# Fuses after Circuit Breakers



## Fixastang (Sep 4, 2012)

I have worked at a WWTP for the past 2 years as the electrician and I have noticed this plant has several pieces of equipment where they have connected a pump or other motor in an odd manner. They start in the traditional manner of going through a circuit breaker, to the across the line starter, through the heaters/thermals then out into the field. Next, they have connected the motor leads to a local fused (using low peak fuses) disconnect then onto the motor/pump. There are other areas of the plant where they have used a local disconnect in lieu of the fused disconnect. The MCC bucket has a new Diversified Electronic phase monitor installed, but it has never reflected an issue when we have trouble with this pump. 

We have been having problems with a completely submersed 480v, 3 phase, 4 amp, 3HP grinder style submersible pump failing after about 3 months of use. We are using Barnes pumps and have similar pumps which have been installed for up to 10 years without giving us any problems at all. All of the pumps end up succumbing to failures including locked rotors or shorted windings. We installed a new pump in early July of this year, and it blew two 5 amp fuses and tripped the mcc thermals. 

The pump was checked and nothing was found inside the pump. It meggs out at 300M and is drawing about 2.5 amps after we replaced the fuses and reset the mcc thermals. We replaced the mcc thermals in July when we installed this pump. We know the line the pump discharges to is clear as it goes into a effluent sampler assembly.

This is one of the many oddities I have seen at this plant as it appears some of the prior "electrical personnel" performed some creative wiring in other areas of the motor control circuits.

I am wondering if the combination of fuses and circuit breaker is causing these issues. I also wonder if anyone else has seen this combination of motor protection on their jobs. 

Please let me know your thoughts regarding this matter. Thanks.


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## JRaef (Mar 23, 2009)

The fuses are redundant (assuming the MCC bucket has a breaker or fused disconnect itself), therefore a waste of money really, but the existence of the fuses themselves would not be a cause for motor failure. I would not waste a whole lot of time on that aspect of it. 

But I would do a thermal scan (if you have one) to make sure you don't have a bad fuse clip or other resistance problem in the disconnect switch. Having a resistance in that switch could cause a motor current imbalance, but one that might not be seen by the MCC starter or the Diversified unit (which is voltage based anyway, so practically useless for motor imbalance protection). When a motor sees a current imbalance, it causes additional heating that is more than what the current would indicate, meaning you can overheat the motor without ever tripping the overload relay. Adding superfluous devices, such as those fuses, increases the risk of something like this.

Also, make sure you have the correct heater elements in your starter. #1 cause of motor failure in the US is improperly selected heater elements. People fail to read the manuals for the starters, the part that says _*"Select the heater element for the motor FLA"*_. Instead, they read the NEC, which is talking about generalities, and see the part about how the OL protection CAN be starting at 125% of the FLA, so they take FLA x 1.25 and THEN buy the heaters. What they don't realize is that the OL relay mfr _*had ALREADY factored the 125% into their selection chart*_, that's why they SAID, *use the FLA*. 

I'm no longer shocked at how many people do this exact thing, but I know that motor rewind shops absolutely LOVE it!


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## Fixastang (Sep 4, 2012)

*Yep, we do it wrong too....*

A majority of our equipment is Square D. The Mcc buckets have a 7 amp breaker in them, then the size 00 starter, complete with thermals that have been oversized. The motor draws 4 amps, so we have put in thermals for a 5 amp draw. We did the same thing with the fuses, we took the FLA and multiplied it by 1.3 as the main boss said to do this. Therefore, we have 6 amp fuses AFTER the motor thermals.

I will see if we have thermal images of the disconnect as we just completed our thermal imaging survey for the year. The fuse clips all seem tight, but one never knows.

Thanks again for your prompt response and for helping educate those of us who do not fully understand these topics.

Very Respectfully,

Kurt.:thumbup:


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## micromind (Aug 11, 2007)

The guy who said to protect a 4 amp motor with 5 amp heaters is an idiot, and should be paying for the burnt up pumps out of his own pocket. 

A slight voltage imbalance will cause a large current imbalance, and if the heaters are oversized, the motor will burn up. 

To properly select heaters, several factors need to be considered;

1) Use the right table in the manufacturers literature. This seems to be a very common mistake, a number of factors go into using the right table. 

