outlets that were previously on the same circuit are now on separate circuits, and those which were previously on separate circuits now are on the same circuit. The washer and dryer now share a circuit with two bedroom outlets whereas those 2 bedroom ones were previously separate from the laundry outlets. The lights were all previously on one breaker, now they're split between 2 AFDDs

Is it common for the circuit 'allocation' to be changed when getting a new switchboard? Do I need to be concerned and do I need to call the electrician back?

the electrician may well have rearranged the circuits in order to prevent a nuisance trip from taking out all circuits in one area,and to spread the loadsmore evenly across the MCBs etc. I'd not worry.

 

[Mikegh]

Am i right in thinking these AFDD devices are only going to come in handy at the CU end where a wire isn't properly terminated?

They should detect arc faults anywhere on the circuit afaik

 

[littlespark]

They should detect arc faults anywhere on the circuit afaik

except in an rfc. if one leg is still connected, and the other is arcing.

 

[pc1966]

except in an rfc. if one leg is still connected, and the other is arcing.

Generally you won't see an arc as the voltage between the broken end points in an open RFC is typically only a couple of volts max. The still should trip if it is a series arc on an appliance cable though (if big enough, as John Ward showed they don't really do small arcs well).

But it does point out that they don't stop an overloaded cable getting hot and even starting a fire, just so long as no arcing is involved...

 

[freddo]

What brand of consumer unit was used? Never seen an AFDD in real life.

 

[Sparkswillfly]

What brand of consumer unit was used? Never seen an AFDD in real life.

Schneider electric