ATS Size

I have done multiple switch designs.
Each one is different.
More switches gives more redundancy.
Sizing; Be sure that the switches are sized to the utility supply.
I have been faced with the situation where someone else bought an undersized generator and I had a 40 Amp rated ATS to switch 100 Amp mains. More than once!
I don't have a rue of thumb. I would cost out the various options.
Transfer switch pricing;
Best price, buy the transfer switches as part of the generator package.
Medium price, roll your own. I have bought a reversing contactor set in an oversized enclosure and added a phase loss relay and a timer for cool down.
Highest price, buy the ATS from the set supplier as a single stand-alone piece of equipment.
Good luck.

Bill
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"Why not the best?"
Jimmy Carter

Thanks. I've seen government docs limit ATS size to 800amps- I'm guessing its cheaper to service while having short order parts around.

Question- how did you prevent your reversing contractor from chattering on inrush? That is my fear with using those over mechanically held devices.

Alternatively - where LVPCBs are already used, transfer systems can be programmed within a PLC or microprocessor relays for controlling the breakers. Usually provides the best flexibility and highest rating.

True. What are the advantage over a PCB approach?

What are the advantage over a PCB approach?

Click to expand...

Not sure what you mean.
What are the advantages of a PCB approach?
If you have a feeder supplied by a suitable PCB it may be used as one side of the ATS.
No need to buy two PCBs.
I always want to see a mechanical interlock.
I don't always get what I want.

Bill
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"Why not the best?"
Jimmy Carter

Yes, think a piece of gear with a main PCB and a gen PCB. Loss of POCO trips the main PCB, starts the gen, the closes gen PCB.

Voltage monitoring relays
Simple and effective monitoring

Eaton’s voltage monitoring relays are solid-state protective relays that automatically operate when one or more phases of the three-phase supply is lost, or when the phase sequence is reversed. These relays monitor three-phase voltage and respond to abnormal or overcharge conditions, a pre-selected undervoltage threshold and a pre-selected percent of phase imbalance to protect equipment against voltage fault conditions.

Link
One of these for the mains and a second for the generator side.
Add a couple of timers or a PLC and you're good.
I would used purpose built voltage monitoring modules even with a PLC.


Bill
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"Why not the best?"
Jimmy Carter

But the questions is- would I use these on an ATS or on a PCB? Or maybe even a contractor? Snoo would like know.

Depending on the type of facility and the nature of the loads, certain codes actually mandate the segregation of generator-supplied loads across multiple ATSs. The motivations for such requirements typically have to do with load prioritization and ensuring that an overload or a fault on a branch of lesser importance doesn't compromise a branch of higher importance. Multiple branches can also aid in keeping your generator size smaller by allowing you to transfer each branch at varying times, meaning that the generator only needs to accept a portion of the total load at each step.

In an installation containing multiple, code-required branches, it's typical to see one ATS per branch. If your generator is solely an optional standby unit - in other words, if it's not serving any emergency loads or other loads mandated by a code to remain energized upon loss of utility service - the decision to use one, or several, ATSs is yours to make. Without having additional information, I would suggest one ATS. What type of loads are being served? Any concerns for motor inrush? Are some loads significantly more important than others?

At the end of the day; Look at your options and cost them out. Cheapest may not be best. Sometimes a small cost premium over cheapest is worth it for a much more robust system.

Bill
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"Why not the best?"
Jimmy Carter

JM said:

The motivations for such requirements typically have to do with load prioritization and ensuring that an overload or a fault on a branch of lesser importance doesn't compromise a branch of higher importance.

Click to expand...

But isn't that where selective coordination comes in?

Hi Mbrooke.
To me, selective coordination is coordination of series connected devices.

ensuring that an overload or a fault on a branch of lesser importance doesn't compromise a branch of higher importance.

Click to expand...

To me this is separating the branches onto different ATSs. Then a fault on one branch is not seen by the protection on another, more critical branch.

Bill
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"Why not the best?"
Jimmy Carter

How do more ATSs aid in selective coordination? They don't in my book- assuring each device in series will coordinate 100% through the fault spectrum at any point in the system prevents a branch of lesser importance from taking out one of more critical importance.

I'm curious, what does PCB stand for in this context? I'm pretty sure its not printed circuit board.

EDMS Australia

Actually LVPCB, Low Voltage Power Circuit Breaker,

Second page of this document gives a nice comparison or the three different type breakers,

MikeL.

I see. That term isn't used here in that context, as they're not called Power Circuit Breakers here.

EDMS Australia

Say I'm dealing with a municipal building where we want redundancy, but its not required by code. Currently only a life safety branch for the emergency lighting is required. The rest can be fed any way.