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.