Shunt trip with multiple push button

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john37 said:
I'm not totally sure how to wire a shunt trip breaker with multiple power cut off push buttons. There are 3 different push buttons in 3 different rooms for a MRI room. Please see attached diagram and let me know if I wired the push buttons correctly. Thanks.
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Besides of the wiring showing incorrectly, I disagree with the use of Shunt Trip as the EPO. Since it requires power to operate, it will be ineffective when the 120V is not present.

A true EPO would use a safety contactor with series wired STOP buttons keeping it energized/closed all the time. In case of loss of control power or the breaking any of the wires the contactor will open.
 
weressl said:
Besides of the wiring showing incorrectly, I disagree with the use of Shunt Trip as the EPO. Since it requires power to operate, it will be ineffective when the 120V is not present.

A true EPO would use a safety contactor with series wired STOP buttons keeping it energized/closed all the time. In case of loss of control power or the breaking any of the wires the contactor will open.
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For MRI's, it's not typical to make the EPO fail safe. Actually, the danger from Estopping an MRI is probably greater than the danger from needing an Estop and having it not work.

Estopping an MRI will quench the unit, causing super cooled helium to suddenly boil. The boiling helium expands in volume, and normally blasts out a quench vent, but sometimes it can fill the room, displacing the oxygen and pressurizing the room so it may be hard to open the door.

Steve
 
steve66 said:
For MRI's, it's not typical to make the EPO fail safe. Actually, the danger from Estopping an MRI is probably greater than the danger from needing an Estop and having it not work.

Estopping an MRI will quench the unit, causing super cooled helium to suddenly boil. The boiling helium expands in volume, and normally blasts out a quench vent, but sometimes it can fill the room, displacing the oxygen and pressurizing the room so it may be hard to open the door.

Steve
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I said a TRUE E-stop. An EMERGENCY-stop's function is to override a failure where NORMAL stop failed to operate.

Your reasoning then would remove the need to have a stop, if the hazard cxreated from it overrides the ned to stop it in the first place.

The doors can open OUT. You have added an additional red herring here as not only something went wrong, so the E-stop was needed, but created another failure where the emergency vent failed, even though those are constructed in the MRI to be unobstructed and failure safe. Where double failure needs to be considered other measures should be taken to eliminate the likeliness. A critical system will be as reliable as its weakest component. An energize to OFF is not a failsafe unless it is done with UPS and redundant paths and monitored.
 
weressl said:
I said a TRUE E-stop. An EMERGENCY-stop's function is to override a failure where NORMAL stop failed to operate.

Your reasoning then would remove the need to have a stop, if the hazard cxreated from it overrides the ned to stop it in the first place.

The doors can open OUT. You have added an additional red herring here as not only something went wrong, so the E-stop was needed, but created another failure where the emergency vent failed, even though those are constructed in the MRI to be unobstructed and failure safe. Where double failure needs to be considered other measures should be taken to eliminate the likeliness. A critical system will be as reliable as its weakest component. An energize to OFF is not a failsafe unless it is done with UPS and redundant paths and monitored.
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You are completely missing the point. This is how MRI's are wired. You don't want to quench the unit every time power is lost.
 
steve66 said:
You are completely missing the point. This is how MRI's are wired. You don't want to quench the unit every time power is lost.
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No Sir,

I have responded to the OP. I perfectly understand your point and said two things:
? The E-stop - as the OP considered it - is not an appropriate designation as it is wired. Then I described how an E-Stop should be wired.
? If the E-Stop creates additional hazard, then it was inappropriately applied for this machine in the first place.
As I said you created a red herring - unnecessarily - as the problem with the venting is a separate problem and would not come into play if the use of the E-stop is resolved. If the use of E-Stop would be an overriding reason - for whatever foolish reason a Government Bureaucrat decided so - then it can not be denied based on the improper functioning of the vent. The emergency relief went is created to relieve the overpressure and if it does not function as intended, then it need to be fixed, separately.
 
Thank you very much guys....Being a new Engineer I wasn't totally sure how to wire the shunt trip with multiple pushbuttons. In the mean time I asked my supervisor to explain it to me.
Ronaldrc thanks for the diagram. That really made it more clearer for me to understand it.
I didnt' mean to create a debate about it. It was interesting to read though!
 
I often design EPO's because the code requires them and not because I think they are needed (645.10). I often design them so that when the CB is off, the EPO doesn't work.
The 645.10 application is different than a conveyor belt application in my opinion. I would design the two solutions differently.
 
ron said:
I often design EPO's because the code requires them and not because I think they are needed (645.10). I often design them so that when the CB is off, the EPO doesn't work.
The 645.10 application is different than a conveyor belt application in my opinion. I would design the two solutions differently.
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Perhaps a defintion would be helpful to distinguish different applications. One may be emergency stop, the other emergency power off?
 
weressl said:
Perhaps a defintion would be helpful to distinguish different applications. One may be emergency stop, the other emergency power off?
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I think you are correct in differentiating the two types of systems, with an Emergency Stop, it effectively stops the process the machine is initiating, where Emergency Power Off completely removes power in case of a catastrophic failure, even though it effectively stops the machine also. I think this is the intention of this requirement. I always try to power the shunt trip from the same source as the breaker (preferably load side) so that if interuption of power to the breaker will not affect operation of the shunt trip. If theres no power to the breaker, tripping the shunt trip would have no effect, as the power is already off.
 
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