Shunt trip/contol wiring fusing

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Clint1000

Member
Hope this is the correct area to post this. Would someone be able to steer me in a clear direction in the code book as to the proper method of fusing shunt trip control wiring in general. One application is an x-ray unit 480 volt, 3 phase with an 80 amp shunt trip circuit breaker. I am trying to clarify the required way to wire the shunt trip with respect to fusing the control conductors. I seem to be getting two differences of opinions.

First idea is that it is required to come off the equipment breaker so that there is little to no chance that the circuit will be interrupted, loss of power and subsequently will not be able to operate the shunt unit in an emergency via the emergency switch. Is it permissible to connect the control wiring to the load side of the 80 amp circuit breaker and then wire through the emergency shutdown switch and back to the shunt unit? Wouldn?t there be a potential issue with multiple wire terminations on the circuit breaker lug? I checked the manufacturer?s instructions and it doesn?t appear that there are provisions for this but nor does it indicate it is not permissible. Also I did not see any tabs/stabs to connect cable terminals (crimp on) to the load lugs as some times motor starters provide them. If this arrangement was done, would the conductors need to be fused to their normal current ratings, #12-20amp or #10-3 amp?

The second idea is that an independent control voltage source be used and then ultimately wired to the shunt unit. The concern is, if the breaker was turned off (or install lock on handle), or tripped without knowledge, the emergency shutdown would not work.
Would someone please clarify this for me as to the NEC code requirements? I have resolved the state?s requirements but am having a hard time with the NEC requirements, rather than the ?it would be nice? scenario. Thank you
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I've read that you can get a breaker that trips upon loss of control-circuit power rather than upon energization. Perhaps this would work with a circuit modification.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I've read that you can get a breaker that trips upon loss of control-circuit power rather than upon energization. Perhaps this would work with a circuit modification.
That type of breaker function is called an undervoltage trip.
 

buddhakii

Senior Member
Location
Littleton, CO
I agree with Larry. Most of the ones I've done in the past couple years would trip with loss of voltage. We ran the shunt trip power off of a seperate breaker in the panel and put a lock on it.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I agree with Larry. Most of the ones I've done in the past couple years would trip with loss of voltage. We ran the shunt trip power off of a seperate breaker in the panel and put a lock on it.
Your statement is a little confusing.

In molded case breaker parlance:
Undervoltage trip means loss of control voltage.
Shunt trip means application of control voltage.

These two trip terms are not interchangeable.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Typically, when a Shunt Trip is used on something such as an X-Ray machine where you have a large power supply, it is for what they call an "EMO" circuit; EMergency power Off. There are specific rules within different industries with regards to what you can and cannot use/do in EMO circuits. Shunt trips are the rule, UV trips are the exception to the rule because the EMO circuit is typically a "last resort" circuit to be used only when life-safety is at risk and may end up causing significant equipment damage when activated. So the logic behind the circuit is going to require POSITIVE action on someone's part, as opposed to the LACK of action, ABSENCE of power or a circuit component FAILURE to act. Be sure to check with the X-Ray machine supplier to see if there are any special rules as to how this circuit is configured.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
So the logic behind the circuit is going to require POSITIVE action on someone's part, as opposed to the LACK of action, ABSENCE of power or a circuit component FAILURE to act.
Of course, a normally-closed switch or contact could be used.

Be sure to check with the X-Ray machine supplier to see if there are any special rules as to how this circuit is configured.
Absolutely!
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Of course, a normally-closed switch or contact could be used.

Absolutely!
Larry
I usually enjoy and learn from your responses to these forum questions.
But lately I have noticed that you have 15,000 posts since 2005. Do you have a life other than this forum or do you just like to hear yourself talk?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
But lately I have noticed that you have 15,000 posts since 2005. Do you have a life other than this forum or do you just like to hear yourself talk?
I'm just trying to cathc up with Bob. :wink:

Besides, it's only just over 11k. :cool:

By the way, where Marc? (mdshunk) :confused:
 

Clint1000

Member
Thank you for all of your feedback. An emergency shutdown, shut trip is required and I was wondering on the fusing requirements of such installation with respect to connecting the control wiring and proper fusing size for it. It is the wiring from the circuit breaker shunt, over to the emergency control knob and then back to the shunt (switch loop). If it was to be connected to the load side of the 80 amp circuit breaker (factory installed shunt trip in this device), how/would the control wires, #10 thhn in raceway be required to be fused? It would be a 480 volt shunt trip since the breaker is 480 volt and would be an easy source of power for the shunt. Would someone please indicate which article in the code calls out these or other provisions? Thank you
 
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