Railroad Generator Breaker discussion.

[itsmoked]

Situation:

108Hp Turbocharged 4cyl Cummins, approx 108hp.
Nameplate:
55.0kW
68.8kVA
240V 3-Phase

Generator is underslung on a railcar.
It is mounted in an enclosure with a protection circuit breaker.

This circuit breaker originally was a thermal/magnetic 150A 3Ph
breaker. It's relatively large:
8" high, 6"wide, 4"deep; handle 2" long, 1" wide, 1/2" thick.
Making me speculate it's a 600V breaker.

Anyway, it regularly tripped. This required the train to stop so someone could reset it. <big frown>

Someone changed it to a 200A breaker. Now it nuisance trips whenever the car is running in high ambients; 105+.

This breaker, as mentioned, is in the enclosure with the turbocharged diesel, its turbo, its radiator, and the exhaust manifold.


Questions:

1) Why is the factory breaker on this 55kW generator 150A?
Isn't 55kW/240 = 230A?
Isn't 230A/3phases = 76A?

2) Can this scheme ever be made to work based on a breaker in an weather ambient that swings regularly from -30F to 120F meaning the breaker ambient probably swings from 70F to 300F?

3) Any suggestions?

 

[davidbeach]

55000W/(240V*sqrt(3))=132A

132A*1.25=165A (Minimum CB size), round up to 200A.

My guess is that the breaker is applied in an ambient temperature far higher than the 40 deg C for which it is rated. Thermal Mag breakers needed to be derated for higher ambients; best bet would be to separate the breaker from the heat.

 

[rbulsara]

davidbeach is right. Or replace the breaker with one with electronic trip unit that is insensitive to ambient. or derate further..if you know actual load does not exceed the rating.

 

[itsmoked]

The generator supports two large refrigeration compressors and all house loads. So there may be some case where both compressors start simulataneously or one starts after the other has been running for a long time and the breaker is already "warm".

I am bothered by further derating because in cold weather the breaker may actually run at its nominal value providing poor protection.

Anyway I think you guys are correct, the breaker should be moved out of the engine compartment. Even if it's only into a Hoffman box a foot away.

Thanks for the sqrt(3) reminder David! Makes a bit of a difference...

 

[ScottyUK]

David,

Does the US code use generator kW for breaker sizing or kVA? Does this explain the 1.25 multiplier to account for the typically-quoted 0.8pf for small gensets?

I agree with moving the breaker away from the engine, and additionally suggest putting an anti-condensation heater and a stat in the breaker enclosure to keep the cold and damp at bay. This is just good practice for electrical panels in cold environments. If the box is a sealed type, put a breather in the base to prevent condensation forming.

The electronic tripping units typically offer much more flexible protection than a thermal-mag unit can: loss of phase (or phase imbalance) and G/F protection are usually included for a relatively small premium over a thermal-mag type.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

Got a lead to one of these electronic trippers?

The plan is to now remove the breaker and move it into the car's electrical closet 50ft away, which is part of the air conditioned space.

Scotty, the breaker is mounted in the generator wiring box (the sheetmetal hump on the generator) which is never wet when the generator is running and is open 5 ways from Tuesday.

 

[ScottyUK]

My current favoured MCCB range is the Siemens 3VF and 3VL series. I pity you doing battle with the Siemens catalogue for the first time - the paper one is hard work, and the website is one of the most user-hostile sites I've ever used. I'll have a look in the paper catalogue when I get back to work - their website has just stolen 25 minutes of my Saturday morning and given me nothing for it.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[rbulsara]

Try this link:

http://www.sea.siemens.com/circuit/product/CircuitBreakers/MCCB-Family Brochure.pdf

I beleive its Siemens JD series Sentron breaker that has electronic trip. So does GE's Spectra series with Microversa trips and Square D or Merli Gerin's with micrologic trips.

 

[itsmoked]

ScottyUK, I hate that webstealing! I know what you mean. In fact earlier this week I was at Siemens' crummy WEB site and made the same comment in another thread. Wish they'd get their heads outta the sand! Thanks for the effort.



rbulsara thanks for the Siemens link I shall study it.
I'll check the Square D and Merli Gerin's

 

[davidbeach]

ScottyUK, well, I went back and looked at the NEC text, and it basically just says that generators should be protected.

Constant-Voltage Generators. Constant-voltage generators, except ac generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses, or other acceptable overcurrent protective means suitable for the conditions of use.

and

Ampacity of Conductors. The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator....

Conductors from Generator Terminals. Conductors from the generator terminals that meet the size requirement in 445.13 shall be permitted to be protected against overload by the generator overload protective device(s) required by 445.12.

So, actually, you get to pick a size as long as the generator is protected against overload and the conductors are protected at their ampacity. Pick a value. The values I quoted in my previous post are those I've used for many years without problems.

 

[itsmoked]

Can the breaker be moved 50 ft from the generator? So the wires leaving the generator compartment in EMT head 50ft to the breaker? No protection...

 

[itsmoked]

BTW thanks for those quotes davidbeach.

 

[ScottyUK]

ItSmoked,

For this situation it is possible to use some form of differential protection and place the cables from the generator to the switchgear as part of the protected zone. Big sets typically use a true differential scheme with a CT at the neutral end of each winding and another at either the line end or at the switchgear. Small sets tend to save a couple of CTs at the neutral end by employing restricted earth fault protection. A protection relay like Areva's P342 would be well suited to a small set.

David,

Those must be the US breaker range - they're definitely different to the new European range although they have a passing resemblance to the older 3VF series, which itself has an uncanny resemblance to the even older Westinghouse HCMP breakers. The US range is probably more use to Mr. Smoked than the one I was searching for - thanks.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!