Cat5 - Shielded or not

[tony sayer]

In article , Roy Brophy scribeth thus

If I were doing this I'd just use ordinary CAT5 cable for that length. OK I know the "regs" have their views on CAT 5 being next to mains cables but unless the sheathing on your SWA isn't earthed;-? It won't be electrically any different to a water pipe in effect!.

It will be fine. We've seen and used this in many adverse applications and really the only problems we have had are in radio field strengths at levels you just will not get in your back yard!.

Fibre is very good stuff and we have used that over distances of around

1 KM or so and very good it is too but for this application its really overkill.

I wouldn't worry about lightning either, we use CAT 5 on some very elevated structures and have yet to have a problem..

And you won't have any of the problems that might beset wireless outside in range of other peoples wireless networks....

[mick]

At 15m and if trenching allowed, I'd just bury some hosepipe a minimum of

12" (preferably 24") away from the armoured & thread cat6 through it. The pipe lets you pull another through if you need to (include a draw wire!). You should be able to use gigabit ethernet over that - something that wifi can only dream about. It's simpler and cheaper than fibre. OTOH, if you can't allow that sort of spacing, then fibre is really the only answer.

[tony sayer]

Why would you need to be 2 foot away from a cable thats very effectively shielded?.

CAT 5 is a very good balanced transmission system and is inherently good at rejecting other electric fields, so why so far away?..

[Tim Watts]

On Friday 07 June 2013 20:59 mick wrote in uk.d-i-y:

Cat5e is sufficient for gig (deja-vu I detect...)

[RzB]

Hmmm... another option is this...

Is there any reason why that wouldn't work down the SWA and through the CU in the studio?

Roy

[Dave Liquorice]

Both plugin so you have something like:

House ring > MCB/RCD > CU > RCD > SWA > CU > MCB/RCD > Studio ring.

It's the MCB/RCDs that contain series coils that will mess things up. These things work best on the same ring, noticable degredation across rings on the same CU. It might work but don't expect 200 Mbps... Not that you get that through put anyway, that's the raw rate and possibly both ways added together. I notice that the ethernet side is

100 Mbps max so what is the point of a 200 Mbps link? Marketing pah!

Not to mention the amount of RF crap the spray all over the Shortwave Bands.

[The Natural Philosopher]

You can run two mains plug setups and bridge them with an ethernet cable somewhere.

But running cat 5 aint that hard. remember if you use ethernet switches, you can run pretty long cables, or even fibre.

[John Rumm]

I do something similar to get data to my workshop. Throughput is not great, but ok for browsing the web or accessing a few files remotely.

I found I needed to use some "hi power" homeplugs to achieve the link reliably.

(mine goes through three CUs, three RCDs, one fuse and two MCBs)

[tony sayer]

And elsewhere fM and DAB too!. Why ever does the OP not just pull a lump of CAT 5 cable, after all his mains cable is SWA thats very well shielded agin egress and ingress. Seems a steam hammer to crack a nut;!...

[RJH]

I'm running some cat6 alongside mains cable in places - obviously, it's the easiest route to follow through a house. I did read that it's not advisable, but thought I'd give it a try and so far, so good. Do you think that ethernet-next-to-mains-equals-disaster is an urban myth?

[The Natural Philosopher]

no. Its just rather poor practice, because potentially under very unusual circs. you might get a shock inserting your todger into a cat 5 socket.

[John Rumm]

The main complaint is from radio hams, since they can stick lots of interference into the HF band, and mains wiring is not shielded or balanced, so it does not prevent the wire broadcasting to the neighbours.

[mick]

It's no urban myth. Although the signal down the network cable is balanced, that balance can never be perfect. Interference from the mains cable induces a 50Hz hum in the cat5 - which it can handle most of the time. The problems tend to arise when you get spikes on the mains - sometimes the result of dimmers etc. Those aren't always filtered out completely. As the network is self-correcting it will keep asking for packets to be repeated if they are corrupted. The overall result is that the network slows down.

The more current is on the mains supply, and the worse the loads on it, the more noise it will radiate. Consequently you'll find that in distribution systems in buildings any data cables will be spaced well away from power cables.

The risk from shock on the network system is as much as grabbing hold of the (insulated) mains cable. :)

[mick]

By the way, SWA isn't a very good screen at networking frequencies. It's better not to depend on it.

