Disconnect Fuse Switch Testing

[Jonsailor]

I used a FLuke 43B power analyzer to read the line and load from a fuse disconnect switch. The results were mind boggling because the load was 7.4 amps and the line was 14 amps. The device at the end of the circuit is a fan with a 7.4 FLA at 5hp. Currently the fuse disconnect switches has 10 amp fuses installed that blow constantly among other units. Can anyone explain how I'm getting 14 amps at the line and 7.4 at the load?

Also with a blown fuse I'm still seeing the unit draw up 6 amps on the line side but nothing on the load side?

[Jonsailor]

I used a FLuke 43B power analyzer to read the line and load from a fuse disconnect switch. The results were mind boggling because the load was 7.4 amps and the line was 14 amps. The device at the end of the circuit is a fan with a 7.4 FLA at 5hp. Currently the fuse disconnect switches has 10 amp fuses installed that blow constantly among other units. Can anyone explain how I'm getting 14 amps at the line and 7.4 at the load?

Also with a blown fuse I'm still seeing the unit draw up 6 amps on the line side but nothing on the load side?

To expand on this there are 3 phases that are connected to a motor. However, at the fuse switch the line side is jumpered over to the next 3 fuse switches that feeds to another motor. I believe this follows kirchoff law but unsure.

[anonymous]

To expand on this there are 3 phases that are connected to a motor. However, at the fuse switch the line side is jumpered over to the next 3 fuse switches that feeds to another motor. I believe this follows kirchoff law but unsure.

I'm not sure I'm understanding you correctly. Do you mean you're measuring like this? This would be completely normal.

          V (incoming to switch)
          |
          O-- 14A (line side of switch)
          |_____
       sw1 \    \ sw2
          |    |
     7A --O    O-- 7A (load side of switch)
          |    |
          M    M  (fan motors)

I hope this bad drawing makes sense. Basically if your line side measurement is upstream of where the jumper taps at the switch, then it will include current from the other switches. The load side measurement would include only one motor, but the line side would include that motor plus everything fed off the jumper.

[Jonsailor]

I'm not sure I'm understanding you correctly. Do you mean you're measuring like this? This would be completely normal.

          V (incoming to switch)
          |
          O-- 14A (line side of switch)
          |_____
       sw1 \    \ sw2
          |    |
     7A --O    O-- 7A (load side of switch)
          |    |
          M    M  (fan motors)

I hope this bad drawing makes sense. Basically if your line side measurement is upstream of where the jumper taps at the switch, then it will include current from the other switches. The load side measurement would include only one motor, but the line side would include that motor plus everything fed off the jumper.

That is correct. One thing that is questionable, is why use a fuse disconnect switches for a 3 phase motor? Wouldn't you use a 3 phase breaker so it isolates all 3 phases at once instead of isolating just one phase. The problem now becomes you still have 2 phases powering a motor which over works the motor.

[anonymous]

That is correct. One thing that is questionable, is why use a fuse disconnect switches for a 3 phase motor? Wouldn't you use a 3 phase breaker so it isolates all 3 phases at once instead of isolating just one phase. The problem now becomes you still have 2 phases powering a motor which over works the motor.

There's nothing wrong with using a 3-pole disconnect switch as a disconnecting means for a 3-phase motor. If you're talking about three separate single pole switches then that's definitely a problem. It's also okay to use properly sized fuses for short circuit protection. I recommend reviewing NEC Article 430, there are too many considerations to list them briefly.

[Jonsailor]

There's nothing wrong with using a 3-pole disconnect switch as a disconnecting means for a 3-phase motor. If you're talking about three separate single pole switches then that's definitely a problem. It's also okay to use properly sized fuses for short circuit protection. I recommend reviewing NEC Article 430, there are too many considerations to list them briefly.

So I'm experiencing a system that has multiple separate 3 phase disconnecting fuse switches. When one fuse pops you still have the other 2 phases still powering the motor. Which is why I'm thinking of replacing these with a 3 phase breaker that will trip all 3 phases at the same time.

[SecondGen]

I'm not sure I'm understanding you correctly. Do you mean you're measuring like this? This would be completely normal.

          V (incoming to switch)
          |
          O-- 14A (line side of switch)
          |_____
       sw1 \    \ sw2
          |    |
     7A --O    O-- 7A (load side of switch)
          |    |
          M    M  (fan motors)

I hope this bad drawing makes sense. Basically if your line side measurement is upstream of where the jumper taps at the switch, then it will include current from the other switches. The load side measurement would include only one motor, but the line side would include that motor plus everything fed off the jumper.

Just want to say that your plain text one-line drawing is badass. Never seen it used in the forums before.

[anonymous]

Just want to say that your plain text one-line drawing is badass. Never seen it used in the forums before.

Thanks! If you use this method, code blocks (# symbol on the editor) do monospaced fonts so it's easier to lay it out.

So I'm experiencing a system that has multiple separate 3 phase disconnecting fuse switches. When one fuse pops you still have the other 2 phases still powering the motor. Which is why I'm thinking of replacing these with a 3 phase breaker that will trip all 3 phases at the same time.

Normally when a 3-phase motor is fed through a fused disconnect, there would also be a motor starter with an overload relay downstream. Since losing a phase will cause an increase in current in the remaining phases, phase loss causes the overload relay to trip after a time delay. There are also solid state overload relays that provide phase loss protection so they trip without an intentional time delay on phase loss. If you don't have a motor starter between the disconnect and the motor, a circuit breaker is probably the cheapest way to avoid phase loss. But keep in mind this setup is only code compliant for smaller motors, and depends on other considerations such as whether the motor is continuous duty or intermittent duty, based on parts of NEC 430/440 that I'm not 100% sure of for your application.