Old C-Bus relays

If I am planning to replace some old non-DIN rail units with some units, should I buy powered replacements or passive ones?

As I understand it, the old C-Bus relays (non-DIN rail) did not supply C-Bus power, is that correct? If so, would I also be right to assume that whilst I could opt for the passive replacements it might not be a bad idea to use powered units and remove some of the old power supplies, given their age?

 

Correct. If the relay units are in either a cream or black box then they don't supply power to the C-Bus network.

There has been the occasional hassle with the old cream-box power supplies causing some comms problems as the power supplies get older, so replacing them is not a bad idea, especially in a commercial job.

You'll need to watch your power supply sizes if you replace any power supplies. The cream-box stand-alone power supplies are 320mA, the ones integrated into DIN dimmers/relays are 200mA and the stand-alone DIN power supplies are 350mA. You just need to remember to keep under the 2000mA limit for the network.

I personally prefer to see stand-alone power supplies on commercial jobs, simply because it makes maintenance easier. If a power supply goes down, then it's easy to just swap out the power supply. If a breaker trips on one of your relay units and it takes out all the power to that relay, then your network loses one of it's supplies (although this can be overcome by wiring separately to the loads and supply). Whether this is an issue or not will depend upon how much spare capacity the power supplies on the network have. It also makes maintenance on the dimmers/relays easier because you don't have to go looking to see if it's one of the special ones that includes a power supply; they're all the same, i.e. passive. Having said all that, if you're pressed for DIN space or cost is an issue then there's nothing intrinsically wrong with the integrated supplies either.

One final point on power supplies is to make sure that you don't have all your power supplies grouped together at one end of the network with all the other units some distance away. The voltage drop can be significant so it's best to put the power supplies distributed around the network in a couple different locations.

 

Thanks for the detailed answer.

Putting all of the parts of the answer together, it looks like I should recommend that all the upgrades are to passive units (like-for-like), and that we separately recommend replacing the power supplies across the site. Looking at powered units without fully understanding the locations of the units being replaced at this stage over-complicated things.

When it comes to recommending replacing the old power supplies, is there anything you can give me to make my case? Eg are we talking about capacitors failing and generating noise, or something like that?