topic Re: Install question: Run CSP-ONT fibre under my floor? in Full Fibre

Install question: Run CSP-ONT fibre under my floor?

markv — Mon, 04 May 2026 14:42:30 GMT

Currently, the elderly copper phone cable comes through a hole in the front wall of the house.
Then down through a small hole in the floor, where the unprotected cable runs 4-5 metres, emerging back up into a cupboard where the master socket is located.
That cupboard is the nexus of my network (incoming cables from MESH extenders, Local DNS Raspberry Pi. mains).
Keen our compulsory FTTP upgrade follows the same route and installs the ONT in the cupboard where the existing Master Socket is.

I don't expect the OR engineer to crawl about under the floor space, but if I get down there, is it feasible to get the engineer to feed the CSP-ONT cable down the hole to me, pause while I worm under the joists and poke it back up, where he can replace the master socket with the ONT?

Does the CSP-ONT fibre connector start life with plug-in connectors at both ends, or does the engineer splice bare fibre into the CSP?
Might my sub-floor routing contaminate the end of the fibre and render it useless?
Would he insist the under-floor run is completely ducted, from down-hole to up-hole (tricky).

There are other possible ways of doing it, but none are anything like as neat.

TIA

Mark (on behalf of my Plusnet-customer SO)

Re: Install question: Run CSP-ONT fibre under my floor?

Baldrick1 — Mon, 04 May 2026 15:01:43 GMT

@markv

The ONT and router can be up to 100m apart if interconnected with CAT5e or better Ethernet cable. By far the easiest solution, that takes all negotiation with the installer out of the argument is to have the ONT installed next to a mains supply on an outer wall, and install your own Ethernet cable to the desired Router location. You can do this prior to the fibre installation so that it's all ready to go.

If you don't have a mains socket available in a convenient location then have an extension lead available for the ONT. Once the installer has gone you can swap this out by using POE on the WAN cable. I have suucessfully run an ONT over CAT5e cable using a TP-Link TL-POE150S Injector and POE 10R splitter

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Mon, 04 May 2026 15:20:20 GMT

Hi @Baldrick1

Thanks for your prompt response. I'm aware of the alternative installation options. Your suggestion is one option, but not preferred.
As per my OP, I'm asking about the practicalities of running ~5 metres of fibre from the new ONT, under my floor, through the dirty, dusty footings, up and out through the wall to the CSP.

I know almost nothing about the physical aspects of fibre connections.

e.g. does the CSP<-> ONT fibre 'cable' have plug-in connectors at each end, or does the engineer attach the connectors (otherwise, how would he get a fibre with an attached connector, through a hole in the wall that is only big enough for a fibre?

Mark

Re: Install question: Run CSP-ONT fibre under my floor?

bmc — Mon, 04 May 2026 15:22:19 GMT

@markv

If you're willing to take a chance on the ONT being where the copper cable comes into the house then you could pre install duct under the floorboards with a draw string in place. If you need to use ethernet cable with plugs then you would need to ensure both holes and duct are wide enough to pull cable through.

You might, just might, get the installer to run the fibre cabke thought the duct for the ONT at your desired location but you would have ethernet as your back up.

The "house" cable is spliced at the CSP - which funnily enough is called a Customer Splice Point.

Brian

Edit : View https://www.openreach.com/help-and-support/full-fibre-broadband-installation-checklist#accordion-b43490109a-item-56113228ba

Re: Install question: Run CSP-ONT fibre under my floor?

MisterW — Mon, 04 May 2026 15:29:30 GMT

@markv

I know almost nothing about the physical aspects of fibre connections.

The cable has a plug connector pre-fitted at the ONT end. As @bmc says, the other end is spliced at the CSP

The cables come in pre-cut lengths up to 10m. Any excess length is coiled in the CSP

Re: Install question: Run CSP-ONT fibre under my floor?

Dan_the_Van — Mon, 04 May 2026 15:34:50 GMT

@markv

This kit might be worth considering https://shop.premitel.uk/product/fibre-termination-point-relocation-kit/

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Mon, 04 May 2026 16:27:34 GMT

Some really useful info coming in; many thanks.

So in practice, the fibre cable would start at the ONT end (ideally in my network cupboard). Down, under the floor, up and out through the wall where the engineer wipes it clean and splices in the CSP.

I could prepare a duct/pull-cord in advance to facilitate things.

