Location of Units with Power

I have 6 relays and 14 dimmers located on 4x15 pole sub-boards in a comms room (all within a few square metres, other system units are present too). Is there any benefit in spreading the units with power around the different boards or is the spreading of the units only for voltage drop on noncentralised installs (so in this case it won't make any difference).

Thanks

 

In short - generally you'll be fine.

In long...
If all of your units are located within the same physical area then it won't make too much difference, provided you haven't exceeded the maximum of 10 x 200mA supplies on any single network. This is because the length of cable between one unit and the other will be quite short, and any voltage drop will be minimal.

Voltage drop is a function of resistance and current (Ohm's Law). If you have lots of current going through a large resistance (read: long cable) then you will get a large voltage drop. The worst case is all power supplies at one end of a cable and all the load units at the other end.

The most reliable network with the smallest voltage drops will be one where the power supplies and the load units are all perfectly evenly distributed around the network. Of course, the real world is far from perfect!

The only other thing you might want to consider is that if all the power supplies are lumped together in 1 panel, and that panel goes down for some reason, then your whole network will go down too.

There's an Application Note on the CIS website that goes into much more detail about network voltage drops. You can find it here.

 

I do have a couple of points ~50m away from these main boards but I have configured it so that there is one relay with power supply at one of these points (200mA) and one dimmer with power at the other (200mA). Although this doesn't make the system completely balanced I am hoping it allows enough distribution. Total power will be around 1800mAa, so within limits.

I've placed a powered relay/dimmer on each board so hopefully there will be something working if a board goes down.

Many thanks for the feedback, particularly the reference to the voltage calculations.

As an aside, I was intending to run the C-Bus power for all units from an APC UPS (and from a second one, the first dimmer's main power to allow some lights to work during a blackout). Is there anything special needed to ensure the UPS power will be ok for the C-Bus network?


Cheers

Andrew

 

Sounds to me like your network topology will be fine.

A UPS will be OK with the C-Bus DIN Relay units but it's no good for your C-Bus Dimmers. This is because the connection to the power supply side of the dimmers provides the zero crossing signal for the dimmer circuit. If the phase of the mains on the power supply side of the dimmers is even slightly out with the real mains connected to the dimmer channels then the dimmers won't work properly. You'll have flickering or even no dimming at all.

If the power supply and the dimmer channels on a dimmer are both connected to a UPS then it will likely be OK, although not all UPS's are made equal so you'd need to do a practical test to ensure the dimming works smoothly.

 

I believe you'll need a true sine wave UPS, as opposed to a modified sine wave one, for your dimmers to work properly.