Why does AFDD protection need to be per circuit?

[spud1]

Hi again!

Why does AFDD protection where provided need to be installed per circuit instead one AFDD covering several circuits, like for example RCDs in domestic split load CUs?

Is there a technical reason or is it another manufacturers money making myth!?

Thanks.

 

[SparkyChick]

Probably for the same reason RCBOs are preferable to split RCD boards... we should be looking to minimise disruption in the event of a fault. Plus, it makes fault finding a hell of a lot easier if you know which circuit you're looking at to begin with.

 

[spud1]

Probably for the same reason RCBOs are preferable to split RCD boards... we should be looking to minimise disruption in the event of a fault. Plus, it makes fault finding a hell of a lot easier if you know which circuit you're looking at to begin with.

True and I absolutely agree with maximising selectivity for minimising inconvenience during a fault and facilitating easy fault finding, but I was under the impression that AFDD protection HAD to be per circuit?

 

[SparkyChick]

From the regs:-

532.6 Arc fault detection devices (AFDDs)
Where specified, arc fault detection devices shall be installed:
(i) at the origin of the final circuits to be protected, and
(ii) in AC single-phase circuits not exceeding 230 V.

 

[SparkyChick]

And.....

421.1.7 Arc fault detection devices conforming to BS EN 62606 are recommended as a means of providing additional protection against fire caused by arc faults in AC final circuits.

If used, an AFDD shall be placed at the origin of the circuit to be protected.

NOTE: Examples of where such devices can be used include:
• premises with sleeping accommodation
• locations with a risk of fire due to the nature of processed or stored materials, i.e. BE2 locations (e.g. barns
woodworking shops, stores of combustible materials)
• locations with combustible constructional materials, i.e. CA2 locations (e.g. wooden buildings)
• fire propagating structures, i.e. CB2 locations
• locations with endangering of irreplaceable goods.

 

[spud1]

And.....

421.1.7 Arc fault detection devices conforming to BS EN 62606 are recommended as a means of providing additional protection against fire caused by arc faults in AC final circuits.

If used, an AFDD shall be placed at the origin of the circuit to be protected.

NOTE: Examples of where such devices can be used include:
• premises with sleeping accommodation
• locations with a risk of fire due to the nature of processed or stored materials, i.e. BE2 locations (e.g. barns
woodworking shops, stores of combustible materials)
• locations with combustible constructional materials, i.e. CA2 locations (e.g. wooden buildings)
• fire propagating structures, i.e. CB2 locations
• locations with endangering of irreplaceable goods.

Thanks for this SparkyChick, So its confirmed then, the regs definately want us to install per circuit, but I just wonder why? Is it purely to do with Selectivity or is there a more practical/technical reason for it?
If it was to do with Selectivity surely the same argument could be made for earth fault (RCD) protection and split load/dual RCD boards would become a thing of the past.

 

[Pete999]

From the regs:-

532.6 Arc fault detection devices (AFDDs)
Where specified, arc fault detection devices shall be installed:
(i) at the origin of the final circuits to be protected, and
(ii) in AC single-phase circuits not exceeding 230 V.

What SC said

 

[AndyL]

RCD's are also at the "origin of the circuit" I don't take that as necessarily individual to each circuit. It may be more to do with the operation of the device itself, but cannot see why split load AFDD's would not work

 

[spud1]

Very good point !

RCD's are also at the "origin of the circuit" I don't take that as necessarily individual to each circuit. It may be more to do with the operation of the device itself, but cannot see why split load AFDD's would not work

Very good point. At the origin of the circuit is positioning, not Selectivity Light bulb moment!

 

[Nebu1a]

So you would not protect the sub main circuit to a barn but would protect the individual final circuits from the barns consumer unit??

 

[spud1]

So you would not protect the sub main circuit to a barn but would protect the individual final circuits from the barns consumer unit??

Sorry don't understand?

 

[Nebu1a]

Are only final circuits to be protected and not sub main/ distribution circuits?
Difficult to compare AFDDs with RCDs as a sub main circuit usually has additional protection (SWA) so may not need RCD protection.
Surely an Arc can occur on a sub main circuit as well as a final circuit.

 

[Darkwood]

It is possible that multiple circuits may have too much noise on them, the unit has to decide what is an arcing fault and what isn't, take your vacuum motor or fridge, it needs to know the arcing on the brushes is not a fault so I expect the more it is looking at the harder it is and possibly nuisance tripping thus follows.
This is my theory anyway, don't take it as gospel but it would make sense.

 

[Nebu1a]

Agreed

 

[Grant1987]

It is possible that multiple circuits may have too much noise on them, the unit has to decide what is an arcing fault and what isn't, take your vacuum motor or fridge, it needs to know the arcing on the brushes is not a fault so I expect the more it is looking at the harder it is and possibly nuisance tripping thus follows.
This is my theory anyway, don't take it as gospel but it would make sense.

Just reading all of this topic I’ve been installing new CCU’s since 18th came into force and noticed couple of changes one being the new rcds have an earth fault indicator on the rcds another being the Rcd trip times on the Hager fuse boards are rather high although complient typical values are x1 89.5 and x5 40.0 these readings are far higher than the old Hager fuse boards which i typically would get x1 19.8 x5 10.5 these are just averages but changed dramatically since the new fuse boards came out