Loose connections

[JdoubleU]

We kept having a fuse blow on phase A of a 3 phase HVAC unit. I found a loose connection at the disconnect above the fuse that kept blowing. I believe the fuse blew because of heat from the arc rather than overcurrent. Is this true.

 

[Minuteman]

The loose connection drew more current on that phase. So yes, the fuse failed because of overload.

 

[jim dungar]

We kept having a fuse blow on phase A of a 3 phase HVAC unit. I found a loose connection at the disconnect above the fuse that kept blowing. I believe the fuse blew because of heat from the arc rather than overcurrent. Is this true.

If fuses regularlly 'opened' due to loose connections, we wouldn't need to infrared scan them.

 

[Cow]

We kept having a fuse blow on phase A of a 3 phase HVAC unit. I found a loose connection at the disconnect above the fuse that kept blowing. I believe the fuse blew because of heat from the arc rather than overcurrent. Is this true.

Well, without megging the load or putting an amp clamp on it, I couldn't tell ya.

I know a loose connection could've caused the fuse to heat up and pop it, but without knowing more, no one can be sure.

 

[brian john]

More than likely the issue was a thermal related, but it must have been one hot puppy.

Did you perform an FOP test across all 3 phase, line to load?

 

[kwired]

Loose connection on line side of disconnect often doesn't have enough heat make it to the fuse to blow the fuse. May want to make sure the switch is not the weak connection and the loose lug was a casualty of the heat. If so the problems are not over. If the lug or switch blades are discolored the problems are not over either.

 

[brian john]

Loose connection on line side of disconnect often doesn't have enough heat make it to the fuse to blow the fuse. May want to make sure the switch is not the weak connection and the loose lug was a casualty of the heat. If so the problems are not over. If the lug or switch blades are discolored the problems are not over either.

Hence why the first thing I would do (after making sure everything is safe and donning proper PPE) is a visual and then a FOP, or in my case de-energize and utilize a Ductor/DLRO/Micro-ohm meter.

 

[zog]

The loose connection drew more current on that phase. So yes, the fuse failed because of overload.

How would a loose connection cause more current flow?

 

[RichB]

Hmm--Brings back memories--at a couple of places I have been in the past we used fuses with replacable links--always seemed to have randon blown fuses--started with the investigating thing--then went to IR --and what I found with the IR was the fuses themselves were hot---hhhmmmmm me thinks-- went around asking and watching how the other 'lectricians were changing blown fuses--they just put in a new link , spun the end caps on handtight and stuck them back in---did a cost survey and decided( less then 5 minutes into the survey --which I conducted while standing there) to go to non replacable link fuses-once we convinced the head of maintenance-99% of the problem was solved

 

[MadWulf]

How would a loose connection cause more current flow?

The energy to create the heat had to come from somewhere.

 

[kwired]

The energy to create the heat had to come from somewhere.

The energy comes from the power source. The loose connection ends up being a resistance in series with the normal load.

Putting a resistance in series with the load will reduce the voltage applied to the load. True a motor will draw more current if the voltage is reduced, but the resistance of the bad connection will also limit amount of current flow. If anything the other two phases will see an increase in current since the motor in this thread was a three phase motor. A single phase motor will see the bad connection as a current limiting device.

 

[kwired]

The energy to create the heat had to come from somewhere.

The energy comes from the power source.

Resistance of the bad connection since it is in series with the load will lower voltage applied to the load. True a motor will draw more current if voltage is reduced, but the resistance of the bad connection will also limit how much it can draw to some degree. Since the motor that is topic of this thread is three phase motor it is likely that the other two phases are where an increase in current will take place.

Something goofy going on here, hopefully this does not create double post.

Looks like it did, for some reason I was not able to see post 11 before.

 

[JdoubleU]

Thank you for all your input. I changed out the disconnect and will post later the results. I do have a question in relation though. How does a loose connection cause more current flow. In theory isn't the loose connection acting as a resistor to current.

 

[brian john]

Thank you for all your input. I changed out the disconnect and will post later the results. I do have a question in relation though. How does a loose connection cause more current flow. In theory isn't the loose connection acting as a resistor to current.

So you randomly change a piece of equipment without know why?

I understand the need to keep production going and the logic in replacing. BUT NOT SPENDING 1 minute to investigate why it needed replacing totally baffles me.

 

[kwired]

Thank you for all your input. I changed out the disconnect and will post later the results. I do have a question in relation though. How does a loose connection cause more current flow. In theory isn't the loose connection acting as a resistor to current.

Absolutely correct.