4-Core Fibre

You may never come across 4-core fibre, but it can be useful, so here is the information in case you need it

Long Distance Spurs and 4-core fibre

There are some isolated spurs over a mile away from their nearest Core chamber. Fibre cannot easily be blown that distance, but you can have an intermediate chamber. The fibre is blown to this chamber and then on to the house from there. You end up with a small chamber of 1 ring which will contain loops of spare fibre and a small junction box about 4 inches square.

If you have 2 spurs, both a long way away from your nearest Core chamber but in the same general direction, then you can lay a single 7 mm duct to the first house and another 7 mm duct from there on to the second house. There is a 4-core fibre that is the same diameter as the usual 2-core. This is blown to the first house down one 7 mm duct - to a junction box on the wall, where it divides into the connection for the first house and a 2-core fibre is blown from there on to the second house.

We have used 4-core in an instance where 2 spurs went to roughly the same place and had a long and really difficult route. It was a lot easier to bury a single 7 mm duct rather than two.

4-core could also be used for a house that decides very late in the day that it wants 2 connections rather than the 1 you have ducted for.

Both of these installations feature a junction box which has an input of a 4-core fibre and two 2-core outputs.

(4-core is exactly the same size as 2-core and is blown in the same way as 2-core)

The left hand installation (picture from Jack Pickup) features a small box with a simple join. The larger black box on the right features a white plate (as contained in a bullet in a chamber) which contains spare fibre, and can be used for say a 12-fibre input and 6 2-fibre outputs

The 4-core to two 2-core splitter box (about 6X4 inches )

This appears to do the same as the black box

An intermediate house fibre blowing chamber. (ignore kneeler pad)

The junction box to protect the splice.

Example of 4-core use

4-core can be the answer. As you nearly always use 2-core it gives you the opportunity to increase capacity without having to start from scratch with digging duct in.

The man at Farm A has recently decided to try and sell his farm as three separate buildings and has applied for planning permission for 3 dwellings. What if he does get the go-ahead for this plan? We have given him only 1 spur – he wasn’t interested in a connection, he doesn’t own a computer (so discussion of how many connections to plan for didn’t arise).

If he only wanted one more connection, it would be simple enough to send 4-core fibre down the duct to his house.

But he wants 2 more. We can’t easily lay another duct all the way back to the chamber, it has a very complex route incorporating 3 other land ownerships. In this case we are lucky because there is one more spur passing to the last house of the bunch, at C.

They are live on B4RN. We have to put in a 1-ring chamber at B. Chop their duct and put 4-core into from the original chamber to the new chamber. That gives us 3 spare fibres for the farm. The new chamber contains the fibre splices that have to be made.

It takes a bit more planning to minimize the down-time for house C.

1. Chop the duct to Farm A at point B

2. Blow 4 core down this tube from the original chamber

3. Splice this at the original chamber

4. Chop the duct and fibre to house C. C has been disconnected

5. Splice C’s fibre to 2 of the 4-cores at chamber B, C should be live again. Maximum 30 minutes downtime .

6. Now you can remove the 2-core that used to supply C and blow 4-core along it.

7. After some more splicing at the original chamber all you have left is to get 2 more ducts the short distance from B to A, get fibre in them and splice to finish.

(It probably takes a bit more thought to decide exactly where to cut the duct to give the right amount of spare fibre)

Another Example of 4-core to the rescue

This is from our early days where we suffered from lack of experience.

We had planned to connect a set of 6 houses in area A from a chamber as a group and had laid most of the duct to them but not fibre. Later, another group of houses at C hit a problem – they had been intended to be part of the group at B, but the landowner didn’t want their track cut through. We re-routed them to the A group. Putting a new chamber in at D we could blow 4-core to there so that there was enough fibre at D to supply all 10 houses.

If we had known more about moling under tracks, then we could probably have got agreement to get under the track at E in the first place (we moled under the track at F eventually – owned by the same landowner)

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