UPS Power distribution - Selective coordination of protective devices

[krisys]

Good day!

I would like to know anyone has some good write up or share some experience on the Selective coordination of protective devices for UPS Power distribution.

What are the aspects to be considered for selectivity study?

My UPS rating is 40kVA and the system voltage is 110V AC, 1-phase. I am not sure whether one of the conductor is earthed or the system is floating.

 

[ScottyUK]

The most significant thing to determine is whether you want to be able to clear a fault while on inverter power, or whether you intend to rely upon the bypass supply for fault clearance. On a 40kVA unit I wouldn't expect to trip an MCB much larger than 40A Type B or 20A Type C off inverter power. The inverter typically can deliver 3x rated output current for one second and electronically limits the current to this value.

 

[krisys]

Scotty

Thanks for your response. It really enlightened me that the short circuit level and its characteristics vary widely with inverter output or the bypass output feeding the load.

Yesterday only I came to know that the MCBs for the electronic loads have different characteristics than the MCBs used for conventional loads. Class B or Z which are suitable for Electronic /low current consuming devices. As there is no significant inrush current, the tripping time of 2 to 3 times the rated current is sufficient for such types of application.

The more I think on this, I am realizing that I know less than what I don't know!

I would appreciate if someone can share their views or way forward on the selectivity study of UPS supply distribution network. As I noticed above, the criteria for this study may to totally different. Now a days, I have noticed that the UPS sizes are getting larger and larger. The distribution networks even more complicated.