Switched mode power supply

[PdL 71]

In an installation on a ship we used Mean Well switched mode power supply 230VAC in 24V DC 5A out in every panel for PLC inputs, output relays and panel motor starters coils. No real big load, perhaps 1A max. The 230V AC line for this power supply also feeds the PLC CPU power supply and comes from the ships power distribution. 2 days ago after they had some power problems and changed over from shore power to generator power, or vice versa. The story was a bit vague. The shore power comes in through a straight transformer. Not immediately but the next day 4 of 7 of these PSUs from various panels througout the ship were dead. At least when load was connected, the output voltage seemed to be switching on/off at a rate of about 5-10 Hz. At the same frequency you could hear a loud switching noise out of the PSU. Anyway this switching of the voltage resulted in the inputs of the PLC going on/off at the same rate and the main alarm system going absolutely crazy. The output voltage measured 9 volts, but when I switched off all downstream 24VDC breakers, the voltage was 24VDC. Now I could switch on one breaker serving one digital input card and then I could hear the same switching sound very weak, but switching on the next breaker resulted in the same. There is absolutely no fault or short circuit in the field contacts connected to the input card, and this was confirmed when we exchanged the PSU for a Weidmuller which is of about the exact same ratings and size. After this everything was to normal again, steady 24VDC out, all breakers on. Luckily nothing else seemed to be damaged. Now of course the question is, why did these PSU die. To find this out, I would like to know what you think could be a reason, or perhaps you have had similar breakdowns. If I read the datasheet they should be well protected against over voltage (1,5 KVAC, 1 minute) but this is what is written in the specs, who knows what they can really handle. How would they handle extreme under voltage, say 50VAC ? Any thoughts are appreciated. Mean_Well_SMPS.pdf

[P Daniil 0]

Most probably they got partially damaged during the "power problems" and as a result could not supply their loads with the necessary current (and hence the 5-10 Hz power-up "hiccup"). Most chances are that their dc-bus electrolytics got "savaged" from the line fault (typically high voltage 300-400 V inductive spikes riding on the 230 VAC line) as they are the most vulnerable part in a power supply. With a partially working electrolytic the power supply can only store and convert minimum/unloaded power. My further guess is that the line neutral became unstable during the coupling/uncoupling of the shore power and, just as it is always good practice, I would regardless power the PLCs through a 400/230V transformer and stay away from the neutral. Most faults involve the neutral and in a ship the presense of the hull (also at neutral) makes matters worse by offering a catch-all connection to the line.

[PdL 71]

There actually is a 400/230 power transformer in every panel and normally we tie everything behind it but this separate 230VAC supply used for the PLC CPU and the PSU serving the 24VDC circuits comes from the UPS system because the PLCs are required to stay powered during ships black out.So would you recommend to put an additional 230/230 transformer at the incoming supply ?

[Ken Moore 23]

I would think that the UPS system would absorb the spikes so that nothing connected to the UPS system would ever see the spikes. Sounds like a very poor UPS system.

[PdL 71]

Youre right, if there were any spikes or surges on the UPS supply this should never have happened. Or the UPS system must have had problems itself.

[Ken Moore 23]

Would it be possible to connect shore power into the UPS outlet by mistake?

[leonuce 0]

I do not know what kind of UPS you have in your system but they are not that good, they should be able to absorb the spikes, apperently they didn't and it took a while for the system to stabilize.

[PdL 71]

It's not a small UPS we are talking about here, I would not know the exact capacity but it serves all important electrical control systems on a 225 ft. vessel. I need to find out more about the UPS setup, it seems we are the only system that suffered damage but I don't know for sure. Thanks for your replies so far, most likely I will pay another visit soon and try to find out some more details.

[P Daniil 0]

Edit: Good luck, please keep us posted. Edited 3 Dec 2007 by P Daniil

[PdL 71]

To follow up, it turns out after my visit some more parts throughout the ship gone defect, which indicates a serious UPS problem. As always I was called away on another job so my colleague has returned to finish the job. After some discussion with the company that does the ships electronics most likely the UPS voltage dropped slowly to a level where the PSUs current draw became (too) high and they gone bad. Still we feel these particular PSUs as well as the brand labeled Weidmuller version are not of the best quality when it comes to lifetime and input signal range. Next step would be to look in to some kind of protection device. Anyone has a good suggestion for a under/over voltage slash surge protection ?

[P Daniil 0]

My approach would be first to have two universal, galvanically isolated, wired-OR connected signals, "Fault" and "Ready", driving all the devices (PLCs, supplies, I/O etc) so that when any supply/device has a problem (load fault or source problem) it will signal the others to shut-down/wait graciously (as opposed to chattering to death). This arrangement has the advantage of enhanced robustness as you have one signal that there is an operational fault and a signal that all is working OK and being able to handle the four combinations accordingly. In more complex set-ups (such as in a ship) where you have separate power and control supplies it may also be worth having respective "Ready" signals for each line. Following this you can then look for suitable surge/UV/OV devices/controls.