Shore Power Breaker Tripping

[Saturn_Europa]

The boat I work on is connected to shore power using an 400 amp Eaton Breaker. KD3400 breaker with a KT3400T trip unit. The breaker has been tripping at around 250-300 amps. We do not have any short circuits on the boat and the breaker is not tripping when starting large motors. I am pretty sure its tripping on thermal. Last night the breaker tripped, then tripped again shortly after resetting it. When the breaker tripped after being reset, the amperage was right at 250A.

From the trip unit manual:

"Thermal Trip: In accordance with standards requirements the thermal element trips the circuit breaker within 2 hours for an overload of 135 percent and trips inless time for higher over loads. For all currents inexcess of the magnetic setting, the tripping action isinstantaneous. In the overload trip region (up to 5 x lth),the trip current times are the same for AC or DC."


The Line to Line voltage is 480 and 277 to ground. The amps are mostly balanced across the three legs and phase rotation is correct. The breaker is new as of last year. It was replaced because the old breaker was tripping at 250 - 300 amps.


Any ideas on what could be causing this?

 

[Dennis Alwon]

It sounds like a bad breaker again. What is the environment of the breaker.... Outside ? Is the panel buss in good shape?

 

[retirede]

Time to investigate the panel and breaker using a thermal camera.

 

[jim dungar]

That breaker has a removable trip unit, is it also adjustable?

The trip unit contacts are some what delicate and may not like the marine environment.

 

[Saturn_Europa]

Its in a NEMA 4X enclosure. Its bone dry with no corrosion. There is no buss. Its lugged on both ends. The breaker feeds a 500 kva isolation transformer that we have on the deck. Its 480v to 480v delta delta. I guess that would be the next place to look.

The trip unit is adjustable for magnetic.

 

[jim dungar]

I guess that would be the next place to look

It is not likely the transformer based on your OP.
A transformer is not suddenly going to have an intermittent short circuit that would cause the breaker to open.
Transformer inrush is only a concern when the transformer is energized.

I agree, with retirede, thermal scanning would be a good idea.

 

[ptonsparky]

Fall of potential across the breaker contacts.
Thermal is much safer though.

 

[hillbilly1]

When the breaker was replaced, was the rating plug also replaced? Those are sold separately, and unless ordered with the breaker, will come without one.