I'm sorry Dave, but you are wrong. If this was correct: Every exterior piece of equipment in the should designed able to surive a direct lighting strike. Every car should be designed to survive a tire blow out at 55 mph without damage. Every airplane should be able to fly through a flock of...
We are moving off the topic a little, but there are always things one can do to improve the quality and durability of a design. Where does it end?
Your phrase "a design which knowingly overstresses equipment by intent" doesn't describe the situation in my mind. The odds that the system would...
You need to use Article 430 of the NEC. Table 430.250 gives indicates 94A for a 75 Hp motor and the conductor should be sized for 125% of that, or 117.5A. That's just a hair over the 75 degree table 310.16 rating of 115A for #2 AWG. I would use #1 AWG.
The NEC requirs the motor to have...
Anyone wanting to know more about this should follow the links in the first response by alehman. They were very help full. One of those links led me to Mike Holt’s forum and a post by rbalex, who signs as “Bob, Robert B. Alexander, P.E.”
http://forums.mikeholt.com/showthread.php?t=110459...
One needs to think about the sequence of events when closed transition ATS switching occurs. On a power failure there is no utility to contribute, so going to generator power is not an issue.
When the utility returns the ATS waits until it is synchronized with the generators and the ATS...
Thanks for the links. I'm glad to see that the argument is well reasoned. As I see it;
Several engineers have questioned the need to increase the ratings of all of the distribution equipment in a facility to protect against a very rare event occurring during the 0.00078% of the time the ATS...
My company designed synchronizing switchgear to bring four 1 mW generators on line to a single bus to supply power to a large hospital's emergency and stand-by systems. We calculated the available fault current from the generators and specified the short circuit withstand rating well above this...
Our technicians have been at the site and have found that I was wrong about several things. First both the switchboard and MDP main circuit breakers are Square D type JL with over current adjustment only. They were both set to max at 10X250A=2,500A.
The ATS is a single blade type with a phase...
The circuit breaker supplying the transformer is 200A, molded case without adjustment. The panelboard main circuit breaker is 250A molded case also without adjustment. The switchboard circuit breaker is a 250A electronic power breaker with long and short trip adjustments. Because this is the...
The ATS switching performed with charged springs, breaking and making in 2 or 3 cycles.
The ATS is open transifition to break the connenction to the generator before making the connection to the utility. Nothing is done to attempt to match the phase conenction.
There are two small UPS...
In general the generator output circuit breaker does not protect the generator. That is the function of the generator's operating and safety controls.
The function of the output circuit breaker is to protect the feeder the generator is supplying. I would coordinate the generator circuit...
Thanks for the reply.
I am also puzzled by the tripping of the switchboard circuit breaker which has adjustable settings that are reported to be maxed. We will verify them tomorrow. The breakers on the panelboard are thermal magnetic without adjustments and should have tripped in advance of...
The site is provided with 100% back-up generator power. After working fine for 6 months last Friday there was a power failure. The open transition ATS energized the generator and closed to it without a problem. On utility power return, the circuit breaker supplying a 150kVA transformer...
