Clean power for LED's from a Original Strand CD80 Rack
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Scenemaster60
Well-Known Member
BillConnerFASTC
Well-Known Member
mikeydoesstuff
Active Member
Sorry, the F was a typo.
Ford
Sr Product Manager, Chauvet Professional
At Cornell U., we had large CD80 Racks in our main stage, Flexible, and Black Box theatres (the Dance theatre, and light lab used portable CD80 packs). I had about a dozen relay modules, that I swapped in when we were locating a projector, or hazer, or LED fixture, or even a computer in various spots throughout the theatre.
They looked very similar to the standard dimming modules, except that they were grey (instead of black), and weighed a fraction of a standard 2x2.4k dimmer module because they had no chokes or SCRs... just relays.
The modules were manufactured by Strand. They were just dry contact relays (like Bill said). As a default, they turned on at 50%, though I sometimes modified the channel in the console to "on at 1%", too.
Because they were relays, and not SCRs, they did not modify the sine wave, at all... they worked great for powering electronics throughout the nooks and crannies of the theatres.
If you have big CD80 racks, this is what you're looking for. If you have the smaller CD80 dimmer packs (like the photo above), you'll need to get replacement cards, and install them. these are not as easy to work on as the big racks.
HTH,
-Ford
They looked very similar to the standard dimming modules, except that they were grey (instead of black), and weighed a fraction of a standard 2x2.4k dimmer module because they had no chokes or SCRs... just relays.
The modules were manufactured by Strand. They were just dry contact relays (like Bill said). As a default, they turned on at 50%, though I sometimes modified the channel in the console to "on at 1%", too.
Because they were relays, and not SCRs, they did not modify the sine wave, at all... they worked great for powering electronics throughout the nooks and crannies of the theatres.
If you have big CD80 racks, this is what you're looking for. If you have the smaller CD80 dimmer packs (like the photo above), you'll need to get replacement cards, and install them. these are not as easy to work on as the big racks.
HTH,
-Ford
STEVETERRY
Well-Known Member
The bigger issue here is fault current coordination. If the original rack contained no non-dim modules, then the fault current coordination was done using the SCCR of dimmer modules with current-limiting chokes. Back in CD-80 prime time in the 1980's, fault current coordination might not have been a big issue. Now it is.
If you change a rack from all dimmers to some dimmers and some non-dims or CC (breaker only) modules, you need to make sure that the fault current coordination is still valid.
BTW, this is a life-safety issue.
ST
BillConnerFASTC
Well-Known Member
I forgot that not every rack had silver sand fuses which I think solved this. Maybe not.
STEVETERRY
Well-Known Member
If the non-dim or CC module has a current-limiting fuse, that can solve the fault current coordination issue by maintaining the original SCCR of the rack.
ST
Guys, an observation: I read the thread, and a safe and legal solution made by the OEM has already been found, and suggested to the original poster. There is no reason to replace the current system. I'm sure that this solution is drastically inferior to a brand new ETC contraption, but replacing the whole rack does seem like a solution in search of a problem.
Also, "clever" sales tactic.
STEVETERRY
Well-Known Member
Mike-
I'm sorry if you read my post as "clever sales", but that was surely not my intention. Nor did I mention any potential replacement product by name or features--on purpose. For those that know me, "clever sales" is not in my repertoire or MO. Rather, I view this as a pure safety issue, where "caveat emptor" comes into play for the informed consumer, especially where the relatively new issue of fault current coordination is involved.
ST
BillConnerFASTC
Well-Known Member
I know a number of folks at ETC that wish Steve would engage in "clever sales" and not be so helpful and so product neutral here. Where else in our industry would people designing, installing, and operating theatre lighting systems learn about Available Fault Current Selective Coordination?
Question of curiosity here...
Why would retrofit modules, supposedly designed specifically for the CD80, but using mechanical relay contacts, have an SCCR issue?
If they are being sold as a drop-in replacement, shouldn't they already be designed to have the necessary chokes / PTC-thermistors / fast-blow fuses or circuit breakers, so as not to compromise the dimmer pack's safety ratings?
(And if they don't, how are they being sold as approved modules?)
Why would retrofit modules, supposedly designed specifically for the CD80, but using mechanical relay contacts, have an SCCR issue?
If they are being sold as a drop-in replacement, shouldn't they already be designed to have the necessary chokes / PTC-thermistors / fast-blow fuses or circuit breakers, so as not to compromise the dimmer pack's safety ratings?
(And if they don't, how are they being sold as approved modules?)
Last edited:
Oh and another question...
Why is the "switch-on level" (be it 50%, 53%, whatever) even an issue? I'd think if you're using a light channel as a simple on/off, you want to stay as far away from the threshold levels as much as possible, to ensure the system doesn't go into oscillation. (Are the hysteresis characteristics even specified for these units??) Anyhow, that would mean your lighting controller puts out 0% when you want the channel off, and 100% when you want it on. So wouldn't the finer details of the threshold, be it 50%, 53%, 10%, 90%, etc. be irrelevant?
