This project is a replacement for the existing ATS. The facility has three 4000amp 3phase 480v switchboards built in the late 60s. The single generator has 6 ATS from the three different switchboards in one cabinet…not ideal at all. Furthermore, all three switchboards have to lose power before the ATSs tell the generator to come online.
We have a spare 1600 amp breaker in one of the switchboards so I designed a busduct to feed a new emergency switch gear (PZ4) main tie tie main. Only life safety and security circuits are to be on the new emergency power switch gear.
With draw out breakers, a single circuit can have maintenance without needing the entire gear de-energized. Due to the size of the generator, we can only allow for a minimum number of future use breakers. Those breakers can be used to power downstream distribution Switchboards.
I’m not a huge ATS fan either but I will beat you on both price and reliability every time.
At one end of the spectrum we have an ATS. It can have just one moving part if it’s a double throw design. They can be quite high reliability. But if it breaks the whole plant is down.
The counter argument is draw out switchgear. Let’s leave aside the UL 489/1077 argument fir a moment. In switchgear every device is s single throw device. So to switch loads or sources we need two devices. These are mounted in drawout frames/cassettes. These go into shuttered cells with some kind of control wiring mechanism. The number of parts roughly triples just going from bolt in to draw out breakers. At least 50% of the failures I encounter in the field are in the drawout mechanism. Other than lubrication issues, breaker problems are not very common except near end of life. Trip unit electronics and drawout mechanism failures constitute the bulk of failures. Don’t believe me? When is the last time you had trouble with your residential panel?
So looking at arrangements we need access to not just the breaker but the cell itself. SOME work can be done on the cell but it’s limited, too.
Plus non-drawout breakers used sealed long life bearings. Drawing breaker manuals recommend service annually and make extensive use of the drawout function. For instance with many of them the lockout point is to rack out the breaker and put a lock through the rails, thus operating the least reliable parts every time we do LOTO!!
So the real solutions are in the arrangements. With MTM we can isolate either main cell or even a main if we can accept losing hAlf the loads. Going to
MTTM also allows both ties to be serviced, again losing half the load. We can keep the loads if we go to a double MTM but that’s just a double bus or ring bus. At this point I’m just going to say we just blew up the capital budget.
Rings exist. They are implemented in Europe using an arrangement called an RMU (ring main unit). This consists of a mix of 3 switches or breakers. Build wise they are either metal enclosed or switchboards. The device can be used as two mains and a feeder…MFM. But once you have more than one breaker/switch can be for a source or a load Nothing is draw out. It doesn’t need it. The whole thing can be isolated. And they are much cheaper than drawout switchgear in double MTM or MTTM arrangements. So this shows that the equipment arrangement, particularly multiple bus or ring bus is the solution to maintenance access and reliability. And it does not need to bust the budget.
It does not need to be RMUs which are best suited to campus distribution systems but the concept of a ring or double bus arrangement and the fact that shutting down an entire section is more valuable than only the breaker is the key here. Many medium size systems use switchboards with PLC control over breakers. So it doesn’t need to be metal clad with protective relays either.
Enter the ATS. One ATS can be very reliable but can’t be serviced and when it does fail downtime is a big issue. As with switchgear the arrangement is the entire key to everything. With a double bypass arrangement there are no single points if failure and everything can be isolated with simple and cheap disconnects.
Only ONE actual ATS is required with a double bypass. The switchboards tie back in with either redundant systems or some kind of distribution, redundant or not.
This gives you the effect of a ring bus in the style and price point of an RMU, vastly less than a double MTM and more reliable.
Ok as for UL 489 vs 1077. Metal clad is vastly over rated and overpriced The isolation is very limited in real world designs. The impact on arc flash for instance is nil in actual tests. The big difference is that UL 1077 gear is for distribution. It has a 30 cycle rating so you can coordinate with ANY load. UL 489 in contrast has a 3 cycle rating. As such the breakers are required to trip in 3 cycles. Even if you turn off instantaneous in an electronic trip unit, there is always a hidden one. It can only coordinate via series combinations or ensuring that you never reach 20xLTPU. So it severely limits the downstream equipment manufacturer to the same brand at a minimum. Granted nobody considers this going in. They just look at the price tag, and somebody may argue for draw out as a personal preference not based on science. So at that point it’s all switchboards, with drawout as an option. This is where we are going towards in the distribution market as far as feeder breakers.
There are cheap 3 and more expensive 30 cycle ATSs and most aren’t involved in handling faults directly…that’s the job of fuses or breakers. They don’t do distribution…that’s the switchboards job. They handle redundancy.
Using your design goals I would put in a single switchboard fed from the utility. Bolt in is fine This feeds the 3 existing loads plus the new emergency load. The emergency load is a double bus ATS feeding another switchboard. This second switchboard probably should be drawout such as ILine since offline access is limited
This is cheaper and uses standard parts, not custom. Everything is short lead times. Since it’s not brand specific in the overall scheme supply chain issues are less of a problem. You can work on every part of the system while maintaining power on the emergency bus.
