Thermal Mag Bkr vs HMCP 4

[JBUDA54]

Distinguished Forum Members!

I have a 250HP 480V Centrifugal Circulation Pump that supplies water to a Heat Recovery Steam Generator (HRSG) that we have in our specification to Supply an HMCP for short circuit protection and Motor Overloads for Long Time Thermal limit protection. I have been advised by the Eaton Sales Engineer that he has had to swap out HMCPs for High Inertia Loads and High Efficiency Motors due to the high inrush and that a HMCP does not have the flexibility that the thermal magnetic breaker has for the adjust instantaneous settings.

The ABB Baldor Motor is a high efficiency motor that has 1765A Inrush and 276 FLAs. That is slightly less than 6.5 x FLA. Per Eaton the HMCP model number is HMCP600L6CW which has a magnetic trip range of 1800-6000.

The 250HP is also at the top end of the pump curve so this pump could operate at a lower rating

The owner is requesting we spend $10,000 ($5K x 2 (for 2 motors) (approximate price difference between the Thermal magnetic Breaker vs HMCP Breaker)) more for a thermal magnetic breaker scenario because this is a critical motor that needs to not nuisance trip, and have a backup line of protection for the Thermal overloads. I have had other clients direct us to use the thermal magnetic breakers for size 4 and above to bullet proof their facilities so this is not uncommon.

Is it wise to promote to the client to save the $10,000 since the data for the HMCP points to a very feasible design or go with the Thermal magnetic breaker for the above mention requirements?

Keep in mind that the Corporate Elec Engr is the only promoting the Thermal mag breaker and the Project manager is having to answer to the Program Director why we are spending more for the equipment than was estimated at the lower cost.

Your thoughts are greatly appreciated. Thanks in advance.

 

[electricpete]

I have been advised by the Eaton Sales Engineer that he has had to swap out HMCPs for High Inertia Loads and High Efficiency Motors due to the high inrush and that a HMCP does not have the flexibility that the thermal magnetic breaker has for the adjust instantaneous settings

That sounds a little bit like this:

http://www.powellind.com/main/Uploadpdf/01.4TB.039 Motor Branch Fault Short-Circuit Protection.pdf

So what can be done to take care of the problem? As of now, I know of no good answer, but there are
several possibilities:
 Use a thermal-magnetic circuit breaker rather than an MCP. NEC §430-52, Exception No. 2C,
allows an inverse time circuit breaker rated up to 400% of the motor full load current to be used
for motors with less than 100A full load current. Small circuit breakers, up to 150A rating, have
fixed instantaneous settings, with minimum pickup varying from 7.5 to 12 times rated current, so
the instantaneous pickup can be from 30 to 48 time the motor full load current, rather than the 13
times required for an MCP. You have degraded the protection of the circuit, but you have met the
Code

The Powell document I quoted is old (1993). Maybe someone can dig up more recent NEC. Are you in the US?

To my mind it applies to high efficiency machines. I wouldn't think high inertia affects instantaneous tripping considerations.


=====================================
(2B)+(2B)' ?

 

[JBUDA54]

Yes I am in the US. Project is in Ohio.

I enjoyed reading that technical write up and it looks like it supports the argument to advise the client to save the dollars.

Thanks for sharing.

 

[jraef]

There is NO difference, I repeat NO difference, in the magnetic trip elements in an HMCP compared to a Thermal-Mag breaker of the same frame size. Whomever is telling you this is either not telling you the entire story, woefully uniformed, or is attempting to scam extra money from your organization. Assuming you are looking at an LD frame on both versions, the magnetic trips on either one are adjustable up to 10X the maximum frame rating, in this case 600A, which is why you get to 6000A. At 276A FLC, your MAGNETIC inrush current (not "starting" current) can in theory be as high as 2000%, so that is 5520A. Your HMCP (or LD frame Thermal-Mag) is adjustable to 6000A. You are fine. The NEC will not, however, allow you to adjust the trips that high, you are going to be limited to 1700% or about 4700A. But chances are, that will be fine too.