2) Temperature difference at the starter vs. the motor. If the starter is in a cool room and the motor is in a hot environment, using the standard heater will not protect the motor. 

3) If properly selected heaters trip, find the problem; never increase the heater in order to make it not trip. It's there for a reason. Measure current, using a recording meter if needed, before making any adjustments to overloads. 

Another commonly overlooked factor in protecting motors; voltage. And even more important, voltage balance. Always measure voltage as close to the motor as you can get. Measure fall of potential across all devices in the circuit. The breaker, the starter, the disconnect, the fuses (including the clips), even the bus clips of the MCC. Do this test cold as well as after running for a while.


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## Jlarson (Jun 28, 2009)

The small Barnes pumps are not at all tolerant of oversized heaters. They also do better when the liquid level is kept above the lifting bail to help cool them. 

I agree with doing an IR scan, the logger and FOP everything, check all field splices between the MCC and pumps too.


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## Southeast Power (Jan 18, 2009)

Fixastang said:


> I have worked at a WWTP for the past 2 years as the electrician and I have noticed this plant has several pieces of equipment where they have connected a pump or other motor in an odd manner. They start in the traditional manner of going through a circuit breaker, to the across the line starter, through the heaters/thermals then out into the field. Next, they have connected the motor leads to a local fused (using low peak fuses) disconnect then onto the motor/pump. There are other areas of the plant where they have used a local disconnect in lieu of the fused disconnect. The MCC bucket has a new Diversified Electronic phase monitor installed, but it has never reflected an issue when we have trouble with this pump.
> 
> We have been having problems with a completely submersed 480v, 3 phase, 4 amp, 3HP grinder style submersible pump failing after about 3 months of use. We are using Barnes pumps and have similar pumps which have been installed for up to 10 years without giving us any problems at all. All of the pumps end up succumbing to failures including locked rotors or shorted windings. We installed a new pump in early July of this year, and it blew two 5 amp fuses and tripped the mcc thermals.
> 
> ...


I would imagine that the designer added them to the line up for fast acting short circuit protection.fuses might also be specified by the manufacturer for warranty.
So my official answer "dontknow"


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## varmit (Apr 19, 2009)

The fuseable disconnects could have been more available, when installed, than nonfused. I see this a lot.

The fuse clips would be my first thing to examine for looseness or corrosion.


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## tates1882 (Sep 3, 2010)

My vote is high resistance in the cable going to the pump. I typical see so cord that is water logged and one of the conductors is going to hell. Or more common on fresh water wells the connections from pump to cable are just crimp connectors and heat shrink and have started to fail due to voltage leakage/water logged.


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## John Valdes (May 17, 2007)

Sometimes the electrician or buyer orders a disconnect and forgets to ask for a non-fusible type. The purchaser may not know the difference and they get what they don't really need.

These are buying/purchasing errors and most likely the mistakes were made by the in house guys when ordering.
There is no need for fuses and circuit breakers for the same circuit.
Every fused disconnect protected by a properly sized breaker could be a non-fused type unless they have a specific reason to do so.


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## JRaef (Mar 23, 2009)

A simple to execute first test would be to look at current on the three phases at the MCC bucket where the wires go into that MCP breaker, then read the currents on all three phases where the wires come out of the local disconnect going to the motor. Compare those values and you should see any imbalance that is being created in the circuit components in between. Do this with the pump running at full flow. You won't know exactly where the problem component is from that, but you will know where to look.

Back to the OL issue. Check the manual for the pump, but most likely they will dictate that you must use Class 10 quick trip overload protection and there is no service factor (or SF = 1.0). If when the MCC was ordered originally, that was not specifically asked for, your OL will be Class 20, and the heater chart for it will assume 1.15SF. So for your own sake, get the correct OL heaters. 5A on a motor rated 4A FLC means the heater is ALREADY 25% too big, so that relay will not even BEGIN to count down to trip until the current is at 156% of FLC, then from that point it will still take around 2hours to trip. That motor is not designed to take that kind of abuse.

Side issue #2 (or is it 3 now?); flow. If, when you do the above, you find that the pump is drawing balanced current but more than 4A in all 3 phases, that might indicate a severe leak somewhere. Load on a centrifugal pump motor is based on flow. If a pipe breaks, or leaks significantly, flow increases and the pump overloads. Then because you have inappropriate overload selection, it is allowing that pump to burn itself out in attempting to do its job.


