So to be clear - your UPS is connected to mains. You should not connect surge protectors to ups plugs. If you need more plugs than ups has then use of PDU is way to go, or power strip without surge protection.
Using surge strips with APC's Back-UPS and Smart-UPS products.
Issue: What is Schneider Electric's policy for using surge strips with the APC Back-UPS and APC Smart-UPS product families?
Product Line: All models of Back-UPS and Smart-UPS
Environment: All models of Back-UPS and Smart-UPS
Cause: Schneider Electric recommends against the use of any surge protector, power strip or extension cord being plugged into the output of any APC Back-UPS and Smart-UPS products. This document will explain why.
Plugging a surge protector into your UPS:
The noise filtration circuitry in a Surge Protector can effectively "mask" some of the load from the UPS, causing the UPS to report a lower percentage of attached load than there actually is. This can cause a user to inadvertently overload their UPS. When the UPS switches to battery, it may be unable to support the equipment attached, causing a dropped load.
Surge protectors filter the power for surges and offer EMI/RFI filtering but do not efficiently distribute the power, meaning that some equipment may be deprived of the necessary amperage it requires to run properly causing your attached equipment (computer, monitor, etc) to shutdown or reboot. If you need to supply additional receptacles on the output of your UPS, we recommend using Power Distribution Units (PDU's). PDUs evenly distribute the amperage among the outlets, while the UPS will filter the power and provide surge protection. PDUs use and distribute the available amperage more efficiently, allowing your equipment to receive the best available power to maintain operation.
Plugging your UPS into a surge protector:
In order for your UPS to get the best power available, you should plug your UPS directly into the wall receptacle. Plugging your UPS into a surge protector may cause the UPS to go to battery often when it normally should remain online. This is because other, more powerful equipment may draw necessary voltage away from the UPS which it requires to remain online. In addition, it may compromise the ground connection which the UPS needs in order to provide adequate surge protection. All APC Back-UPS and Smart-UPS products provide proper surge suppression for power lines without the need of additional protection.
Maintaining EPP and Warranty:
Plugging any non-APC surge protector, power strip, or extension cord into the output of an APC brand UPS could void your Equipment Protection Policy (EPP). However, the standard 3 year product warranty is maintained. If, after taking into consideration this knowledge base document, you choose to use an APC brand surge protector in conjunction with your APC brand UPS, your warranty and Equipment Protection Policy will be maintained.