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Anonymous Poster #1

UPS Main Power Input

10/30/2023 10:19 AM

Bear with me for this dumb question:

Why UPS main input power is fed from Emergency power (DG) though it has batteries?

Similarly, Why the Emergency light fixtures (with Integral batteries) are fed from Emergency power supply(DG)?

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Guru

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#1

Re: UPS Main Power Input

10/30/2023 10:52 AM

The battery backups are used for the brief time it takes for the emergency generator to kick on and stabilize...the changeover event...

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Guru
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#8
In reply to #1

Re: UPS Main Power Input

11/10/2023 8:09 AM

...and if the emergency generator starts the second or third time or not at all until someone has kicked it, then the batteries will prove invaluable.

GA

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#2

Re: UPS Main Power Input

10/30/2023 11:42 PM

Think about what Solar said. The batteries are going to be charged again after they provide the power in an emergency. They must be rechargeable by the emergency power equipment on that emergency bus. The Emergency bus will (or should) be disconnected from the mains in the case of an outage.

So. The UPS main power input is supplied through the emergency bus AND the mains during nominal operation.

and when the mains drop out, the batteries are still connected to the load/charge bus.

I maintained an APC brand "always on" UPS system for a radio station here in northern California. It was hooked up the same way for those same stated reasons. Our facility was an emergency broadcast facility. We broadcast to San Francisco and the North Bay area. We had a generator in the basement that would kick in way before the batteries ran out and start recharging the batteries while providing operating power. Usually within 30 seconds though we had and hour of battery time.

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Guru

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#3
In reply to #2

Re: UPS Main Power Input

10/30/2023 11:44 PM

Some lighting must be on the emergency panel circuit. When the lights go out, you still need to be able to see!

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#4

Re: UPS Main Power Input

10/31/2023 1:25 AM

This is to keep the battery reasonably small.

If the UPS as well as Battery backed light fixtures have to be sized for the complete duration as required by the designers (what is known as autonomy period for battery), these could be bulky and expensive.

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Anonymous Poster #1
#5
In reply to #4

Re: UPS Main Power Input

10/31/2023 5:28 AM

But from what I have seen in my limited experience, irrespective of the time it takes to bring the emergency DG on line, the batteries are always sized for the full autonomy.

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Anonymous Poster #2
#6
In reply to #5

Re: UPS Main Power Input

10/31/2023 8:08 AM

Exactly. What happens if the DG fails to start?

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Guru

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#7

Re: UPS Main Power Input

11/07/2023 7:08 AM

From the systems I have worked on the emergency lights are sized so the lights provide enough illumination for and orderly evacuation to occur in a timely manner.

UPS batteries are sized to enable the control system to be shut down in an orderly manner. Usually with an outage the rest of the system has lost power and so has stopped. UPS supplies usually feed critical safety systems to allow them to be maintained until a safe shutdown can be managed.

The systems I have worked on did not have the luxury of backup generators for the loads on a washplant could be 5MW. UPS supplied the control system for the black start turbo jet generator supplying 30MW to enable a boiler to be brought back on line for it could supply half a boiler power to enable enough MW to be generated when a total station trip had occurred. The rest of the station could be brought back on once the first boiler was brought back to half capacity.

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