Switches on twin and earth?

Discussion in 'C-Bus Wired Hardware' started by Mike Costello, Mar 12, 2019.

  1. Mike Costello

    Mike Costello

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    Hi all,

    I've installed CBus for years but it has always been in new builds or refurbs using CBus pink Cat 5 cable for all switches, sensors etc but have been asked today about replacing a Carlo Gavazzi system with CBus however the system is star wired using standard twin and earth to the switches from where the dimmers/relays will be.

    Has anyone any experience of this and will it work or should I just steer clear without insisting on a rewire which it sounds like the customer won't do.

    Thanks in advance
    Mike
     
    Mike Costello, Mar 12, 2019
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  2. Mike Costello

    jboer

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    I wouldn't try to run C-Bus over T+E, without knowing what rules you have over there I think the options would be either making the switches DALI and bringing it into a C-Bus ecosystem, or using a heap of Aux inputs and just having dry contacts. However you wouldn't get many indicator options with that.
     
    jboer, Mar 13, 2019
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  3. Mike Costello

    Mike Costello

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    Thanks for the reply.

    I'm not doubting you at all but the electrician is asking me what is the reason it might not work as is it not just still copper transferring data? Really sorry if this is a stupid question but I can sort of see what he means as although we use 2 twisted pairs for CBus I presume this is just due to how thin a Cat5 strand is so if there's twin and earth this will be thicker than a twisted pair anyway.

    Sorry again if it's a stupid question!!

    Thanks
    Mike
     
    Mike Costello, Mar 13, 2019
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  4. Mike Costello

    Ashley

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    Resistance is only one of a number of properties of any cable (and to be fair, generally the only one of concern for electricians). Cbus cable carries dc power with a high frequency data signal superimposed on it, so things like capacitance, characteristic impedance and noise immunity come into play. Cat5 cable is manufactured to strict specifications and the signal is designed to expect these conditions. Changing any of these properties will certainly compromise the performance. When an ac signal hits the end of a wire a certain amount is reflected back along the cable. The amount is determined by the characteristic impedance. These signals add to the existing signal and if large enough can corrupt the data. Also, cat5 cable is accurately twisted. This improves noise immunity as both sides of the signal are exposed to the same interference (that's a bit of a simplification, but good enough for us!). Even using cat5 cable, it must be segregated from mains wiring by at least 100mm and cross at right angles. Using existing t&e you will have no control over segregation.

    Will it work? Maybe. Cbus is very resilient to interference. Would I do it? No way. :)

    You are going to be held responsible for the result.

    You could also as your electrician why he doesn't use t&e for his computer network cables? Or why different cables exist in the first place.
     
    Ashley, Mar 13, 2019
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  5. Mike Costello

    znelbok

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    There was a reply many years ago that was C-Bus is robust and will run on a rusty bit of fence wire or wet string (do the search and see how many times fence wire comes up..).

    C-Bus topology is very flexible and the star wired infrastructure you have is fine for C-Bus.

    Will it work - IMO yes, but I would only do it for my own place because it is not as per the design of C-Bus and I would take on the hassle of fixing it when there were problems - would I do it for a customer - not unless others had proven it to be reliable before me

    As for an answer for the electrician - C-Bus is a data network and needs a twisted pair for comms (just like an Ethernet network does). Your electrician is showing his lack of understanding of data cabling, noise rejection, twisted pairs etc.

    Options
    Wireless switches and a bridge (if its still around)
    Aux input modules with standard switches (not cheap)
    Re-wire as you have indicated is the best option.

    Cheers
     
    znelbok, Mar 15, 2019
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    Dave_F likes this.
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