Contactor replaces Shunt Trip Circuiit Breaker 2

[electrageek]

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.

 

[busbar]

For frequent operation, that does seem line kind of a lame control scheme. NEC Article 427 bears review. Consider that local power-circuit isolation is not a prerequisite for setup/pre-run adjustment.

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.

 

[electrageek]

Thanks busbar. I was trying to find a electrically-held contactor but not having much luck. Siemens has a 400A motor contactor but it is mechanically held. Probably work ok though. Is there any concern for the interrupting rating of the contactor or breaker for faults inside the heating unit. I am going into a meeting on Monday to try and talk them into this change. It would seem that the other 400A breaker in the electric room would suffice for the short circuit protection. I could put current limiting fuses in series with the contactor I suppose if they were really concerned. My opinion is that they are not any worse off without the 400A shunt trip CB other then redundancy.

 

[busbar]

The overcurrent redundancy should be of little benefit.

 

[jbartos]

Suggestion: Please, would you post why the CB trips. Is it because it is designed to trip or because of overload? If it is designed to trip, that is o.k to be changed to the contactor. If it is not designed to trip, than the contactor is not right to install since you are losing one protection device.

 

[electrageek]

jbartos

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.

 

[peterb]

Electrageek -
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.

 

[electrageek]

Thanks everyone I appreciate the help.

 

[jraef]

I'm curious,
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!

 

[jbartos]

Suggestion: According to UL, the circuit breaker loses is UL Label, if it clears three short circuit faults. If there are shunt trips at the CBs and faults happen, one may try to reset the CB into a short. This means the CB lost two short circuit trips against its UL Label certification. Is it not a waste of the CB value in terms of UL Label certification?

 

[electrageek]

jraef,
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.