For single cord loads I can see the need for an STS. However, for dual cord loads my thought is that switching between power sources can be performed right at the load level. I have someone else telling me that this is not as reliable...
I can see two sides to this discussion.
The points provided above are based on all the loads having dual power supplies (redundant power supplies) and that they all work. However, all these redundant power supplies become single points of failure if one or the other electrical service is down for either service or mishap.
Placing a single STS between the main electrical services does indeed add another device to the system. It also reduces the risk of downtime due to single source loads and/or failed redundant power supplies.
Placing STS at the load, either at Rack level or at individual load level (or by inherent redundancy) adds many devices to the system and places many more single points of failure in the system.
In either case, I believe adding STS is going to raise the availability of the system regardless of where it's placed. I would prefer a single STS at the sources.
always keep in mind that, the set-up has to be as simple as possible. less components in, more reliable. more equipments, more points of failure, more downtime if trouble. also, easier to identify the trouble if any.
My opinion: if you have dual power supply loads, keep one side directly on the UPS, and the 2nd directly on Normal AC Source. you choose which is where to be tie.
so, actually the STS function is already present in your dual power supply load. ;)
i agree with the above post. with dual corded power supplies, i don't think you need an STS since it will be done at the load level as you mentioned. and to echo the above poster, the more pieces of equipment introduced, the more points of failure.
It depends on what TIER level you want to have in your DC. If you want a TIER 4 you need an STS at every level of your power distribution.
We use 2n UPS connected to STS, connected to RPP PDUs, connected to rack ATS for single cord loads.
Dual cord loads are connected to 2 rack PDUs which are connected to different RPPs, so there's always an ATS/STS between a power cord and the UPSs.
Our DC is TIER 3.
What you could encounter, if you have a high load server connected to 2 PDUs and no ATSs is, that one PSU can't take the whole load and breaks in case of a power path failing. Maybe because it had a defect which didn't come to light because it ran at > 50% load all the time. I have seen that in our DC.
I guess that is what the person meant who talked to you.
I don't agree with mihail on the direct connection to the utility. You should always connect devices to UPSs. They will filter the current and your devices will live longer.
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