Can a piece of string be attached to a fibre cable for pulling?
AI suggests I will need something called a 'Pulling grip', but more reading says that is only used when pulling fibre for hundreds of metres.

To repeat a moan I have read here hundreds of times... why is it so complicated/difficult?

Re: Install question: Run CSP-ONT fibre under my floor?

bmc — Mon, 04 May 2026 18:34:12 GMT

@markv

To be clear you only find out on the day as to what the installer considers acceptable in terms of fibre route and CSP / ONT location.

Given the distance string may well work. If not, it could be used to pull something more suitable. Note that if using the existing holes in the floor then you would need to sacrifice your copper connection so the the FTTP fails for some reason you'd be left with any service.

Brian

Re: Install question: Run CSP-ONT fibre under my floor?

Baldrick1 — Tue, 05 May 2026 07:26:17 GMT

@markv

There is a minimum bending radius on fibre and I doubt that an installer would be happy for it to be pulled through floorboards and along an unsupported route as you presumably plan.. However, as has been repeatedly said, it depends on the installer on the day.

In most cases, it is considered complicated/difficult as people don't accept that this is a new connection to the property and it may not be practicable to install the ONT in the same location as the original BT Master socket, which in many instances was installed during the house build.

The bottom line is that you leave it to the day and try your luck and/or hedge your bets and install your own Ethernet cable from a convenient location from the installer’s viewpoint to your ideal location as a back up. Even if not used, for the WAN, this could be useful in the future for providing a backhaul for a mesh satellite.

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Tue, 05 May 2026 09:12:59 GMT

Coming to the conclusion that running the fibre below the floor is not feasible unless I pre-install trunking fixed to the joists, and a draw-string (far friom easy). Even then, possible bend issues going through the floor.
Will have a harder think about Plan B and Plan C.

Many thanks for painting a clearer picture.

Re: Install question: Run CSP-ONT fibre under my floor?

RobPN — Tue, 05 May 2026 15:07:09 GMT

@markv

No need to actually fix trunking to joists if you get flexible trunking such as this (other sizes, suppliers available). Just run it under the flooring from entry point to exit point but make sure each end is fixed so it cannot fall away into the underfloor space. Best to feed the draw rope through the trunking before installing. That way will ensure there are no tight bends within the trunking, and no excessive force will be used pulling the fibre as it slides through very easily.

My own installation (though different to today's methods, before CBTs were used) involved the external fibre cable being pulled through a 17.5m run of flexible metallic conduit starting from under the eaves above an upstairs window, through the loft, down a chased out wall, under floorboards then finally terminating at the indoor CSP in a meter cupboard on the ground floor. Then the internal fibre goes through aproximately 2-3m of plastic flexible conduit (similar to what I linked to above) through the floor/ceiling space to the room above where the ONT is fitted.

That was a 2-stage installation, first stage by MJ Quinn, second stage by Openreach, both saying words to the effect that it was one of their easiest jobs.

So IMO, if you prepare properly and make the fitters job easy rather than harder, I think it would be unreasonable for them to be uncooperative.

Good luck!

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Tue, 05 May 2026 17:24:35 GMT

@RobPN That's a neat idea, TVM.

Would need to find a way of fixing the flexible trunking to the underside of the floorboard, but I'm sure Wickes/Screwfix will have something suitable.

That does revive Plan A, so thanks again 🙂

I'm very much onboard with making peoples lives easier - it always pays dividends.

Re: Install question: Run CSP-ONT fibre under my floor?

TimSmall — Fri, 08 May 2026 09:34:16 GMT

For new housing developments, Openreach usually get the house builder to run the internal fibre, and then they come along and just put the CSP on the wall outside, fusion-splice the external fibre to the internal fibre, check the optical path quality and then install the ONT.

Look for the Openreach document "Fibre Developer Handbook" / "How to build a fibre network". In the March 2026 edition, the pages you need are on page 26 to 38 (not all relevant). Particularly relevant is page 27 "Openreach ONT in non-standard position".

I know this isn't intended for householders and existing properties, but if anything new builds are held to a higher standard, so a good one to have in your back pocket.

If you want to make things as easy as possible for the installer, then as well as installing a pre-run duct, you could buy the indoor fibre cable in advance (about £15) and run it to the location where you expect they will be happy to drill through the wall (i.e. what the house builder would normally do).

