Where to position AFDD's in HMO subs

[Jimbo6871]

Hi all,
Looking a bit of input.
First HMO install since AFDD's became mandatory in HMO's.

Install details: Main Board: 18 way TP&N with overall SPD feeding out via 20A RCBO's through individual MID meters to 14 rooms, CU in each room feeding 1x 16A radial to 1 twin 13a socket and 1x 6a for light circuit.

So the question is do I use combined AFDD/RCBO's at the main DB (giving AFDD cover to everything down stream - cable, CU, light and socket) OR just RCBO's at the main board and use a combined AFDD/RCBO on the radial to the socket in the CU located in each room?

I would have thought that the further upstream the AFDD is the more protection it is giving but the regs require AFDD only on the socket circut in this setup if my interpretation is correct.

Go easy, first post here!
Thanks in advance

 

[Phoenix]

Why are you providing RCD protection to the sub mains? If they trip, not only do you loose everything, but if they are in the main board, someone now has to find the building maintenance chap (or call him in if out of hours) to unlock the plant room to reset. Add AFDD protection in and you are making it more likely!

MCBs at the main board, at least type C, if Zs in spec for it and there are no reduced CSA cpcs on the subs, might even spec it with Ds- submains installed in such a way they don't need RCD protection. (you'll never get complete discrimination between MCBs, but you might be able to get partial discrimination at the low end where the fault current is only a couple of hundred amps or so)

Combined AFDD+RCBO device on the sockets, and a just a plain RCBO to the lights

 

[Jimbo6871]

Why are you providing RCD protection to the sub mains? If they trip, not only do you loose everything, but if they are in the main board, someone now has to find the building maintenance chap (or call him in if out of hours) to unlock the plant room to reset. Add AFDD protection in and you are making it more likely!

MCBs at the main board, at least type C, if Zs in spec for it and there are no reduced CSA cpcs on the subs, might even spec it with Ds- submains installed in such a way they don't need RCD protection. (you'll never get complete discrimination between MCBs, but you might be able to get partial discrimination at the low end where the fault current is only a couple of hundred amps or so)

Combined AFDD+RCBO device on the sockets, and a just a plain RCBO to the lights

That is a fair point and not dissimilar to what my original spec was..... howeverrrrrr...... Landlord has all the sub lines first fixed in 6mm T&E, ground floor in metal trunking, 1st floor clipped through roofspace.
(I spec'd all subs in SWA - LL didn't like the price, went to Bodge-it & Leg-it who have since been told to clear off - another horror story for another day).

So, as far as I can make out since the drops to the subs are T&E, they need RCD protection at the main DB (at least for the first floor), although I'll happily stand corrected if it can be done with MCB's as discrimination is non existent with two 30ma trips in series, so yes a recipe for a lot of nuisance tripping call outs and general tenant unhappiness - the LL has already been warned.

 

[UNG]

So the landlord didn't like your price got someone in to bodge and now you are picking up the pieces because the landlord ditch the bodger
So I would be looking to install as your original design if it costs the landlord more that's tough on him and his poor decision making the alternative is you walk away and leave him to it IMO jobs like this are not worth tarnishing or losing your reputation on

 

[mort2376]

 

[sirsparkabit]

View attachment 14923

got to get one of these for the job I an on at the moment

 

[Fleeting]

Whether the cables need additional RCD protection would depend on the installation method.

 

[binky]

Whether the cables need additional RCD protection would depend on the installation method.

And to add to that, sub mains don't have to be RCD protected if I recall correctly.

 

[Fleeting]

And to add to that, sub mains don't have to be RCD protected if I recall correctly.

It may well do if the cable is installed in such a way that requires it but from what that have so far quoted it doesn't.

 

[Phoenix]

OP, what is the exact routing of the 6mm T/E submains? if its just clipped through the roof space and drops striaght into the top of the board, then I can't see any requirement for RCD protection, if it drops down the wall to the board at a lower level in the room, then as I'm assuming its still at first fix stage? then metal adapt box behind the board with a earth link to go into the earth bar, 25mm steel conduit drop bushed into the box with female bush on the void end for the submain to drop down, and a couple of plastic conduit risers for( RCD protected) final circuits.