So the best possible benefit of the doubt I can give them is that what they REALLY are offering you is an ND (800A) frame T-M breaker with a smaller trip unit, because it will have a mag trip that is adjustable up to 8000A. But again, the NEC is not going to allow even 6000A, so the higher 8000A setting capability is irrelevant.

That said, there may be another PERFECTLY legitimate reason for you to NOT buy an HMCP breaker, but it has nothing to do with the setting capabilities. You cannot "roll your own" motor starter assembly with an HMCP, it can only be used as a part of a FACTORY built starter assembly that is going to be tested and listed as an assembly. So if you are making your own custom starter, you can ONLY use a T-M breaker (or fuses).

"Will work for (the memory of) salami"

 

[wroggent]

Assuming you are looking at an LD frame on both versions, the magnetic trips on either one are adjustable up to 10X the maximum frame rating, in this case 600A, which is why you get to 6000A. At 276A FLC, your MAGNETIC inrush current (not "starting" current) can in theory be as high as 2000%, so that is 5520A. Your HMCP (or LD frame Thermal-Mag) is adjustable to 6000A. You are fine. The NEC will not, however, allow you to adjust the trips that high, you are going to be limited to 1700% or about 4700A. But chances are, that will be fine too.

The limitation on setting the instantaneous trip only applies to instantaneous only breakers not inverse time breakers.

Read about this at:

http://ieeexplore.ieee.org/xpl/logi.../iel3/4013/11530/00523957.pdf?arnumber=523957

 

[oldfieldguy]

The MCP needs to be set at no more than 13 X FLC except for Design B energy-efficient motors, where it is allowed to be set at 17X per NEC 450-52 C 7.

Many people try to set these things too tight, using the "Change the trip point, try a start, it trips, go up one notch, then try again, stop when the motor starts." In my experience, this is where the nuisance tripping comes in.

Relying on the NEC and your motor FLA, since your energy efficient motor is allowed 17x FLA, multiply your FLA x 17, or 276 x 17, getting you 4692. Your MCP probably has settings from 1800-6000 in 600-amp increments. Set the trip point to the first setting below 4672, which should be 4200 amps. That should get you away from nuisance trips.

You will have complied with NEC, the MCP still provides the needed instantaneous protection for short circuits, and you have side-stepped the nuisance involved with the occasional high-current start.

old field guy

 

[jraef]

Wroggent,
Interesting take on it. I'm having trouble grasping the concept that I have been misinterpreting that clause in the NEC for 30+ years...

OFGs point is where I was going with it, but this new wrinkle, if I get the point, says that by GOING to that larger frame T-M breaker, you are allowed to ignore that 17x rule altogether, so setting the mag trips at 8000A becomes possible. Interesting.

"Will work for (the memory of) salami"

 

[jraef]

Now that I'm at a different PC and have my NEC with me, I looked at the referenced section. I can't believe I have never picked up on the subtlety of the placement of the "exceptions" that allow higher settings on the mag trips as only being pertinent to IT breakers. Wow... learned something new today.

That said, I do NOT think that was the intended result, in fact I have read some info discussing this code section in general years ago alluding to the fact that even the 1300% exception was a "mistake" and yet was allowed to remain, with statements to the effect that it would be removed in the next code cycle (written in in the '90s). That of course flies in the face of the subsequent addition of the 1700% exception that was added because of energy efficient motor issues. So maybe none of this was a "mistake", or maybe it turned out that it was a fortuitous mistake that solved a problem that they had not yet thought of back then so it was left alone. Either way, it looks like it works.

"Will work for (the memory of) salami"

 

[waross]

The Canadian code is simpler;
The rule for instantaneous settings for inverse time breakers refers to the rule for instantaneous only breakers which states in part:
"-rated or adjusted, for an ac motor, to trip at not more than 1300% of the motor full load current or at not
more than 215% of the motor-locked rotor current, where given-"

Bill
--------------------
"Why not the best?"
Jimmy Carter