[quotemsg=18151340,0,2008285]When APC writes 550VA or 330W , what exactly is the intended capacity of load backup. Is it able to deliver around 330W of power , or in other terms can run 5 bulbs of 50-55W simultaneously.[/quotemsg]
Yes, that's what it means. If the UPS shuts down due to overload, it either means the UPS is defective, is not meeting its specification or the load you connected to it is actually higher than you thought it was. You may need to double-check the actual power consumption using some alternate means, such as a "kill-a-watt" type plug-in power meter.
[quotemsg=18151739,0,410752]photo 8/40, it's worte in the manual to do not use a power bar before it![/quotemsg]
It does. Nearly all power bars also tell you not to daisy-chain them. But who follows instructions when they aren't convenient? We can pretend that the UPS plug was a small appliance instead.
The biggest problem with daisy-chaining is some people's lack of common sense: don't daisy-chain a dozen power bars to create a makeshift extension cord and don't run heavy loads (total) from such a contraption. I have nearly a dozen power bars around my workbench to put outlets where I want them to be but most have less than 100W worth of loads connected to them. I also have three daisy-chained power bars going around my workbench so I can switch all of my lighting and stuff I do not want to leave unattended or wasting power while I'm not there on/off with a single switch. (150W of CFL+LED light pouring out of the door is usually a blaring hint that I may have forgotten something if I get distracted after walking out.)
The other problem is some power bars' contacts getting loose over time and insert-pull cycles, especially those "flappy" ground connections: insert plugs crooked and wiggle them while unplugging a few times, the ground flaps get bent so far that they don't make reliable contact anymore. Much of the same goes with the slotted flat strips on live and neutral, they don't stand up to abuse as well as standard wall outlets do. Loose connections will be an issue for anything that gets plugged into a compromised outlet and daisy-chaining power bars compounds the problem. All three of my old (as in 10+ years old) SurgeArrest Performance power bars had one or more outlets in the center strip that were no longer usable until I opened them and bent the contacts back into shape when I did my SurgeArrest repair and follow-up tear downs.
It is possible to safely mix and match power strips for whatever purpose you have in mind, but you need to keep tabs on what gets plugged where. For manufacturers covering their arses and occupational hazard legislation, it is much simpler to say "don't do it" than worry about how all of the possible variables can pile up to create trip, shock, fire and other hazards.