OT Tripping pump breaker

rta53 · Dec 7, 2005

We go a call from a customer this past weekend saying a pump was down on their water system. The service technician found that the feeder breaker had tripped. He reset the breaker but it tripped again the next day. The pump is a Grundfos submersible type although it is mounted on a frame and is not submersed in water. It is 17.5 HP 3-phase 480VAC. The FLA is 42. The running amps on each phase is around 33. When he checked the starting current 2 of the phases was around 77 amps but one was 95. The pump has been in operation for 4 years with no problems. One thing the tech told me was that the feeder breaker is 80 Amps. According to the NEC this could be sized at 100 Amps. I was expecting to see a difference in the running current but there was none. Could the higher starting current in one of the phases indicate a pump motor problem or is this normal? I am considering just replacing the 80 Amp breaker with a 100 Amp since this is what it should have been in the first place. What do you think?

theDave2 · Dec 7, 2005

According to the NEC this could be sized at 100 Amps

Please give the article number that supports that.

Is the breaker the only OCPD for the motor ? Is there not a starter with overload for it ?

The wire from the breaker to the motor has to be sized (& derated depending on many conditions) high enough for a 100 amp breaker.

Find the cause of the trips. It may be just a loose wire somewhere. Check ALL connections. Check upstream of the breaker too incase the feed is iffy. If you can't find anything, then replacing the breaker with the same size as it could be going.

elevmike · Dec 7, 2005

17.5 hp 480vac 3 phase. Ok so my little chart here shows 25 FLA & maybe a 50 or 60 amp breaker. However your specs (voltage & current & breaker size)match a 15 hp motor @ 230 volts right in the dime.

rta53 · Dec 7, 2005

17.5 hp 480vac 3 phase. Ok so my little chart here shows 25 FLA & maybe a 50 or 60 amp breaker. However your specs (voltage & current & breaker size)match a 15 hp motor @ 230 volts right in the dime.

First of all I need to correct the HP rating. It is actually 25HP and the FLA is indeed 42. The pump motors that we typically use almost never match up with standard motor HP charts.

As for the NEC, if you look at Table 430-52 you will see that the code allows Inverse Time Breakers to be rated at 250% of Full-Load current. If you look at Exception No. 2 the code allows you to go even higher if the breaker size is not sufficient for the starting current of the motor.

elevmike · Dec 7, 2005

That's better.

If you have only one high current leg, it may indicate a partial ground. Maybe a current leak through the insulation or tape on the motor connections?? Two high legs may indicate a short in the windings but no ground. Time to do a megger test, and check the connection box on the motor.

Tark · Dec 7, 2005

As elvmike pointed out, I would megger the motor first and check connections, if all looks good this is the NEC code that will allow you to increase the size of the breaker, and how it is computed.

When determining the ampere ratings for conductors, switches, and short-circuit and ground-fault protection devices you need to use the values given in Tables 430.247, 430.248, 430.249 and 430.250, not the FLA of the motor, Article 430.6(A)(1) You only use the FLA in determining the overload protection, Article 430.6(A)(2).

So assuming you have an Induction-Type Squirrel Cage motor, a 25HP @ 480V is 34amps. Using an Inverse Time Breaker, 34 x 2.5 = 85amps. 85 is not a standard size as shown in Article 240.6, so using 430.52(C)(1) Exception No.1, you could go to a 90amp breaker to start, not a 100.

Note that 430.52(C)(1) Exception No.2 states “Where the rating specified in Table 430.52, as modified by Exception No.1, is not sufficient for the starting current of the motor:..” So if you are using an 80amp breaker, you are not using a rating as modified by Exception No.1. The code is saying that you need to try a 90amp breaker first, if that is not sufficient then you can use a larger breaker, as long as that breaker does not exceed 400 percent of the full load currents, which in this case would be 34 x 4 = 136amps. Note that you are not permitted to use the next highest standard size in this case.

MikeW · Dec 7, 2005

Something's amiss...

This pump has been running for 4 years with no problems and now is tripping the breaker, something has changed. Don't upsize the CB as this will just postpone the eventual failure, whether it be pump motor, loose wiring connection, etc.

elevmike is on the right course, it's time to roll up your sleeves and figure out "what has changed" using standard testing methods.

If this is a submersible pump and has only been in the air, perhaps the motor insulation has broke down over four years of use since there probably was less heat transfer from the pump/ motor to the ambient than Grundfos designed it for. See what the fluid temperature range specifications are for the pump to see if this may be a contributing factor.

elevmike · Dec 7, 2005

MikeW said:
If this is a submersible pump and has only been in the air, perhaps the motor insulation has broke down over four years of use since there probably was less heat transfer from the pump/ motor to the ambient than Grundfos designed it for.

