Too much protection?
This article is about a rather geeky aspect of power distribution so if that is not your thing, maybe stop reading here. Or not - you might be surprised about what I am about to tell you.
Power distribution racks (a k a distros) are rarely (if ever) used to their full capacity in terms of current per outgoing channel. Most distros have 16A capacity per channel but on average they are loaded with less than half that, 7-8A.
A big distro typically has 72 channels at 16A, meaning it could draw as much as 384A per phase on a three-phase supply. But only 150 - 175A on average. This makes it possible to connect more than one distro to a common 400/500A three-phase supply.
Most often this is done by daisy-chaning the distros as they typically have a three-phase out (thru) connections fitted.
Now it gets interesting. Should the out/thru connection be protected, i e fed from a circuit breaker? Surely it can't hurt, right? Well, perhaps not hurt but can certainly create confusion. The image above shows unprotected out/through. The link between the in and thru connections should of course be rated for the same current as the connectors and the link cables between racks rated for that current too. There is a single circuit breaker (MCCB) that acts a main breaker for the whole distro.
Now recently we have seen a trend of fitting a circuit breaker to the out/thru connection. The suggestion is that this better and that it makes the system more safe.
But does it really?
If you follow the supply from the start to distro #3 you will see that current will be passing through three main circuit breakers. This is far from ideal as you end up with no discrimination at all (discrimination in breaker/fuse selection being a good thing!). An over-current situation can trip any or all of the three breakers in series, you can't predict which ones will trip first.
So I wonder, how is this "better" or "more safe"? No one I have spoken to has been able to answer. Yes, you could get away with a 250A rated cable between the distros but only if the connectors didn't allow for a greater current, which they do.
I'd love to hear what you think or feel about this.
Cascading MCCBs can be used to provide increased ability to handle large available fault currents in fixed installations, but are for this daisy-chain application as described a very bad idea in my opinion. The feed-through MCCB in Distro #1 would always carry the largest load and tend to trip first in most transient or continued overload situations. That would make it more difficult to locate the source of the problem and break (no pun intended) the common sense rule in temporary power systems that the closest breaker upstream from the fault should be your first clue to the fault’s location.
Project Manager at Stadium FX Ltd
6 年I know companies where they have link through without breakers but they have added additional powerlock outputs with there own mccb. The idea behind it is to hve a owerlock distro integrated in your 72-way which allows you to connect to a dimmer rack that usually commes without an mccb. Personally I don’t see the point of a breaker on th throug of the powerlock
Electrical Engineer /Project Manager FIA e-Safety Delegate - Motorsport and EV- Safety Specialist
6 年I’m with you - confusing regulations with pragmatic discrimination and protection only adds unnecessary risk. It is unfortunately a result of bad engineering and a lack of understanding.....
Operations Director at Empro LLC
6 年Never have been a fan of linking through....often when supplying stage power for lighting maybe the Lx Guys choose to do so once I hand them over power...personally if the situation allows I prefer a breakout panel local to backstage to split supplies individually
Entertainment & Event Production Guru | Lighting and Production Design
6 年In most any distribution situation, each rack should have its own mains breaker making any type of breaker on the pass through unnecessary. I'm not aware of this trend, but as you've discribed it, it would seem pointless. There may be an argument for adding a switch (not a breaker) between the input and output so if you are not using the pass through you could disconnect power from the output cam locks, but assuming they are covered or capped (as they should be) this would also just be pointless redundancy.