A question about PSU APFC inrush current, APC UPS, and MCB

[Anonymous user]

Hello, I have a question about inrush current. Currently I have:

• a PC with 400 W maximum power draw and a 700 W PSU which reportedly has 29 A inrush current[1],

• a DOMAE MCB C type 4 A 1 Pole 4500 A 230 V (DOM12252SNI),

• and an APC Back UPS 230 V 550 W 1.1 kVA (BX1100LI-MS).

The PC is working perfectly right now and doesn't trip the MCB or the UPS circuit breakers. Suppose I replace the PSU with 1000 W one that reportedly has 37 A inrush current[2]. Would the new PSU trip the MCB or the UPS circuit breakers? As I understand it, the MCB only has 4 A rated current, but the inrush current from the PSU is much higher. Thank you for your time.

[1] https://www.techpowerup.com/reviews/NZXT/HALE82-700-V2/5.html

[2] https://www.techpowerup.com/reviews/Corsair/HX1000i/5.html

[Anonymous user]

Anyone?

[Brad_C · Accepted solution]

Patience. You posted to a commercial support forum on a Sunday. Not only that, but you posted a question that's not really relevant to the forum.

If you get the data sheet for your circuit breaker, you'll see a trip-current/time curve. This is to allow for high inrush current in solid state devices, but also mechanical apparatus (such as motors). That curve will tell you whether or not you'll have an issue. I attempted to look up a data sheet for the breaker, but could not find anything in a language I could get my head around, and given it was a Sunday I gave up and opened a beer instead.

You'll most likely be ok. Inrush current is a bit variable and can change wildly depending on where you are in the mains cycle when you switch it on.

Try it, the worst thing that happens is you need to get a breaker with a more tolerance to severe short term impulse.

(edit) actually, given you're powering it *through* an APC UPS you'll find the common mode inductors in there will help take the edge off the inrush spike anyway.

[Anonymous user]

I apologize for my impatience and for posting an irrelevant question. I've been researching about inrush current and whether it trips an MCB, but I couldn't quite find what I was looking for and this forum seemed like the right place to ask.

Also I found the curve you were looking, but the image[1] resolution in the data sheet[2] is very low. It is said in the data sheet that in C type curve, the magnetic trip units operate between 5 in and 10 in. So if the rated current is 4 A, then the MCB trips at 20 to 40 A? Nevertheless, it'd be great if the UPS could filter the spike so I don't have to replace the MCB. Thanks for the insightful reply!

[1] http://imgur.com/a/fiUn0
[2] http://www.seewide.com.my/webshaper/pcm/files/Datasheet/schneider-DOM11341.pdf

[Brad_C]

Nice, so you have a class C breaker and your analysis of the 20-40A trip would be about right in a perfect world.

Ideally you want either a class D breaker or a better rated circuit. Having said that, I honestly don't think you'll have a problem. The inrush is going to be less that 100ms. At 50Hz, 100ms is 10 half cycles. If you scoped that inrush I bet you'd see the worst of it gone in 1 or 2 half cycles as the main filter cap charges up. Much more than that and you'd be smoking bits of the power supply. So you shouldn't have an issue.

If you were going to have an issue, you'd probably be seeing it with your existing PSU anyway on such a small breaker.

Do you really get 1000W circuits where you live? Here we see 6-8A for lighting (1-1.5mm2 cable) and 16-20A (2.5mm2 cable) for a standard power circuit (nominal 240V mains).

[Anonymous user]

Well before purchasing the UPS, the PSU would sometimes trip the MCB when I hit the PSU switch after it had not been used for hours. So I went on to buy a UPS and thank God there haven't been any nuisance trips ever since.

And yes, I only get 920 W, i.e., 4 A for the entire room and 10 A for an air conditioner. I'm a college student living in a small apartment. Unfortunately it's not my property so I don't think I can get the MCB replaced. So I wanted to make sure it can handle the 8 A increase in inrush current. But yeah I will try it. Thanks!

[Brad_C]

I was having a think about this over dinner. *If* you find your new PSU is causing grief, unplug the APC UPS from the wall so it switches to battery before you turn the PSU on. An APC UPS on battery will cope with huge inrush requirements. It'll cause massive output distortion briefly, but nothing to worry about. Once you've clicked the PSU on you can plug the UPS back in and let it switch back to mains.

It's a pretty awful workaround, but I'm constantly amazed at the abuse a BackUPS or SmartUPS will tolerate.

[Anonymous user]

Hi Brad, thanks for being so thoughtful. The new PSU has been working perfectly so far. There has been no nuisance trips, even when using a 4 A MCB. I guess the 8 A increase in inrush current is tolerable. I can expect nothing less from such good UPS and PSU. Thanks again for your help!