That's a good point! The motor is designed to be submerged, so cooling the motor takes place via contact with the fluid, not a fan. It's likely the motor was running hot and now it's shot, or at least short lived.

rta53 · Dec 7, 2005

elevmike said:
That's a good point! The motor is designed to be submerged, so cooling the motor takes place via contact with the fluid, not a fan. It's likely the motor was running hot and now it's shot, or at least short lived.

Just to clarify. These Grundfos pumps are factory modified standard submersible pumps. Pump and motor are housed in a stainless steel sleeve with water lubricated bearings.

Gerry M · Dec 7, 2005

Definitely being out of water is a problem.

Just curious,

If a submersible pump is not in the water, how are you pumping fluid? I assume this is a special design?

Ahh! the old tube trick!! That sounds better!!

Tark · Dec 7, 2005

Whoops. I failed to see in your orginal post that the pump has been running for 4 years. Don't change the breaker size, as everyone else has posted, there is something else going on.

leitmotif · Dec 7, 2005

Will reply after each statment to gt em all

rta53 said:
We go a call from a customer this past weekend saying a pump was down on their water system. The service technician found that the feeder breaker had tripped. He reset the breaker but it tripped again the next day.
REPLY OK the pump is starting OK runs for a while then trips the breaker??

The pump is a Grundfos submersible type although it is mounted on a frame and is not submersed in water. It is 17.5 HP 3-phase 480VAC.
REPLY Are you sure this is the REALLY correct installation for theis pump?? Granted it has run for 4 years BUT ,,,,,,,

The FLA is 42. The running amps on each phase is around 33. When he checked the starting current 2 of the phases was around 77 amps but one was 95.
REPLY I would start saving money. With this much imbalance on starting current something is wrong somewhere. What gets me is the the running currents are fairly close. I would take another set of clampons and get a real good idea of waht the run current is and starting current - possible he misread the clampon??

The pump has been in operation for 4 years with no problems. One thing the tech told me was that the feeder breaker is 80 Amps. According to the NEC this could be sized at 100 Amps. I was expecting to see a difference in the running current but there was none. Could the higher starting current in one of the phases indicate a pump motor problem or is this normal? I am considering just replacing the 80 Amp breaker with a 100 Amp since this is what it should have been in the first place. What do you think?

REPLY IF the pump is starting OK and runs for several hours OK
I would assume
1. there is nothing wrong with the breaker - it is doing exactly what it is supposed to be doing.
2. I would check all connections
3. I would meggar that motor
4. IF nothing was apparently wrong and IF you have one handy I would put a data logger in there and record line volts, line current to the starter and currrent from starter to motor.
5. Have you made sure there are no mechanical problems ie binding etc - what shape are the shaft seals in??
WHOOPS it is submersible WITH water cooled motor IF you lost water cooling to motor windings I would think you would burn one out pretty quick -- I don't think this is the source of your problem.

IF the data logger indicates that all voltages and currents have been steady AND the breaker trips at a currrent less than what it should THEN replace it.

Dan Bentler

Logix · Dec 7, 2005

I hope that there is a motor starter circuit with thermal overloads on a motor this size. If the breaker is tripping, it's an indication that there is an instantanious short somewhere. Usually the thermal overloads will trip if a motor single phases, or is overloaded, before the breaker trips. Meggering the motor is a good idea, I'd also megger the wires that run to the motor to ground with both ends disconnected. There may be damaged insulation inside the conduit.

rta53 · Dec 8, 2005

Thanks for all the advice guys. As of yet I have not been to the customers site to check the motor or connections. This pump is mounted on 1 of 2 Reverse Osmosis machines in this plant. So far they are able to get by.

I hate to admit this but I don't own a megger. When we do a water system we usually get plant maintainence personel or the electrical contractor to megger the motors and wiring. However it is a tool I have wanted to purchase. I do know that Biddle is a common one I have seen, but the price range varies a lot. I looked at one at Graingers that had an output voltage of 1000VDC and costs around $900. Is it better to have the crank version or does that matter? If you guys have any recommendations for a good megger I would appreciate it.

elevmike · Dec 8, 2005

Megger is actuall a brand name that became generic. Kinda like "Kleanex" for facial tissue. So if you go to buy a "Megger" hold on to your shorts $$$$. You can get megga-ohm meters of various brands in all price ranges, but they are still pricy. If it's going to sit on the shelf alot (they usually do) then check ebay for a reasonable deal.

OT Tripping pump breaker

rta53

Lifetime Supporting Member

theDave2

Member

elevmike

Member

rta53

Lifetime Supporting Member

elevmike

Member

Tark

Member

MikeW

Member

elevmike

Member

rta53

Lifetime Supporting Member

Gerry M

Member

Tark

Member

leitmotif

Member

Logix

Member

rta53

Lifetime Supporting Member

elevmike

Member

Similar Topics