Why is the "switch-on level" (be it 50%, 53%, whatever) even an issue? I'd think if you're using a light channel as a simple on/off, you want to stay as far away from the threshold levels as much as possible, to ensure the system doesn't go into oscillation. (Are the hysteresis characteristics even specified for these units??) Anyhow, that would mean your lighting controller puts out 0% when you want the channel off, and 100% when you want it on. So wouldn't the finer details of the threshold, be it 50%, 53%, 10%, 90%, etc. be irrelevant?
BillConnerFASTC
Well-Known Member
CD80s were designed and manufactured before the issue was fully recognized as a potential hazard. It was added to the NEC in early 2000s iirc.
Can you keep using it? Yes, just like ungrounded systems and receptacles in wet areas without gfci protection may be continued in use in some circumstances. Should you? Is it a smart choice? Is it safe? Maybe but probably not. And what industries other than entertainment technology would? To many people in our industry are too willing to accept too little.
In this case maybe the current safety standards and codes can be met but I think modifying a system with different components requires it meet the codes and standards in effect today, and perhaps it can. It would require research and I don't know if some of the after market products are even listed. Lots of gear is not, and I won't specify or accept it myself, as many inspectors would not accept it.
STEVETERRY
Well-Known Member
Dimmer modules or non-dim modules that plug into a dimmer rack are UL Recognized components. UL is happy to recognize a non-dim module without current-limiting fuses or a choke. In that case, the SCCR of the entire rack falls to the native AIC rating of the circuit breaker in the non-dim module. Let's say the original rack was full of dimmers with chokes and the rack had a 100,000 amp SCCR. Once that non-dim module goes in, the SCCR likely falls to 10,000A, which is a typical AIC rating for the type of breakers used in these modules.
But what if the engineer laying out the original system specified a transformer, wiring length, and switchgear layout that makes 40,000A of fault current available at the rack? No problem for the original, which had an SCCR of 100,000A. Big problem for the non-dim module, which now reduces the rack SCCR to 10,000A. That breaker in the non-dim may not be able to safely clear a fault with 40,000A of fault current available.
Thus, with a retrofit module, the fault current coordination issue falls to the specifier, engineer, or end-user--not to the NRTL that recognized the new non-dim module.
ST
Well, heck, suppose this thing was installed right in a power plant, on the first step-down transformer next to the turbine hall! The fault current could be astronomical. So it seems a little silly to me for equipment... any equipment... to have its ratings based on an "assumed type of connection" to the AC power mains.
Am I wrong to figure when selecting and overcurrent protection device, the resistance of just the device's own conduction path (power cord, thermistor (if present), rectifier (if present), the protection device itself, etc.) should be used to calculate the short-circuit current? And then ensure the disconnect occurs fast enough before heat build-up melts something in (or near) that conduction path? (I don't specialize in power systems, but I'm genuinely curious.)
This brings up another curiousity:
Given that switch contacts almost always have higher AC ratings than their DC ratings (relying on the zero-crossing of AC power to assist with extinguishing the arc between the contacts)... and I wonder if this applies to fuses too... does AC line frequency become significant when dealing with overcurrent protective devices and their response time? Does a legacy 25 Hz system have far more demands than an aircraft 400 Hz system, for example?
Am I wrong to figure when selecting and overcurrent protection device, the resistance of just the device's own conduction path (power cord, thermistor (if present), rectifier (if present), the protection device itself, etc.) should be used to calculate the short-circuit current? And then ensure the disconnect occurs fast enough before heat build-up melts something in (or near) that conduction path? (I don't specialize in power systems, but I'm genuinely curious.)
This brings up another curiousity:
Given that switch contacts almost always have higher AC ratings than their DC ratings (relying on the zero-crossing of AC power to assist with extinguishing the arc between the contacts)... and I wonder if this applies to fuses too... does AC line frequency become significant when dealing with overcurrent protective devices and their response time? Does a legacy 25 Hz system have far more demands than an aircraft 400 Hz system, for example?
STEVETERRY
Well-Known Member
For more information, see the excellent article "Fault Current Demystified" by Ken Vannice on page 40 of the Summer 2013 issue of Protocol here:
http://edition.pagesuite-professional.co.uk//launch.aspx?eid=d8524bce-4e49-4823-806d-1a3c0166a79d
ST
RickR
Well-Known Member
This is an issue of our industry default control being an analog slider so you need a switch point in the console. Back in the day, 0-10V control was also typical so you need a switch point in the rack. There is always the possibility of getting a transitory level so you can't depend on only 0 or 100%. 50% is as far away from the "normal" settings of on/off as possible. I last used 'on @1%' when I wanted something to be 'first on, last off' as it was a semi-safety issue. Sure I could have done other console tricks, but that was best in that situation.
I'll let others address the electrical engineering questions. Thank you @STEVETERRY for being far clearer than most EEs I've known!
Ford
Sr Product Manager, Chauvet Professional
Thank you Steve, this is a great article!
I had not really pondered that many of the systems that are being "upgraded" for LED fixture distribution were not designed for this, and lack the proper circuit protection. It's a little counter intuitive to think that the lower power draw of electronic fixtures could actually be more of a hazard than running through dimmers and then out to a higher wattage fixture...
Thanks for broadening my perspective.
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