ELECTRONIC CB

[jimbo123]

I have a Eaton/ Westinghouse HDG 3400f circuit breaker with a kt 3300t trip unit. Its a 400 cb with a electronic trip unit rated for 300amps. This cb controls a vfd for a pump motor that is only drawing 195 amps and will las t for about 30mins when the cb trips. I put a amp meter on it while its running with a min/max setting and meter never records more then 195amps. Motor is said to be rated 135 hp at 480v. There is no shunt wiring that i can see. Unit is inservice for at least 2years.
Does anyone if these trip units fail or have a bad history, anything?, they are electronic units

 

[zog]

Your breaker cat# is wrong I think, sounds like a frame only #. But, as far as the trips, what is your thermal pickup set at? Could be that simple. Or you could have a harmonics issue here, these are not true RMS sensing trip units and have issues with things like VFD's at times.

 

[jimbo123]

You are right zog breaker # HKD 3400F the trip unit is a kt 3300t a 400a breaker with a 300a trip unit. breaker is seeing only 195amps. All 3 setting on cb are set at the very min. for the last 2 years. The last 2 days has been tripping out. Don't has enough info on cb or trip setting to change them . I will be trying to locate some spec sheet on them . [ HKD65]
Thanks for pointing me in right direction.

 

[zog]

Well the settings determine when the breaker trips, sounds like (Based on your 30 second statement) it is tripping when it should. The settings are determind by a load and coordination study but pretty simple for LT (Or thermal) setting, should be set 100-125% of motor current. If it has worked fine for 2 years my guess is the motor is seeing a mechanical failure (Bearings perhaps), drawing more current than normal, and your system is working perfectly to do what it is designed to do.

 

[Jraef]

I have a Eaton/ Westinghouse HDG 3400f circuit breaker with a kt 3300t trip unit. Its a 400 cb with a electronic trip unit rated for 300amps. This cb controls a vfd for a pump motor that is only drawing 195 amps and will las t for about 30mins when the cb trips. I put a amp meter on it while its running with a min/max setting and meter never records more then 195amps. Motor is said to be rated 135 hp at 480v. There is no shunt wiring that i can see. Unit is inservice for at least 2years.
Does anyone if these trip units fail or have a bad history, anything?, they are electronic units

Where did you put the ammeter? On the line side or the load side of the VFD? On the load side the readings are meaningless. On the line side your readings may be inaccurate because of harmonics, which may also be causing interference in the way the ETU is seeing the current.

Also, you say that the unit has been in service for two years, but does it really say Westinghouse on it? The name "Westinghouse" has not been put on a circuit breaker for over 15 years now (Eaton bought that unit in 1994, they switched the name immediately but it took a few years to deplete inventory). If so that means someone may have put in a used breaker, who knows it it had been properly tested / calibrated / programmed or if it was even good from the outset.

 

[jimbo123]

I took the amp reading on load side of vfd. Thanks for the info , now Iknow why the key pad on the vfd was off to the amp meter. it did say eaton on breaker . It is a 400a breaker with 300a trip unit. It is a kt 3300t , would you know off hand the starting range of the trip unit ? ?? - 300amp.

Thanks again with the help.

 

[Jraef]

I took the amp reading on load side of vfd. Thanks for the info , now Iknow why the key pad on the vfd was off to the amp meter. it did say eaton on breaker . It is a 400a breaker with 300a trip unit. It is a kt 3300t , would you know off hand the starting range of the trip unit ? ?? - 300amp.

Thanks again with the help.

• It's not an electronic trip unit, it's a standard Thermal-Magnetic trip unit.
• On T-M trip units the thermal trip is not adjustable, it is fixed at 300A
• The magnetic trip is adjustable from 500 - 1000% of the tip unit rating, so in this case 1500 - 3000A

So that leaves up with 3 possible causes:

1. The input current is much higher than you are detecting by measuring the output current, which is a distinct possibility given that you measured the output of the VFD.
2. The harmonics that the VFD is creating on the LINE SIDE are causing disproportionate heating of the thermal elements in the T-M trip unit. I know there are those who will say this can't happen, I am not among them because I have seen it happen. I cannot adequately describe why, I had my theories but other members have shot them down. It definitely happens though and I have fixed it by adding line harmonic filters in front of the VFD.
3. After 30 minutes there is some sort of heating effect somewhere in the circuit that causes a ground or phase current fault and the breaker is tripping on Instantaneous current (magnetic trip).

My advice would be that before you go spending money on solutions, you should get hold of a recording meter. Look at WHAT the breaker trip is associated with by measuring current at the load side of the breaker, because unfortunately, that breaker will not tell you why it tripped, only that it did.

 

[jimbo123]

Jraef, thanks for your help with this breaker. Today i was told that the motor /pump is 134 hp [sounds like a odd size to me ] the vfd is sized at 125 - 150 hp . The key pad has running readings, amp 204 at 60 hz records faults , motor temp , Motor only runs at 60 hz at startup 204amps.

Then for the next 1.5 hrs it ran at about 170 -185amps by the keypad readings. The flc on a 150hp is 180amp. cb is 300a. this is a cutler hammer sux9000 model. When the cb trips and i turn it back on the keypad does not have any faults recorded so i really don't have any idea why it trips.

If you have any ideas on the type of recorder to use or anyother info would be great. Thank you help.

 

[Jraef]

...
If you have any ideas on the type of recorder to use or any other info would be great. Thank you help.

If you can, I would look into renting one, they are very expensive to buy for one little project like this, probably $3K+.
Like this:
http://dranetz.com/portable/powerxplorer-px5
or this:
http://www.atecorp.com/equipment/bmi/4800.asp

Or ask around with your buddies and see if anyone has anything you can borrow, that's what I do now that mine was stolen.

 

[jimbo123]

thanks again for your help