The 2017 Edition of the National Electrical Code 240.67 is extending the requirement for Arc Energy Reduction to fuses rated 1200 Amps and larger. The details of 240.67 state that a fuse shall have a clearing time of 0.07 seconds or less at the available arcing current or else an arc energy reduction method shall be provided.

Although it does not become effective until January 1, 2020, it will be a new requirement for fuses.

Here is this week’s Question of the Week:

Does your company (clients) have fuses rated 1200A or greater?
Yes
No
Many sites/clients - it depends

This may sound strange but I've only come across one fuse manufacturer that even gets to 1200 A at least at medium voltage. Even at 480 V, there aren't very many of these. Almost the entire selling feature of triggered current limiters is because there are no fuses in the 1200 A+ range.

So is a TCL technically a "fuse"? I'd almost say yes. For those that don't know what you have is a CT, a protection relay (possibly self-powered by the CT), a piece of busbar rigged with explosive charges, and a current limiting fuse in parallel to the busbar. During normal operation, current flows through the busbar. When the protective relay triggers, it fires the explosive charges and removes the busbar from the current path, transferring everything onto the current limiting fuse. The fuse in turn then trips shortly after that.

So if we took th fancy TCL and put it in a black box and look at how it works, what we have is a current limiting fuse with a very high current limit. And when it blows, hold out your wallet...these things are very expensive to replace.

Yep. We have some fairly biggun's around here:



These are still in the crate and I'm not man enough to pick the whole crate up with one hand to take a pic like I did with the 2500A:



We have a very large dorm that was built in the early 60's with a 6000A 208Y/120V service in it. I don't know if it ever had 6000A fuses in it. I know it has 1600A fuses now though. We installed a campus-wide metering network back in 1999. After a few years of collecting data we were comfortably able to reduce a ton of main fuse sizes based upon actual load data.

Just 'cause I mentioned it. I was out and about early this morning and took a couple pics:





I'm told this switchboard did indeed have 6000A fuses in it at one time but the actual load was far less and the fuses were changed out to 1600's.

Thanks for sharing! Great photos. The largest that I have seen is 5000 Amps. I wasn't sure anyone actually used 6000 Amps.

You're welcome, Jim.

Believe it or not this dorm actually has two 6000A services (well, really feeders by NEC definition of a service. Our "service" is 34.5kV from the utility company). It was set up to be able to tie the two 6000A boards together in case one of the primaries was lost but the then "future" tie switches and 6000A feeder tie bus was never installed.

This dorm is quite large, 10 stories, 532,000 SF. But still, the company that designed it believed in over-design in every way. They didn't start using 480V until the early 70's from what I can tell. I've not done an SKM model on it because we are tearing it down starting this summer.

Image from Google: