RE: Concept of daisy chaining rack mount UPS's to Server Room UPS

[Anonymous user]

My company is about to replace one of the main UPS's in our server room to resolve an ongoing hardware/ power failing issue which has been taking down our core hospital network. The problem I am faced with is that there are those folks on the clinical side(I work for a large medical center in the Silicon Valley) who are requesting additional external rackmount UPS's to be installed in the server racks. I have tried to inform them that this would not be necessary as in the event that the server room UPS should fail, we have the room configured to automatically cutover to generator power. I believe what they are asking to do is what is known as daisy chaining UPS's; external rackmount UPS to server room UPS. I have heard that you need to be very careful when doing this but don't have all the details. Can anyone help clarify why caution needs to take place and what I need to be aware of? I know amount of load on the external UPS's is a factor and what type of UPS they are plugging into in the rack. I have also heard of either overload and or power drainage if a battery in one of the rackmount UPS's should go bad. Could someone please help fill in the gaps for me????? Thanks in Advance!!

[Cap1_apc]

I'm trying to think of a benefit of "daisy chaining" UPS's, but it is generally just a bad idea, or completely unnecessary.

Especially in the case of a generator setup - daisy chaining creates problems. For one, you will be bottlenecking the available load by the rack mounted units (depending on their size). In any case, when the power is lost to the room, the main UPS will power the rest of the UPS' and they would continue to run on utility power. In the case of the generator, the main 'data center' UPS should run with sufficient time to enable you to turn on the generator. The generator, in turn, would power the UPS and it would return to utility power. This is the typical use of a UPS (bridge the gap between power loss and generator power).

Adding more UPS' will just create more possible points of issues

[BillP]

Hi there Nerd!

You didn't say who makes these UPS systems your company is using, but your advice to them is sound.
In addition to injecting multiple points of failure, here's another techno reason you can use to explain it to them.

When power comes in from the utility company it's a nice clean sine wave, produced by a generator. When utility power fails and the UPS comes on line, a typical double converstion online UPS makes power by converting DC battery voltage to AC voltage, producing a modified sine wave using a 6 or 12 pulse rectifier.

It depends on the manufacturer's design topology of the main UPS and the downstream UPS, but the risk for the clinical folks is that they are wasting money, energy, and space, since the possibility exists that the downstream UPS will see a modified sine wave as a power fault. Or the computer power supply will see a regenerated modified sine wave as a power fault.

This isn't typically a problem when you use an APC-MGE UPS as your main UPS, since we provide a true sine wave output.

[Anonymous user · Accepted solution]

Tech Tuff wrote:
Hi there Nerd!

You didn't say who makes these UPS systems your company is using, but your advice to them is sound.
In addition to injecting multiple points of failure, here's another techno reason you can use to explain it to them.

When power comes in from the utility company it's a nice clean sine wave, produced by a generator. When utility power fails and the UPS comes on line, a typical double converstion online UPS makes power by converting DC battery voltage to AC voltage, producing a modified sine wave using a 6 or 12 pulse rectifier.

Actually, double conversion means the UPS is ALWAYS online, hence the name. The power is converted from AC->DC->AC continuously, so the output is always inverter synthesized and it's output waveform is independent of input waveform. Many online topology however use the line as the timebase for inverter so that line-frequency controlled time keeping device holds time, then switch over to 60Hz or 50Hz internal clock when utility power is lost. There are some units that offer non-line sync and always use the internal synthesizer for locations where power quality is so unstable that frequency swings as well, but if you use this option, the internal clock signal won't maintain the frequency accurate enough to support a line synched time keeping devices.


So, if the daisy chain setup works now, it would be fine since the loss of primary side power is completely transparent to output.