Tripping breaker

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Djglennon62

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Dubuque, Iowa
I have a 60A 3 phase breaker feeding 2 50A 3 phase breakers, that are protecting 2 sewage pumps, that operate alternately. What would cause the 60A breaker to trip before the 50A breaker?
 
When it comes to dead shorts, or exceedingly high amp draws, most breakers will trip closely at the same rate. It's not like the current slowly ramps up over a couple seconds. If the circuit is purring along at, say, 35 amps, then suddenly a short develops, the current spikes within one, maybe two, cycles. If there's 8,000 amps at that point, it matters not whether the breaker is 50 or 60, or 15, or 400. They all will react the same. It's just a matter of which one noses out the others.
 
ramsy said:
3ph 60A sees 2 pumps, or two 1ph 50A breakers see 1 pump each
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He said it's on an alternator, so (presumably) only one at a time .

But this brings up another point, in that if Pump #1 is running for a long time, the 60A breaker thermal sensing elements have been "seeing" that pump current the whole time. Pump #1 shuts down and if Pump #2 starts up almost immediately, the 60A breaker thermal sensing elements are still hot because of running Pump #1, then it has to deal with the starting current of Pump #2. The 50A breaker for Pump #2 however is cold because it has been doing doodley-squat the whole time Pump #1 was running. The starting current for Pump #2 is no problem for it yet that starting current for Pump #2 is ADDED onto the thermal state of the 60A breaker, so it trips but not the 50A breaker.
 
ramsy said:
3ph 60A sees 2 pumps, or two 1ph 50A breakers see 1 pump each
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OP says 3 phase

Jraef said:
He said it's on an alternator, so (presumably) only one at a time .

But this brings up another point, in that if Pump #1 is running for a long time, the 60A breaker thermal sensing elements have been "seeing" that pump current the whole time. Pump #1 shuts down and if Pump #2 starts up almost immediately, the 60A breaker thermal sensing elements are still hot because of running Pump #1, then it has to deal with the starting current of Pump #2. The 50A breaker for Pump #2 however is cold because it has been doing doodley-squat the whole time Pump #1 was running. The starting current for Pump #2 is no problem for it yet that starting current for Pump #2 is ADDED onto the thermal state of the 60A breaker, so it trips but not the 50A breaker.
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Could be , I'd place my money on a bad pump or something failing in the controls like a bad starter contacts. I suppose this is a sewage lift station?
I'd look at everything, I've seen every scenario from bad contacts on the motor starter, bad alternator circuit.
 
I would find out why the pumps are tripping in the first place. 2 60 horse setup as lead/ lag sound like a wet well or biosolid pumping from a clarifier or screen dewatering... they may be getting ragged up, or in the case of dissolved air flotation or gravity belt thickener, too much polymer can make the sludge too thick and difficult to pump.

Pumps tripping at a Wastewater Plant is actually a very rare event, look into the reason why they are tripping, not so much as which breaker is tripping.

Welcome to The Forum
 
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Jraef said:
..if Pump #2 starts up almost immediately, the 60A breaker thermal sensing elements are still hot because of running Pump #1, then it has to deal with the starting current of Pump #2..
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Yes, an overload seems more likely to me if OCP stays closed during startup, since inrush is the most stressful and likely event to trigger short-duration faulting.

Overload can also occur if controls just alternate pumps without an interlocked disconnect to Snubber. Control only opens 1 leg, so for a short time alternating motors share legs, and 60A breaker deals with 2 motors simultaneously. One motor's inductive kick at shutdown increases the next motors inrush at start up.

Investigate replacing 60A Breaker with duel-element short-circuit fuses, relying on overload heaters internal to motor(s).
 
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I agree with Jraef on considering the fact the 60 amp breaker is seeing twice as many starts as each 50 is seeing. If starting frequently enough the 60 is already warmer then the 50 amp breakers which can increase it's chances of tripping on a startup.

Sometimes these alternating setups also are a lead - lag setup - if first pump fails or can't keep up second one starts - that could also cause tripping of the 60 if there is already one running then the other starts.
 
Yes what we don't know is what we dont know.
However we do know this is a sewage lift station as per OP. The way these work is that they will pump down to a level below that of the float level of the turn on. I would think there is plenty of time between starts for the breaker to turn off.
I would think that one of the grinder pumps has a issue or the circuit 60 amp breaker or related connections is failing.

