Simple Redundant AC Input Circuit Issue
Simple Redundant AC Input Circuit Issue
(OP)
Hi Guys,
The attached circuit is an AC redundant input circuit for a chassis that runs several SBC cards. The AC powers four PSUs. For some reason when the AC is turned ON, occasionally the fuses on both AC inputs (one fuse each on each side) burns. Switching the AC inputs individually the circuit works normal. The fuse is 15A and the load is 8A max. The fuse is fast blow type. The PSUs are inrush current limited at 10A a piece.
The circuit works like this. When the top AC input is present the relay turns ON and connects the Top AC input to the PSUs. Now when the top AC input is turned OFF the relay turn OFF and the lower AC input powers the PSUs.
The attached scope shot shows the AC waveforms when both AC inputs are killed at exact same time manually. CH1 and CH2 are AC inputs and CH3 (magenta)is current. This shot is taken at the AC input side before the filter. The shots shows the high current condition happens for 1.25ms which is also current limited since it is flat at the top. Not sure why this is happening and how a fuse can respond to such a short duration of surge current.
Any ideas regarding this failure?
Thanks
The attached circuit is an AC redundant input circuit for a chassis that runs several SBC cards. The AC powers four PSUs. For some reason when the AC is turned ON, occasionally the fuses on both AC inputs (one fuse each on each side) burns. Switching the AC inputs individually the circuit works normal. The fuse is 15A and the load is 8A max. The fuse is fast blow type. The PSUs are inrush current limited at 10A a piece.
The circuit works like this. When the top AC input is present the relay turns ON and connects the Top AC input to the PSUs. Now when the top AC input is turned OFF the relay turn OFF and the lower AC input powers the PSUs.
The attached scope shot shows the AC waveforms when both AC inputs are killed at exact same time manually. CH1 and CH2 are AC inputs and CH3 (magenta)is current. This shot is taken at the AC input side before the filter. The shots shows the high current condition happens for 1.25ms which is also current limited since it is flat at the top. Not sure why this is happening and how a fuse can respond to such a short duration of surge current.
Any ideas regarding this failure?
Thanks





RE: Simple Redundant AC Input Circuit Issue
RE: Simple Redundant AC Input Circuit Issue
RE: Simple Redundant AC Input Circuit Issue
It seems to look like someone didn't bother to find out what the actual transient current from the power supplies looks like.
TTFN
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RE: Simple Redundant AC Input Circuit Issue
The time is only roughly 1.25ms on 46A and 5ms on 27A. The fuse specs chart shows at 240A load current lasting 10ms min can blow the fuse. In this case it is 46A max and it will take about 500ms to blow the fuse per the spec. I did not made this clear earlier but the scope shot TEK0005 is shot on the PSU side at the relay common point and the TEK0004 shot is on the AC side. Not very convinced that this short duration can pop fuses but something is definitely going on and you may be right too.
RE: Simple Redundant AC Input Circuit Issue
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Simple Redundant AC Input Circuit Issue
The primary question now is how this short circuit is taking place. Any ideas?
Thanks a lot.
RE: Simple Redundant AC Input Circuit Issue
However, what exactly is happening from t=15ms to t=20ms? The waveform for CH2 looks like it got delayed by 1.25ms, which suggests that there is something going on with your line voltage.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Simple Redundant AC Input Circuit Issue
Another possibility: there's a miswire not shown on your sketch. It's not uncommon for the actual root cause to be NOT shown in the information provided in the thread.
RE: Simple Redundant AC Input Circuit Issue
When the switch breaks contact, it may carry an arc that is extinguished at the next zero crossing.
Now, if the other throw of that pole is connected to a different phase, you have a phase to phase short through the arc.
Your scope trace seems to be showing a voltage drop on both phases which supports this conclusion.
To sum up, the issue may not be the PSUs at all but an arc carry-over in the relay. Not much clearance in those little relays.
Try a three pole relay: One pole for the neutral, one pole for one phase and one pole for the other phase.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Simple Redundant AC Input Circuit Issue
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Simple Redundant AC Input Circuit Issue
RE: Simple Redundant AC Input Circuit Issue
RE: Simple Redundant AC Input Circuit Issue
Thanks a lot for valuable comments.
RE: Simple Redundant AC Input Circuit Issue
Two standard relays will be a lot cheaper than a special vacuum DPDT relay - I don't even think they exist in the size you need.
If you look at pull in and drop out times for AC coil relays, you will find that drop out usually is about half the pull in time. So you will have something like 5 to 10 milliseconds safety. But make sure you check the timing before buing!
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Simple Redundant AC Input Circuit Issue
Depending on several unstated variables (DC current, cost, size constraints), there may be a point where it would be simpler to double-up the number of PSUs, simplify the AC circuits back to normal, and then use diodes to provide the redundancy on the DC side.
If you start getting into vacuum relays, it'd probably be cheaper to buy another bunch of cheap PSUs. This is assuming that your "several SBC cards" aren't requiring (for example) 30A each. If the SBC cards just need a couple of amps, then such PSUs are extremely cheap. Vastly cheaper than a vacuum relay.
Just something to stop and think about.
RE: Simple Redundant AC Input Circuit Issue
RE: Simple Redundant AC Input Circuit Issue
The PSUs are already redundant and ORed at the ouput. The input redundancy is a separate requirement.
Thanks.
RE: Simple Redundant AC Input Circuit Issue
OMG. What's this for? ...No, just kidding (none of our business).
Of course we can't see your system spec from here, but one can sometimes meet multiple "separate" requirements with one approach. If the redundant PSUs were wired up directly to the separate AC inputs, this one approach might meet both requirements - providing AC supply redundancy and PSU redundancy (allowing either failure mode, not both at the same time).
Unless the system spec has another requirement that specifically excludes such a two-for-one approach (to allow for simultaneous failures of BOTH an AC input AND a PSU), the wording may allow this.
This may be disregarded if it's off the mark.
RE: Simple Redundant AC Input Circuit Issue
Another option may be to feed both relays from the same phase. This may not be a good idea if the relays are fed from different transformers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Simple Redundant AC Input Circuit Issue
Well, if your PSU has a ride-through like 20 ms, it will probably be a problem.
I have a Kepco 72-6. Four quadrants of DC +/-72 V and +/-6 A output current. Big, noisy and just great.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.