As for cat5 being balanced, look at it this way: both wires in the pair are being moved up and down in voltage at 50Hz by mains hum. Data is carried in the difference between the voltage on the pairs. At some point the 1s and 0s become 1s and 1s, as the hum voltage overcomes the differential voltage at the top and bottom ends of the 50Hz waveform. That's when your network dies.

You can get screened cat5, which helps with the hum problem a lot, but it also slows down the network due to capacitance to earth. Don't use it on long runs. Better to avoid running close to mains wiring for more than about 3m.

[Dave Liquorice]

Does seem very OTT, in offices the network cabling is simply in a seperate compartment of the dado trunking couple of inches at most.

In the normal home interference shouldn't be a problem. If you or a neighbour are into arc welding and have a BFO welder that might be. I'd keep the network cable away from mains by a couple of inches and if sharing a hole run one or the other through a bit of plastic conduit. That's mainly because the insulation on Cat5 isn't rated for mains voltages but it's unlikely to be a problem in the real world.

[Dave Liquorice]

Agreed.

How? Show the maths.

[mick]

I'll stick with peak-peak voltages. RMS would only confuse things.

The differential voltage between the cores is 2.2 to 2.8v. Say 2.5v If the induced voltage is, say, 6v then both cores move up and down by 3v relative to 0v. So, the maximum voltage on one core is 1.25+3=4.75v. At the same moment the voltage on the other core will be -1.25-3=-4.75

Those conditions are fine if the receiver can accept 5v to earth on both inputs.

As the hum voltage is increased to, say, 18v (which is quite possible with capacitive coupling) you would get +/- 19.25v to earth, even though the differential voltage is still 2.5v.

That would still be ok for a +/-20v input, but now add 5v symmetrical noise spikes on top of that. The signals are now +/-24.25v, which would (hopefully) get clamped by the protective devices. You are now getting 1s and 1s though, as 24.25-1.25=23v (greater than 20v). There isn't enough differential swing to get 0s.

A 5v spike on top of a 20v waveform wouldn't be unusual if the cat5 is close (say 10-20mm) to mains wiring.

[tony sayer]

The problem manifests itself up at VHF on FM and DAB transmissions to and it does have quite an effect if your received signals aren't that strong...

I'm surprised that Ofcom allow the things but then again .. Ofcom;?..

Chapter and verse from BBC research..

[tony sayer]

Have you ever tried this in practice?..

We have, and the only place where we had a problem, now fixed, was running some CAT 5 right next (within 4 foot) on a radio mast to some transmitting dipoles radiating some 100 watts in the FM band!..

That caused a problem for some microwave link units connected to the CAT

5 cable but we now have replaced that with Ubiquity screened cable which BTW is excellent mechanical stuff as well and that cured that completely.,

I rather doubt that any domestic situation would have such a problem somehow;!...

I know of other installations where power cables are right next to bundles of CAT 5 cables and all is well running at 100 and gigabit rates.

I think you'll find that balanced line working such as what CAT 5 is to be very immune to external interferer's...

In fact some 10 years ago I put a SWA cable into my shed some 100 feet away from the house chucked in the same trench as the SWA were a couple of bits of CAT 5 cable (temporary use of course!). One was in use for the phone the other a PC point. Never a moments trouble even when using arc welders and the like;!...

[mick]

I think the problem only really appears where the hum level is quite high and there is liable to be mains noise on top of that. Quite typical in, say, engineering works where there is a relatively high load and plenty of noise from SMPS and motor speed controllers. It becomes very relevant where the bandwidth of the network is important as speed is definitely compromised. Nowadays it's almost as cheap to go fibre instead of cat5e on any long lengths between buildings anyway.

Of course, you don't see the network slowing down unless you monitor it over a period of time. Bursts of noise slow it down, but it speeds up again between the bursts!

As you say, in domestic applications it probably isn't so important. In the OP's case I'd be tempted to bury cat5e or cat6 in hosepipe as I said earlier. The distance isn't great, the pipe gives some protection and also a little bit of spacing from the SWA. The more spacing, the more consistent the network speed will be. Best results would probably be with utp at about 1' away. Perfectly acceptable results would probably be with ftp at a couple of inches away, but the cable is more expensive.