why use cat5

I know why we use cat5 for networking etc, but i can't understand why we use it for c-bus networks.

My understanding is that it is a 'slow' network for reliability, so a cat3 spec cable should suffice. We dont even use cat certified connection in the network, more a cobbled together sort of thing

Standard Cat5 is no good because of the sheath rating

The clipsal cat5 is obviously no good as we need to double up the conductors (not very professional if you ask me)

So why dont we use a cable such as something used in RS485 networks or plain old instrumentation cable like Dekoron. Conductor sizes can be increased to suit the current rating required, It should be too hard to find something that has the correct sheath rating (I could be wrong on this). Shielding is available as well for thise noisy areas.

Using the RJ45 connectors would have to be done away with but there are plenty of connectors out there that would do the job, hell even screwing it to the unit will work, its not as if we dont do that already.

Ps anybody use any sort of rj45 distribution block, much like a hub

That was my thought for the week, time to switch off again

Mick

 

The requirenments for C-Bus cables are:
1) conductor cross-section sufficient to carry 2A without becoming a fire hazard.
2) at least 2 conductors which must be twisted to avoid picking up stray fields from nearby systems (esp. 50Hz ones).
3) maximum TOTAL installation capacitance: 100nF between conductors. This may rule out some shielded cables.
4) Balanced cable. If a shielded cable is used, it should have two conductors within the shield exhibiting the same capacitance to the shield on each conductor.
5) conductor resistance upper limit approximately 200 Ohms for longest point-point distance

None of these requirements specify cat.5 cables. In fact, the original C-Bus system was designed around 2 pair Cat.3 cables, using the 2 pairs in parallel to get the current rating. Note: 3 pair cable is required to use the remote over-rides on DIN and PRO units.

Don

 

Well me old China a browse through the Introduction to C-bus manual on the CIS web site would answer your question.

CAt 5 Un-shielded Twisted Pair Cable (UTP) has a couple of advantages one which leads from the other:

To allow a C-bus network to run for 1k and have up to 100 C-bus units connected the amont of Capacitace that the cable adda to the network is critical, too mutch and the data waveforms will get distorted. If shielded cable is used to eliminate electrical interference it increases the capacitance so the 20 - 40pF/meter of Cat 5 UTP is ideal and the tight twist provides excellent common mode protection against induced voltages. Cat 5 UTP is built to a standard and is both relativly cheap and widely available - sounds perfect to me! Cat 3 by the by is typically 60 - 80pF/meter and does not have twisted pairs.

The Pink sheath on the Clipsal C-bus Cat 5 is mains rated which is important where the cable has to enter a switch board where the DIN output Relays and Dimmers are mounted and single insulated mains cable can be expected to be.

The cross section of the copper in the C-bus Cat 5 is the same as in other Cat 5 cables and doubling it up (being sure to maintain the twist between the +ve and -ve conductors) is not onerous and limits volt drop across the network while also giving more copper for the terminal to contact.

In a perfect world there would be more copper but the advantages of CAt 5 UTP outweigh this.


Now you are correct that in theory any conductor will do but you may well run in to problems with maximum length of cable run, interference, dodgy comms etc. And let's face it if the manufacturer recommends it why would you do anything different it's not as if the cost of Cat 5 UTP would be significant in the overall cost of a project what with C-bus Product and Labour. The cost of getting it wrong by using other cable and having to go back and fix the ensuing problems are just not worth it.

 

so you have stated the cable requirements to me and it is correct, but my question still stand

In my example I mentioned instrument cable such as dekoron. It has a high twist for the same reasons as cat5 etc, the capacitance is at worst .3uF/km and the sheath rating is 0.6kV (this is sheilded too.)

Cost is 23c/m, so a little more expensive, but I believe a better cable for the job as the conductor size can be 0.5mm2 or 1.5mm2. Multipair versions are available.

It is one thing to go with the manufactures recomendations, but when they produce thier recomendation and no one else does, you have to ask sometimes are they doing the right thing by us.

Don't get me wrong I am not trying to suggest that clipsal are wrong, just discussing alternatives.

Mick

 

0.3uF per km will only allow you 333 metres before you hit the 100nF design maximum. The system probably will work at the higher capacitance, but is not specified for it, and you run the risk of distorted communication waveforms. Note that the 333metres is the TOTAL cable length for the installation, not the maximum distance between two points. Fine for a small installation.

Don

 

znelbok

I understand your discussion and motives. What is the sheath voltage rating of the suggested alternative?

 

see my last fpost for the example I gave.

I was not indicating that this was the ideal solution with that particular example, just that there were possible alternatives out there that were possible better suited for the job

 

Sticking with the shielded instrument cable i have a job with an existing automation system that is failing and want to replace it with cbus but the control wiring is a shielded 4 core instrument cable (looks untwisted) and i cant replace it. Will it work for cbus?

 

Often it will BUT there are a bunch of factors to be aware of, read the thread above in terms of how much current the cable needs to carry and the shielding could cause capacitance issues. In saying that I think most of us have put C-Bus over other cables than Cat5 from time to time, but really depends on distance, how many units ect ect.