200E fuse selectable with 150E


So I'm working on finding out what the available fault current is BUT, I've got a very simple 12kV oneline comprised of a single incomming line and two outgoing feeders each to a 2500kVA transformer. Switchgear is to be S&C type fusible switch. So, it's a given that I'll have 150E fuses for the two feeders. The question becomes what is the minimum size upstream breaker I can use and be selective. Would like to keep this as small as possible owing to some bottle necks I'm running into with the utility. Would like to make it therefore 200E. Given a peak load of 4.2MVA and given that that equates to 198A at 12.2kV voltage we're at, a 200E fuse ought to work fine. My concern is the inrush of potentailly two 2500kVA transformers starting up at the same time. If I take that at 2 times 12 times FLA, I get 3000A for an inrush point. That right about on the 200E line. Am I getting carried away worrying about this fuse tripping in the event of a utility outage and then restart? Besides, isn't it only 12 times the FLA on the first start up ever and after that considerably less.




Curmudgeon still using printed IEEE Color Books


IEEE Std 242 Protection and coordination of Industrial and Commercial Power Systems (Buff)

... I get 3000A for an inrush point.That right about on the 200E line.
Yep, that's true

....Am I getting carried away worrying about this fuse tripping in the event of a utility outage and then restart?

Besides, isn't it only 12 times the FLA on the first start up ever and after that considerably less.
IEEE 242 says to figure on 8x - 12x FLA for coordination. They don't consider that it will ever back off
  1. I rarely ever design any of this. Most of my time is spent getting it to work the first time and then keeping it as close as practicable to 100% up-time while running 100+%
  2. Most of my clients would have a freaking fit if any of the critical circuits ever tripped.
  3. For the non-critical circuits, they would have a slightly lower grade fit if they could still remember the last time that one had tripped.
  4. Internet advice is worth what you pay for it. Except mine - it is worth a bit less.
I figure the coordination on 12X inrush.

Littlefuse application guide recommends 140% for E rated fuses. 150E is slightly under that. I would consider 175E for the individual feeders.

As for the 200E for the main, the 3000A inrush hits the curve at 40 ms. That's really tight. I would consider 300E. That puts the 3000A trip out to 200 ms. If the utility won't de-bottle neck that far, consider 250E. That puts the 3000A trip out to 100ms

Consider that a set of fuses is ~$6000. And if one opens, the other two were really stressed. Best to change all three. Also consider down-time to replace. And god forbid, what happens if one opens on the next energization. As for the sun, moon, and stars having to line up for this to happen, yeah, they do that (as in line up). This is where the worm would turn and start to dig.

How often do you get to open these before someone notices?

And I'm thinking you already knew all this. Is there additional context?

the worm


Dear Worm - Thank you for taking the time to write that very thorough response. Going with something larger than 200E is, we are being told by the utility, going to trigger some major upgrades on their end and at the expense of my client. So I was looking for a sanity check to make sure I wasn't being cavalier with our clients money. Your input was spot on and very helpful! Some of it I had thought of - some of it I hadn't. For example, I agree that from a strictly perception point of view, we NEVER want to blue fuses or trip breakers on startup no matter what we know about subsequent startups. What I hadn't considered was that not only is the perception bad but that it is a 6000 dollar proposition. Great input - thanks again.


Curmudgeon still using printed IEEE Color Books
You are welcome. (Don't forget I'm pretty clueless from my side of your monitor)

Some additional thoughts:
Consider asking the utility exactly what are the issues. You are installing a process that requires 4.2 MVA. There is something odd if the utility can't supply that - Utility transformer is small, small transmission lines, end of a long run?? If you know what is bugging them perhaps there are mitting measures.

4.2 MVA is a nice tight number. Sounds well engineered. So what happens next year when the client figures out how to get another +10% on the production knob? Most of my clients fall between 50% and 80%. At 50% there is a lot of money laying on the table in capital costs for equipment that isn't making any money. At 80% the equipment is running hot and there is nowhere to go when they want their +10%. 4.2 MVA on two 2500KVA xfm is 84%. Yeah that is getting up there. Utilities don't mind running their gear hot enough the make the rain steam and occasionally glowing transmission lines - user clients, not so much.

So where am I going with this one. Depending on the quality of service, expected longevity, expected maintenance, ROI, if the client needs the power, they get to pay the bill.

And again, I'm pretty sure you knew all of this.

This next one is out of the box.
The inspiration is from Apollo 13, NASA working out how to start up the command module with the limited power availability.
Replace the 200A main, (200E fuses) with a circuit breaker. For currents less than 3500A, set the trip to delay 100ms on startup. Over 3500A, trip with no intentional delay. Set the LT for 100%, 200A.

Interesting project. Let us know how it comes out