Is the landlord co-signing the design (and probably installation) sections of EIC with you?....

 

[Murdoch]

Very surprised that you agreed to do the 2nd fix …..

 

[binky]

Very surprised that you agreed to do the 2nd fix …..

does sound ike the sort of customer you don't need

 

[SPECIAL LOCATION]

So, as far as I can make out since the drops to the subs are T&E, they need RCD protection at the main DB

There is no regulation that states that all T&E cables must have RCD protection...

In these situations you need to get back to basics and double check BS7671 guidance...
What are the reg's that you consider are applicable to this particular installation design?

 

[Jimbo6871]

Whether the cables need additional RCD protection would depend on the installation method.

That's a fair point to be sure.
The ones in the metal trunking I would say are okay without RCD - but then I didn't put them in and I found one nicked through to copper cable on visual inspection.
The cable runs up to the roofspace have been changed and I now have them surface mounted in metal trunking on the wall not in so that part is okay as well.
There is one section of cable though where they are run in a stud wall so falling foul of the 'less than 50mm depth rule' those will need rcd. As for the cables in the roofspace, they seem to have been clipped on top of the rafters above the ceiling and how they got the cables in there is anyones guess - a large mouse couldn't get through parts of it the space is that limited.

 

[Jimbo6871]

OP, what is the exact routing of the 6mm T/E submains? if its just clipped through the roof space and drops striaght into the top of the board, then I can't see any requirement for RCD protection, if it drops down the wall to the board at a lower level in the room, then as I'm assuming its still at first fix stage? then metal adapt box behind the board with a earth link to go into the earth bar, 25mm steel conduit drop bushed into the box with female bush on the void end for the submain to drop down, and a couple of plastic conduit risers for( RCD protected) final circuits.

Is the landlord co-signing the design (and probably installation) sections of EIC with you?....

That's a good point, the room drops could be pulled back and surface mounted in metal to the cu's without too much hassle I think.
First thing to check on next site visit.
Cheers J

 

[Phoenix]

If its surface mounted, it doesn't have to be in metal as its not concealed. Think of it as surface mounted, or protected by earthed metallic covering (suitable as a cpc), not and

 

[Jimbo6871]

So the landlord didn't like your price got someone in to bodge and now you are picking up the pieces because the landlord ditch the bodger
So I would be looking to install as your original design if it costs the landlord more that's tough on him and his poor decision making the alternative is you walk away and leave him to it IMO jobs like this are not worth tarnishing or losing your reputation on

Very surprised that you agreed to do the 2nd fix …..

Sadly just in too deep with this outfit.
Building my client book back up again ATM so I can walk. I stupidly let more and more other jobs slide by over the past 3 or 4 years as I've been working almost full time for this crowd for a good few years. It was all good when the going was good, whatever a job needed it got, but since money has got tighter they are always looking ways to cut costs (corners), so on occasion they have turned to 'Bodge-it and Leg-it non-electrical Unlimited' until a manager found them using 1.25 flex on a 40A radial - all out war ensued.

 

[Jimbo6871]

If its surface mounted, it doesn't have to be in metal as its not concealed. Think of it as surface mounted, or protected by earthed metallic covering (suitable as a cpc), not and

True, but in this particular building I wouldn't use pvc, (not that I'm saying it is a bad area or anything)

 

[Andy™]

(I spec'd all subs in SWA - LL didn't like the price, went to Bodge-it & Leg-it who have since been told to clear off - another horror story for another day).

in which case, start over to your spec as originally planned. and remind landlord that it'll now be extra because of the additional work of removing other peoples bodge

 

[binky]

There is one section of cable though where they are run in a stud wall so falling foul of the 'less than 50mm depth rule' those will need rcd.

I've always regarded that as meaning plastered into a wall rather than loose in a cavity. 'Buried' makes little sense otherwise.