Contactor replaces Shunt Trip Circuiit Breaker
Contactor replaces Shunt Trip Circuiit Breaker
(OP)
I have an application that currently uses a 480Vac, 400A shunt trip circuit breaker (CB). The load is a 225KVA SCT (saturable current transformer)resistor heating system for annealing metal parts. The problem is that after each cycle they trip the 400A CB to shut off the heating power. After a number of trips the breaker malfunctions and needs to be rebuilt mechanically. I want to replace the 400A CB with a 400A rated vacuum contactor (Siemens part 300HP @ 480Vac). Besides the 400A CB inside the heating unit itself there is also a 400A CB back in the Electric Room that supplies the 400A CB in the unit. I would be keeping that breaker and it is lockable where as the local breaker is not. What do I need to be concerned about if I replace the local CB with the contactor.






RE: Contactor replaces Shunt Trip Circuiit Breaker
For an electrically-held contactor, the control logic changes from an on pulse at shutdown, to continuously present while on. For a mechanically-held contactor, the control logic is closer (but not identical) to the shunt trip.
Something on the order of a NEMA Size 5 Non-Fusible Combination Starter may be appropriate.
RE: Contactor replaces Shunt Trip Circuiit Breaker
RE: Contactor replaces Shunt Trip Circuiit Breaker
RE: Contactor replaces Shunt Trip Circuiit Breaker
RE: Contactor replaces Shunt Trip Circuiit Breaker
The undervoltage release device (UVR) in the CB can be dropped out in three ways. First the loss of input voltage depending on the phase can drop out the UVR. Secondly a normally closed contact from the emergency stop pushbutton is wired in series with the UVR coil. Thirdly there is a normally open contact from the PLC (control computer) in series with the emergency stop PB that is energized to permit the manual closing of the CB then it opens up at the end of a cycle to drop out the UVR which opens the CB. Most customers just don't like change. They think that if that was the way it was designed then there must be a good reason and I am just missing whatever it is. I know that the CB from GE was never intended to be used as a contactor and my experience says it should be tripped only in a true fault condition once in a great while. They are just so use to closing and reseting and closing that my idea scares them.
RE: Contactor replaces Shunt Trip Circuiit Breaker
Busbar is on the right track. If what you want is an electrically held contactor, a standard NEMA size 5 contactor will do the job - check Allen Bradley, GE, ABB, Cutler Hammer etc for rating details. That size contactor will probably have a DC coil, with a rectifier and an economizing control (capacitor or resistor).
I suggest that you take a data sheet to your meeting that gives the rated switching currents and the rated switching frequency of the contactor. For sure, the upstream breaker will continue to provide short circuit protection and the contactor will not be called on to interrupt fault current.
RE: Contactor replaces Shunt Trip Circuiit Breaker
RE: Contactor replaces Shunt Trip Circuiit Breaker
First you mention that this is a Shunt Trip, then later you call it an Undervoltage Release. They are distinctly different devices with different uses. Which is it?
UVR coils on circuit breakers are used to provide protection against re-energizing a circuit after a power failure. Once installed, additional circuits can use it in a similar function to a Shunt Trip, but not vice-versa. If you replace the UVR with an electrically held vacuum contactor, you MUST provide a "3 wire" control circuit in order to duplicate this functionality. This may make in incompatible with PLC control, and that may be why they did not use it in the first place!
A work-around to this would be to add a voltage monitor relay that also drops out the vacuum contactor coil in the event of a power loss, requiring manual reset.
Incidentally, try Joslyn Clark for the vacuum contactor. They make a very inexpensive and quite capable 600V rated VC, one of the only ones I know of that are low voltage, which saves significant cost.
That which does not kill me, makes me stronger... and pissed off!
RE: Contactor replaces Shunt Trip Circuiit Breaker
RE: Contactor replaces Shunt Trip Circuiit Breaker
I was correct the second time, it is a UVR (undervoltage release) device. The original reason it was installed was to meet a customer request that the heater system have a way to take the power off the heater unit after a cycle was completed. The unit already had the 400A breaker so they added the UVR device to it to give the customer a inexpensive way to remove power at the end of a cycle. I will check on the Joslyn Clark contactor.