Obviously there isn't a 100% guarantee that the engineer will agree to use it, but most will - particularly if you make it clear that you've followed the requirements in the Openreach document.

Two different brand options that Openreach use for internal cable are:

EZBend - this is white and allows for a tight bend radius (15mm? - check the individual cable specs). Below floor would be housed inside some Flexicon (or similar) polypropylene flexi conduit (£10 for 10 metres). If you want something even tougher, then go for some "LoFlex" type flexible steel conduit instead (a bit like the outer metal part of a shower hose).

Dexgreen "Inside/Outside" - similar to the EZBend but also has a tough black outer protective sheath. You can remove the black sheath for "indoor" parts of the run - which then reveals a 3mm white inner sheath (almost identical to EZBend) which could be run in visible areas if the black colour is not wanted, and where you don't need extra protection like tops of skirting boards. I think I'd be tempted to use this instead, and skip the conduit, although this wouldn't be allowed on new build, but up to you!

You can buy both readily online in e.g. 5m 9m 10m 20m lengths. Get the type with an SC/APC connector at one end ("single ended"). Should cost £10 to £20 depending on length and type.

Yet another option would be to run your internal fibre using a cable that is pre-terminated at both ends (double ended). Then you have a backup plan. If the engineer is happy to use it, then they'd cut off one end, and splice it just like they would do with the "standard" single ended cable. If they don't want to play ball then they just install the ONT , and then you use to move it later - either with the wall-mounted SC/APC coupler linked above, or a much smaller £1 SC/APC inline coupler to extend the run. A bit of white heat shrink round one of those looks fairly unobtrusive.
Let us know how you get on!

Re: Install question: Run CSP-ONT fibre under my floor?

TimSmall — Fri, 08 May 2026 09:56:42 GMT

"need to find a way of fixing the flexible trunking to the underside of the floorboard".

I would use screw-in cable tie mounts. They're called various things - e.g. "Cable Tie Saddle Mount" "Cable tie bases" etc. Maybe get the self-adhesive type which also accept a screw (but also screw then in). That way you can use the adhesive to position the bases, and then also screw them in (if you use the non-adhesive types, then otherwise it's easy to drop the bases whilst trying to get the screws in - particularly somewhere awkward like a crawl space). Use reasonably generous width cable ties.

Re: Install question: Run CSP-ONT fibre under my floor?

bmc — Fri, 08 May 2026 10:17:33 GMT

This is from my FTTP installation in late 2017.

Black external sheath was stripped back once indoors and then it was stapled round the skirting board.

Brian

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Fri, 08 May 2026 13:28:42 GMT

@TimSmall Brilliant suggestion(s) - TVVM. Have ordered 10m single-ended Dexgreen.
Undecided if I'll run in 'bare' under the floor (it looks tough as old boots), or inside flexible conduit.

Have found that Fibre Developer Handbook and will read and digest the appropriate sections (and have prominently to hand when the OR engineer visits 😉

Would there be any advantage/disadvantage (except cost) of getting an ONT off eBay too, and pre-installing that. So OR just need to install the CSP and splice in my Dexgreen fibre.

Nokia and Adtran seem to be the choices.

Pehaps the engineer would see that as a step too far...?

Re: Install question: Run CSP-ONT fibre under my floor?

TimSmall — Fri, 08 May 2026 14:29:29 GMT

I think I'd leave it to the engineer to supply and install the ONT.

Re: Install question: Run CSP-ONT fibre under my floor?

MisterW — Fri, 08 May 2026 14:42:11 GMT

The ONT is 'keyed' to the connection. On installation the engineer has to register the serial no against the connection. I'd be pretty sure that registering a non-Openreach supplied ONT wouldnt be possible.

Re: Install question: Run CSP-ONT fibre under my floor?

markv — Fri, 08 May 2026 15:00:18 GMT

Very useful to know, thanks. Makes sense. In the copper world, i guess a line was idenified by the phone number. You plug in you ISP-supplied FTTP rouuter and it Just Works (no credentials required) because they know it is YOU.

So the OTN serial no does the same job in the fibre world.

There must be a lot of dissapointed eBay buyers!

Re: Install question: Run CSP-ONT fibre under my floor?

bmc — Fri, 08 May 2026 15:03:06 GMT

@markv

The ONT's belong to OpenReach and should be left in situ if you leave the property. Any second hand ones on sale are technically illegal as the seller does not own the ONT.

Brian