Part of performing an arc flash study may involve reviewing existing equipment.

Many have seen/heard of some interesting/embarrassing finds when looking at existing equipment.

This week’s question is about fusible switches (low voltage/not control).

Have you encountered different fuse sizes/types/manufacturers/class in the same switch?
Yes – different sizes
Yes – different types
No/doesn’t apply
A conductor was used where a fuse belonged – extra bonus points.

SELECT UP TO 3 Stories are always welcome!

I can't recall finding different sizes but same amp rating, same class, different manufacturers yes, more than once.

The conductor question is actually legitimate. Copper bars can be a legitimate item. For instance nonfusible cutouts are just normal cutouts with copper bars installed instead of fuse links (typically SMU/DBU/CMU-20's). So as far as someone doing something illegal, that's not a good example. Different sizes for legitimate reasons also sometimes exists when the loads are single phase fed from a 3 phase system.

But yes different manufacturers and/or mixed types such as a Bussmann low peak fuse mixed with non-low peak fuses and occasionally different sizes is very common. As a motor shop engineer the biggest problem we run into is that no matter what is actually the problem, the call is always that there is something wrong with the motor. You get "this is the third motor we've put in and it keeps burning up, even after we bypassed the overload relay and put in the biggest fuses we had in the warehouse to keep it from tripping them. Your motors are junk." Wonder what happens if the maintenance workers keep burning up? Do they put in bigger ones to fix it?

Next week I'm doing fuses 101 for our sales people.

Keep in mind, the fuse almost never provides overload protection. On a typical 2-5 second motor start (inrush) the optimal fuse size is 150% to 175% of the NEC table value when using time delay fuses, In other cases a non-delay fuse can be sized at 300% with NEC exceptions, as large as 400%. In affect, the fuse is only short circuit protection and sizing should have little impact on overload protection.

Some examples I have witnessed (certainly not all inclusive):

Copper pipe (with ends flattened to fit rejection fuse holders)
Different class & amp rating (one three phase switch with fuse sizes varying from 250A to 400A, all three phases were different amp and class)
AC fuses in DC applications (fuses did not have printed DC ratings and manufacturer confirmed they were not for DC applications)
Fuses held in with zip ties as the fuse holders were too loose.
Nested adapters (30A fuse in 60A adapter in 100A adapter, the turducken of protection)

The fuses are all the same, but that's about the only thing correct in this switch...

Is MikeO'Brien your alias Joe Tedesco's ?

Read the part about motor shop engineer. Very little of what I do does not involve a motor in some way. I love it when customers oversize the protection but I'd lose customers if I didn't always offer the best protection available. So far that means fuses for Type 2 protection of the starter.

When sizing the actual motor protection you are supposed to go by the MOTOR name plate, not the NEC table. Use the table when you are sizing for components and wiring. Going by the NEC table for protection of a specific motor with it's worst case sizes guarantees the motor will be underprotected. This used to be a pain because not so much with fuses but with eutectic overloads in particular frequently you had to change them when changing motors. With today's electronic or RC overloads you just adjust the dial to the FLA and close the door.

Second more than one fuse manufacturer actually suggests using 125% of FLA for fuse sizing which gives both short circuit AND backup overload protection. I've tried it but it seems to cause some nuisance tripping since it doesn't coordinate well in many cases with the overload relay. You can't just get rid of the overload relay unless you have a thermal switch too by Code but conceptually you could.

Flexible fuses?
This is a switch manufacturing error that no one caught. Do fuses maintain their rating when bent?

The bus bars on the end shouldn't be affected as long as the fuse body itself is not altered in the process of bolting it in.

How would you feel about taking the fuse and brazing bus bars directly onto it and bolting it in rather than using fuse clips? Horrified? Well...

https://www.monsterfuses.com/search/?s=55A212942P4RB
https://generalelectricfuses.com/ge-lim ... ter-fuses/
https://www.gescan.com/products/ge-55a212942p12rb-32078

GE does it in their Limitamp starters among others.

I'm not condoning the practice, just pointing out that manufacturers abuse fuses all the time, whether it's a design decision in engineering or a design change on the shop floor.

When performed correctly, soldering, silver soldering, or even brazing, should apply very little heat to the fuse elements.

In the photos above the fuse body on the left appeared a bit distorted.

To add further insult to injury, this disconnect, fused for 400-amps, was connected to 1000 amps of rectifer capacity. This system had two identical 400-amp fused disconnects, (the other disconnect didn't need bent fuses), connecting to separate 48VDC batteries on one side and a 1000 amp rectifier plant on the other side. After a battery discharge all the fuses will clear unless the DC load exceeds 200 amps.

I’m confused? It’s DC? If so not all fuses are tested for DC so there is always a selection problem. That might be the biggest TESTED size. Generally (RMS argument) the DC rating is about 50% of the name plate AC rating. Second sounds like redundancy if it’s not a max size issue by having two battery strings. A lot of utilities do it that way. If it’s all or nothing though then it is pointless. Often with DC fuses I see a lot of goofy, rare European brands with of course CE (not third party tested) marks.

The fuses in the 2 pole switches are DC rated.
The rectifier can produce 1000 amps. The normal system load was approximately 200 amps. Under normal conditions the charge current to each battery is a few hundred milliamps. When normal AC power fails the batteries discharge to support the connected load. When AC power returns the batteries will draw as much recharge current as the rectifier can produce. If all of the load is still running, and if each battery is identical, and each battery provided the same amount of energy during discharge, and the external circuit resistance of each battery is equal; then, each battery will draw 400 amps of recharge current. This has each fuse operating at 100% of rated current for some period of time during recharge. However, if everything isn’t equal, or if the load was reduced, or if one battery was off-line for some reason, the recharge current would exceed the fuse rating and the fuse would eventually open. How fast the fuse opened would depend upon the details of the specific event.