The Little Things Count

I come across this issue more than I should. I'm not sure if it is due to "what is on the truck" syndrome but probably once a quarter someone on our team will report finding improperly sized fuses installed on the electrical disconnect to a mechanical unit. The RTU powered exhaust units that are powered by the disconnect in the photo above were supposed to have 60-amp fuses. Somehow, they received 25-amp fuses.

This mechanical contractor decided to take every shortcut they could find so it wasn't very surprising to find that the fuses were not done correctly. In fact, the wrong fuses and contractors taking shortcuts in the rest of the work do seem to have, at least at some level, a correlation.

In this scenario, the RTU will, at some point, go into economizer, the powered exhaust will turn on to maintain building pressure and then the fuses will likely overamp and break. The building will become over pressurized, and the doors will stand open, possibly whistle, maybe the elevator doors won't close if it is really bad. Maintenance will investigate, determine the powered exhaust failed and then likely replace the fuses like in kind. This will happen again once or twice. After that, the maintenance team will wonder why the fuses keep popping and take a look at the mechanical and/or electrical plans to find out that the fuse size is wrong. They will replace with the proper fuse, hopefully it will fit in the existing disconnect (unlike this example), and all will be well.

Except this isn't okay. The Owner's staff has now chased their tails trying to figure out what is wrong, wasted time they could have been doing preventative maintenance or some other value-added task and spent even more of the Owner's money on this RTU install.

When the opposite happens, and the fuses are larger than they are supposed to be, we now have a situation where equipment protection, or worse yet, people are not protected like they should be.

Always remember, if you are going to check fuse sizes, it is recommended the downstream circuit breaker is locked out and tagged out. If the disconnect is shut off but the circuit breaker is not, the line side of the disconnect is still hot.

Stay safe out there!