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## Big John (May 23, 2010)

I like non-fused disconnects because in the maintenance world it's one less point of failure that needs to be maintained and troubleshot. But often they were a special-order component, so if it was spur-of-the-moment install, I would have to put in fused discos.

The only time I see circuit breakers used necessarily in conjunction with fuses is when a circuit has a very high available fault current. In which case current-limited fuses can be used in front of the breaker to obtain a lower series rating so that the breaker doesn't need as high an AIC.


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## Fixastang (Sep 4, 2012)

Many thanks to everyone for their input regarding this issue. I intend on following JRaef's recommendation to check the current the top and bottom of the starter as well as the top and bottom of the fused disconnect. I will post the results to let everyone know the readings at this location.

JRaef is also correct in stating this is mcc probably uses Class 20 overloads. The mcc is either a Square D series Four or Five Mcc. We are looking into installing either a different contactor complete with Class 10 Overloads or removing the Class 20 overload block from the bottom of the contactor and installing a DIN rail mounted Class 10 overloads.

The pump is continuously submerged under at least 10 feet of incoming sewer water and our first point of connection is inside the fused disconnect. We always replace the cable with the pump. I am not a huge fan of using butt splices anywhere unless absolutely necessary. The pump is also located outside in a wet well that is adjacent to the MCC which is located in the basement of the building next to the wet well.

I am also going to review the literature from Barnes to determine if they have a specific recommendation regarding how the overloads should be sized. I am inclined to believe the information others have posted regarding the overloads being the wrong size as we have found areas of the plant where 60 amp fuses were installed on 1/2 HP motors as the previous plant maintenance personnel could not determine why the correct size fuses were blowing, so they just increased the fuse size until the problem went away. We are slowly correcting these problems as we find them as well as changing the wiring to include the proper motor thermal and moisture detection equipment.

Again, many thanks for the input thus far and feel free to continue to add comments. I am grateful so many of you are willing to share your knowledge to help others learn.


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## JRaef (Mar 23, 2009)

Just to be clear, the pump manual will likely show you what CLASS of overload relay to use, and 99% of submersibles will tell you Class 10. The actual rules on selecting heaters (or setting the dial) on the the OL relay will be part of the RELAY manufacturer's instructions.

And a recommendation: Use a Solid State Over Load relay. Sq. D has one called a Motorlogic that can directly replace a standard OL relay on an 8536 starter, which is what you will have in a Model 4 or Model 5 MCC. The reason I recommend them for things like this is that they provide better phase loss and phase *current* imbalance protection. They can be selected in the field for Class 10, 20 or 30 trip curves as well.


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## Fixastang (Sep 4, 2012)

*Amp/Volt Readings*

Here are the Amp and Volt readings I took using a Fluke T5-1000 just a few minutes ago:

Bottom of 7 Amp Circuit Breaker

Amps - A phase 3.7 B phase 3.5 C Phase 3.2
Volts - A-B 486V B-C 488V A-C 488V

Bottom of Nema Size 1 Starter using new B6.25 thermals sized per Square D sticker on side of cabinet:

Amps - A Phase 3.5A B Phase 3.4A C phase 3.3A
Volts - A-B 484V B-C 484V A-C 487V

Top of Bussman LPS-RK-6SP fuses

Amps - A Phase - Unmeasurable due to wire location B Phase 3.6A C Phase 3.4A
Volts - A-B 484v B-C 487v A-C 486v

Bottom of Bussman LPS-RK-6SP fuses

Amps - A and B phase - unmeasurable due to wire/conduit locations C Phase 3.4
Volts - A-B 484v B-C 487v A-C 486v

I could not measure the amps at the bottom of the fuses as the wire was too tight to fit the Fluke T5-1000 between it and the bottom of the fuse holder on both A and B phases.

Please let me know your thoughts regarding these readings.

Kurt.:blink:


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## tates1882 (Sep 3, 2010)

Fixastang said:


> Here are the Amp and Volt readings I took using a Fluke T5-1000 just a few minutes ago:
> 
> Bottom of 7 Amp Circuit Breaker
> 
> ...


Looks good to me. the current imbalance is proportional to the voltage imbalance.


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