That is my take. :thumbsup:
 
Just my take on why I would be looking for an operational or maintenance problem versus electrical... the waste water plants I worked at were designed by ch2m Hill or other large engineering firms. The capital cost of these plants can exceed 9 figures, and the electrical systems are usually spot on. Unless the pumps were changed, or the electrical was done in-house, I find it unlikely that the design is wrong.

There could be an issue with ultrasonic level indicators or float switches causing rapid cycling, and the 60 amp breaker would already be warm and possible trip when the other pump comes on.

I agree that we need more information from the original poster, such as do both pumps ever run at the same time, how many starts per hour, amperage readings, etcetera
 
JFletcher said:
Just my take on why I would be looking for an operational or maintenance problem versus electrical... the waste water plants I worked at were designed by ch2m Hill or other large engineering firms. The capital cost of these plants can exceed 9 figures, and the electrical systems are usually spot on. Unless the pumps were changed, or the electrical was done in-house, I find it unlikely that the design is wrong.

There could be an issue with ultrasonic level indicators or float switches causing rapid cycling, and the 60 amp breaker would already be warm and possible trip when the other pump comes on.

I agree that we need more information from the original poster, such as do both pumps ever run at the same time, how many starts per hour, amperage readings, etcetera
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There are also facilities that have age on them and maybe when equipment has failed didn't get replaced with identical components to what was originally there. Then there is sometimes the maintenance guy/operator that changes things thinking his idea is better than what was there before.
 
JFletcher said:
Just my take on why I would be looking for an operational or maintenance problem versus electrical... the waste water plants I worked at were designed by ch2m Hill or other large engineering firms. The capital cost of these plants can exceed 9 figures, and the electrical systems are usually spot on. Unless the pumps were changed, or the electrical was done in-house, I find it unlikely that the design is wrong.

There could be an issue with ultrasonic level indicators or float switches causing rapid cycling, and the 60 amp breaker would already be warm and possible trip when the other pump comes on.

I agree that we need more information from the original poster, such as do both pumps ever run at the same time, how many starts per hour, amperage readings, etcetera
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They can be wrong.
We installed a small system similar to the OPs some years with the exception we had a fused SE disconnect ahead of the supplied controller. It was blowing the 60 amp fuses. I had the same question as OP. Why? I talked to the area Bussman fuse rep and we came up with the cycle time was too short. Telling that to the engineer that designed the whole thing was not met well. It took a few months but they changed the float levels. No problems since, other than the occasional 2x4 and underwear that don’t make it through.
 
ptonsparky said:
They can be wrong.
We installed a small system similar to the OPs some years with the exception we had a fused SE disconnect ahead of the supplied controller. It was blowing the 60 amp fuses. I had the same question as OP. Why? I talked to the area Bussman fuse rep and we came up with the cycle time was too short. Telling that to the engineer that designed the whole thing was not met well. It took a few months but they changed the float levels. No problems since, other than the occasional 2x4 and underwear that don’t make it through.
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Yeah, "they" can be wrong. I worked at Nansemond plant when it was undergoing its Phase 1 upgrade, a 100 million dollar project. All the electrical worked well, save for the 6 26 kilowatt lighting towers that all came on at one time. That 156+ kilowatt dead load on the system off kicked our centrifuge offline on under-voltage. Restarting the centrifuge would kick the towers (or an aeration blower) offline, and the vicious cycle was born. It took the better part of the year for them to fix the problem.
 
You are assuming this is part of a "plant". I've seen a LOT of lift stations that were done with no EE involved because the station came packaged by a vendor, then it's simply an installing electrician who decides on the feeder. I've seen this situation more than once, where that EC decides that since only one pump can run, he doesn't need a bigger main, not thinking about the effect of subsequent starts on that main.
 
I've designed more than a few sewage pump stations
elec, mech and sanitary design
in my experience they are typically over sized for peak demand and storm water infiltration

I designed for a duty cycle of 25-30% avg
10-20 low flow
60-70 peak

another consideration is retention time, in both the wet well and force main
too long = septic
you want to evacuate the fm every 4-6 hrs since no air in line
it is a balance: too big = septic, too small friction loss, wasted hp

the whole system needs reviewd
piping
well size
pump size
controls
not only elec

could be as simple as adjusting the level control
 
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