main/tie/main

Status
Not open for further replies.
J

john d

Member
Location
Florida
If i remove the key interlock in the M/T/M so power can flow in either direction what are the code violations? The power comes from 2 different breakers at the same substation. In case there is a underground failure power is still available. This is the current set up seriesing 2 buildings and they want to ad a third building. They are careful to make sure 1 switch is always open.
 
john d said:
If i remove the key interlock in the M/T/M so power can flow in either direction what are the code violations? The power comes from 2 different breakers at the same substation. In case there is a underground failure power is still available. This is the current set up seriesing 2 buildings and they want to ad a third building. They are careful to make sure 1 switch is always open.
Click to expand...

A M-T-M interlock scheme is there to ensure that one of the 3 breakers is always open, this interlock exists because the available fault current in the system when designed with all 3 breakers closed (Both sources are parralled) exceeds the interuption ratings of the breakers.

If you remove or overide this interlock and if (or when) a fault occurs in the system the breakers will not be able to interupt the fault, the entire substation and maybe the building will burn to the ground and when the smoke clears you will be liable for everything that happened.

So, dont do it!
 
Since I'm not familiar with your systems ratings, let's say that it is 1000A mains and 1000A rated distribution bus.
How do you protect the 1000A rated distribution bus from exceeding 1000A, as the mains will likely split the load equally based on their upstream impedance.
 
ron said:
Since I'm not familiar with your systems ratings, let's say that it is 1000A mains and 1000A rated distribution bus.
How do you protect the 1000A rated distribution bus from exceeding 1000A, as the mains will likely split the load equally based on their upstream impedance.
Click to expand...

Ron, in a M-T-M setup the bus is designed to handle the full load for both sides, it needs to be in able to operate in the single ended mode.

The concern here is the available fault currents that are about doubled when all 3 breakers are closed exceeding the AIC ratings of the breakers.
 
I never stated this to be funny just to point out that a closed transition would also be a violation. some people will specify a closed transition depending on the type of facility (not taking into consideration the fault current being possibley 2x the design fault current) because of the "bump" i was just stating the obivious not trying to make it funny. But if your system was designed for closed transition at the time it was installed and six months later the avaliable fault current from the utility increases past the safe distance for closed transition then what? Does the utility notify the user when the fault current has increased? if not then how would you know?
 
Last edited:
I have seen industrial facilities where there is a selector switch to select which main is to trip when the tie to be closed. Upon closing the tie, the main that is selected to trip, trips in 3 seconds.

This provides uniterrupted power to the loads, and minimizes the time the 2 sources are parralled. And allows work to be performed on the main or its upstream equiupment.
 
markstg said:
I have seen industrial facilities where there is a selector switch to select which main is to trip when the tie to be closed. Upon closing the tie, the main that is selected to trip, trips in 3 seconds.

This provides uniterrupted power to the loads, and minimizes the time the 2 sources are parralled. And allows work to be performed on the main or its upstream equiupment.
Click to expand...

Sure that is fine, and not uncommon, but the system needs to be designed that way. If the system has key interocks as the OP stated the system is not designed to operate that way, removing the interlocks would violate more than just the NEC.
 
zog said:
Sure that is fine, and not uncommon, but the system needs to be designed that way. If the system has key interocks as the OP stated the system is not designed to operate that way, removing the interlocks would violate more than just the NEC.
Click to expand...

All comments make assumptions, but state their responses as unequivocal.

We do not know the short circuit ratings or duty, therefore making assumptions that the system is not suitable for parallel operation remains in question.

Establishing WHY the key interlock was installed in the original design would lead to more qualified answers. (It could have been a simple operational requirement.)

An AHJ can always approve a time limited automatic closed transition as the likeliness of a fault while transferring is infinitesimally small. (IEC recognizes this as a valid transitional mode without needing to double the SC withstand.)
 
Any pictures of these interlocks or the application for people like me who understand what your talking about but have never experienced or work on equipment as such.
 
mikeames said:
Any pictures of these interlocks or the application for people like me who understand what your talking about but have never experienced or work on equipment as such.
Click to expand...

A picture would not help you much as it looks like an ordinary cylinder lock. Follow this link to get a better idea what they do and how are they doing it.

This is the grandpappy of them all:

http://www.kirkkey.com/index.html
 
weressl said:
All comments make assumptions, but state their responses as unequivocal.

We do not know the short circuit ratings or duty, therefore making assumptions that the system is not suitable for parallel operation remains in question.

Establishing WHY the key interlock was installed in the original design would lead to more qualified answers. (It could have been a simple operational requirement.)

An AHJ can always approve a time limited automatic closed transition as the likeliness of a fault while transferring is infinitesimally small. (IEC recognizes this as a valid transitional mode without needing to double the SC withstand.)
Click to expand...


Agreed, I am making an assumption, but I bet it is correct. The OP asked about removing the kirk keys, not installing an auto transfer scheme.
 
Let me try to explain this better. Power comes out of a 400a breaker at a outdoor sub. 13.8kv underground to a existing building entering a load interupter switch[ a main without a key] ,busses to a fused interupter switch witch feeds into the building sub. , outdoors after the fused switch there is a switch [tie without the key] than another fused switch going in the building to another sub. than after that switch there is aload interupter switch [main without a key] than out to the next building underground. Picture this with 3 buildings. As long a 1 switch is open somewhere in this senerieo all is good. You would have 2 breakers feeding 2 systems. If that switch gets closed you would complete a circle and that would not be good. There must be a violation here somewhere. You would also be feeding some of these load interupter switches on the load side so if you open the switch to open the door it would still be hot on the load side. This system is serviced by only experienced people.
 
john d said:
Let me try to explain this better. Power comes out of a 400a breaker at a outdoor sub. 13.8kv underground to a existing building entering a load interupter switch[ a main without a key] ,busses to a fused interupter switch witch feeds into the building sub. , outdoors after the fused switch there is a switch [tie without the key] than another fused switch going in the building to another sub. than after that switch there is aload interupter switch [main without a key] than out to the next building underground. Picture this with 3 buildings. As long a 1 switch is open somewhere in this senerieo all is good. You would have 2 breakers feeding 2 systems. If that switch gets closed you would complete a circle and that would not be good. There must be a violation here somewhere. You would also be feeding some of these load interupter switches on the load side so if you open the switch to open the door it would still be hot on the load side. This system is serviced by only experienced people.
Click to expand...

Each time I read this I have a different picture of the system in my head, could you post a sketch?
 
You can not physically lock on all three keys at one time in a kirk key system, the link shows this, for those of you playing at home!
 
zog said:
If you remove or overide this interlock and if (or when) a fault occurs in the system the breakers will not be able to interupt the fault
Click to expand...

Zog,

Why the breaker will not able to interupt the fault if the interlock is removed??
 
Status
Not open for further replies.
Top